Barometer table

[Guest guest]

David, I know how you enjoy handy tools and rules, just don’t rely on them. The best tool you have when out on the water is from the neck gasket up, uncluttered with facts and figures and alert to what is around you. Hope you enjoy this.

Source: National Weather Service

The amateur forecaster should modify the table in accordance with his or her own observations. The following show the wind direction, the barometer reduced to sea level and the character of the weather indicated:

SW to NW, 30.10 to 30.20 and steady - Fair with slight temperature change for 1 to 2 days.

SW to NW, 30.10 to 30.20 and rising rapidly - Fair, followed within 2 days by rain.

SW to NW, 30.20 and above and stationary - Continued fair, with no decided temperature change.

SW to NW, 30.20 and above and falling slowly - Slowly rising temperature and fair for 2 days.

S to SE, 30.10 to 30.20 and falling slowly - Rain within 24 hours.

S to SE, 30.10 to 30.20 and falling rapidly - Wind increasing in force, with rain within 12 to 24 hours.

SE to NE, 30.10 to 30.20 and falling slowly - Rain in 12 to 18 hours.

SE to NE, 30.10 to 30.20 and falling rapidly - Increasing wind, and rain within 12 hours.

E to NE, 30.10 and above and falling slowly - In summer, with light winds, rain may not fall for several days. In winter, rain within 24 hours.

E to NE, 30.10 and above and falling rapidly - In summer, rain probably within 12 to 24 hours. In winter, rain or snow, with increasing winds, will often set in when the barometer begins to fall and the wind sets in from the NE.

SE to NE, 30.00 or below and falling slowly - Rain will continue 1 to 2 days. SE to NE, 30.00 or below and falling rapidly - Rain, with high wind, followed, within 36 hours by clearing, and in winter by colder.

S to SW, 30.00 or below and rising slowly - Clearing within a few hours, and fair for several days.

S to E, 29.80 or below and falling rapidly - Severe storm imminent, followed within 24 hours, by clearing, and in winter by colder.

E to N, 29.80 or below and falling rapidly - Severe northeast gale and heavy precipitation; in winter, heavy snow, followed by a cold wave.

Going to W, 29.80 or below and rising rapidly - Clearing and colder.

Source: National Weather Service

[djlewis]

Hi, Rick:

This ~is~ neat. Is it for real? Or more politely, how accurate is it?

The NWS is certainly in a better position to test its accuracy than we are, so I'm not sure what to make of the injunction to "modify the table in accordance with his or her own observations".

Can you send me the actual pointer or URL for the source at NWS.

Yes, from the neck up...

Thanks. --David.

[djlewis]

I found this in several places around the net, though not at NWS/NOAA. This one seems the best, with a table format and some additional "general statements", which I've repeated here in case this link ever goes away.

http://www.weatherwagon.com/pressureforecast.htm

The text below is paraphrased from the National Weather Service

Here are some general statements of wind-barometer readings that are generally true in the United States. "When the wind out of the south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest.

"When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.

"As a rule, winds from the east and falling barometric pressure indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather, but again there are exceptions and in some parts of the country these rules do not apply."

[jeffcasey]

Since it is too slimy outside to play hookey and paddle, and we are all geeking out **talking** about paddling, here is some perspective on weather and altitude:

Rick's table shows sea level weather variations ranging from 29.8 to 30.2 (inches water). This is about a 1.3% pressure variation, which corresponds to about a 366 ft elevation gain. You can figure that typical daily weather stuff will cause your barometer/altimeter to go wonky on the 50-100 foot precision and constantly need recalibrating due to weather fluctuations...but if you are doing big mountains, the altitude changes will dominate over the weather.

David likes the idea of staying at sea level as a fixed reference so the barometer is a good weather gauge. The 10-20 foot sea variations (waves, tides, and the big rock you eat lunch on) are unlikely to materially change your weather interpretations, so this works.

Working the other way, here are some landmarks:

-Mount Washington, you are down to about 79% of sea level air pressure,

-I've heard (unsubstantiated) that commercial airlines pressurize their cabins to 8000 ft equivalent, or about 75% sea level pressure (Sir Godfrey?),

-Top of the Sierra Nevada or the Rockies, about 58% of sea level,

-my personal high point (about 18,000 ft) was huffing and puffing at just about 50% of sea level,

-Ken Cooper tromping through the Andes probably gasped for breath at 41%,

-Everest tops out at 31%, but that is the zone where you seriously kill brain cells.

[Guest guest]

Jeff:

I've been grinding through a fairly heavy textbook on weather lately (actually using it to fall asleep at night). In the process I've had a series of 'I never realized that' moments.

One of the recent ones was that reported barometric pressures are all by convention "reduced" to sea level. In other words, the decrease in atmospheric pressure due to altitude (which your figures illustrate) are compensated for by using a standard table or formula to convert them back to the sea level equivilent. For exemple, pressure on an isobaric map of the U.S. for, say, Mt. Washington is not the actual barometric reading for Mt. Washington but rather the actual reading adjusted for the fact that Mt. Washington is 6,000' plus above sea level. Conceptually, the pressure equals a point at sea level below the peak of the mountain. Without this adjustment or

'reduction', a regional barometric map over land would be meaningless.

So, while a barometer used for hiking in the mountains can be very useful to track rising and falling pressure (and also the rate of change) to predict weather changes, the numbers on your barometer at altitude will not correspond to the numbers in the chart that Rick and David have posted. With the proper adjustment, however, they should be as valid as they are at sea level.

Of course, we sensible mariners don't have to worry about this effect. We just have to get out of the way of cold fronts, squalls and fast moving nor-easters.

Scott

[Jed]

Scott,

As per the manual from my hand-held barometer (Silva Escape): Prior to use, the barometer is calibrated for this sea-level adjusted pressure. Basically the local adjusted pressure is acquired from some official source, an offset is applied and the result matches the Isobaric charts. As long as the altitude is held constant (ie the barometer is not transported over varying altitudes) then the pressure(s) as read off the instrument will match the numbers in Rick's NWS chart. But as you point out, if the barometer is used for hiking, the altitudes will vary so the absolute readings and trends will not match the charts.

An interesting exercise is to convert the wind directions, pressure ranges and trends in the NWS barometric chart to system / front locations and movements. A barometer can be a great (if somewhat technically demanding) tool to forecast weather when away from NWS services for an extended time. Use of the NWS system (via radio, internet, TV) provides a distinct advantage in speed, accuracy and convenience, so few people take the time to learn how to use a barometer well. Have fun with the weather text, mind-numbing isn't it?

Jed

[Jed]

>Here are some general statements of wind-barometer readings

>that are generally true in the United States. "When the wind

>out of the south and southeast and the barometer falls

>steadily, a storm is approaching from the west or northwest,

>and its center will pass near or north of the observer

>within 12 to 24 hours, with wind shifting to the northwest

>by way of south and southwest.

>

>"When the wind sets in from points between east and

>northeast and the barometer falls steadily, a storm is

>approaching from the south or southwest, and its center will

>pass near or to the south of the observer within 12 to 24

>hours, with winds shifting to northwest by way of north. The

>rapidity of the storm's approach and its intensity will be

>indicated by the rate and amount of the fall in the

>barometer.

Why are the above statements only "generally" true? What is the mechanism? What are the limits of these generalities? There is a larger picture / better model that is much more accurate but significantly more complicated. What is the "between the lines" message?

>"As a rule, winds from the east and falling barometric

>pressure indicate foul weather, and winds shifting to the

>west quadrants indicate clearing and fair weather, but again

>there are exceptions and in some parts of the country these

>rules do not apply."

How does this differ from the previous statements? Where is this true and where is it that these "rules" "do not apply"? What information would be required to transform these general statements into absolute and always accurate rules?

Jed

jluby@teamnorthatlantic.com

Life is too short to own an ugly boat.

[jeffcasey]

"Generally", weather in the mid latitudes (most of the U.S.) comes from the west. A fairly regular exception is that of tropical storms which come up from the south.

Incoming storms are low pressure systems. Coriolis effects are deviations from Newton's laws due to our rotating frame of reference -- in the northern hemisphere, trajectories will be modified by coriolis effects to "bend" to the right. Air rushing into a low pressure zone will bend to the right of the center. Since air rushes in from all sides, this sets up counter-clockwise circulation.

Thus a counter-clockwise storm approaching from the west will indicate its impending arrival with 1) dropping pressure and 2) winds out of the south. After the storm passes to the east, the winds will have shifted around to the north.

Similarly a counter-clockwise storm approaching from the south (such as hurricanes or wintertime nor'easters) will start with 1) falling pressure and 2) winds from the east. The typical nor'easter actually stays offshore, so the winds start from the east (when the storm is way south), then shift to winds from the north (when the storm is directly offshore). These bring in a lot of moisture and are where our big snow dumps in the winter usually come from.

Jed wants certainties....let's see....weather is too complicated...two or more storm systems can combine and anything can happen (read "the perfect storm") - two storm systems can even spit out a third one like a pitching machine, and in nearly any direction. This sort of weird stuff doesn't happen too often, since the chances of big well developed low cells colliding is fairly low. Hence the "generally". Other wrinkles include local topography (mountains, lakes), which usually give different rules to the local weather.

For more specifics, the weather geeks build big computer models (indeed, this is much of the incentive behind the development of some of the large supercomputers). Some of these models are nothing more than huge libraries of past weather....they match up conditions to previous conditions and predict based on what happened before. Some of them look at the regions of high & low pressure and predict where the large scale air transport will occur....thus predicting where existing storms will move to. All of this is very complicated at the detail level, and very expensive (you need a lot of local data to build the weather map).

Jed's last quote, "winds from the east and falling pressure indicate foul weather".... falling pressure indicates an approaching storm. With intact counter-clockwise circulation, an east wind means the storm is to the south. Since foul weather / warm-water-storms often travel south to north, a storm due south is probably aimed head-on for you. Similarly, winds out of the west imply a low to the north. Storms very rarely travel north to south, so the implication is that the storm is headed away (the west wind is probably associated with a rising barometer).

All this is from the hip, so any of it can be askew. Perhaps Scott can retrieve his soporiphic tome from under his pillow and correct any gotchas.

[djlewis]

>A barometer can be a

>great (if somewhat technically demanding) tool to forecast

>weather when away from NWS services for an extended time.

>Use of the NWS system (via radio, internet, TV) provides a

>distinct advantage in speed, accuracy and convenience, so

>few people take the time to learn how to use a barometer

>well.

As an avid consumer of NWS forecasts, I must say that there is at least one good reason the "do it yourself", or at least, to know what is going on behind the NWS projections. That's the problem that NWS forecasts are a best guess, and apart from rain variations, do not discuss discontinuous alternate scenarios, even when they have some significant non-zero cance of happening.

Here is an illustration. A bunch of us were out on -- guess where -- Casco on October 12, with a NWS prediction for a sunny afternoon. As we ate lunch, the weather grew increasingly unsettled, until it became apparent that our predicted sunny day was not going to materialize. In fact, we were getting at least a mild nor'easter. We then confirmed that by noting -- surprise, surprise -- that the wind was coming from the NE rather than the predicted S/SE.

I checked the weather history for Peaks when I got back (see below, from the wonderful wunderground.com) and it showed barometer drop all day (30.07 at 1 am, 29.93 at noon, 28.78 at 6 pm), and the wind shift from SSE to N-NE at 6:45 AM. Well, we didn't need a barometer to note the wind direction, but the combination certainly would have triggered at least suspicion of the official forecat even as we set out.

What happened? I have not reconstructed the actual weather events of the day, but my guess is that some frontal system changed direction, and instead of going out to sea, strayed W and hit us in Casco... or something like that.

Now, did the NWS say anything like "sunny in the afternoon with 5 kts from the S, but with a 20% chance of a mild noreaster with rain and 10-15 kts from the NE". Of course not! That would just confuse most people. (The only probabilistic reporting NWS ever does is the proverbial 40% chance of rain, which probably covers a multitude of possible events, but I suspect usually refers to continuous variations.)

But the erratic front, or something like it, was most likely what really happened. The 20% (or whatever) chance of a noreaster depended on exactly where you were and how the storm decided to track north. So, the prediction for sun and S winds was based on the *most likely* scenario. In this case, however, there was a big discontinuity between the most likely event and the second most likely event.

So, finally a conclusion. It seems to this sub-novice meterologist that we mariners should be aware of what's really going on so we can identify those variables and possibilities that just don't appear in the NWS best-guess forecasts. We need to be alert to signs that the NWS best-guess is not actually happening.

Of course, it would helped a lot in this case just to note the wind direction, and even more to have had that barometer along. At the very least, we could have tuned into the weather channels on our VHFs, but with such a sunny prediction, who needed it?!

I say this all with some personal trepidation since I really was trying to avoid upgrading my status from sub-novice meterologist to at least novice, already having more than enough in my life to keep me busy. But I'm now thinking I need to. Scott, what's that book you fall asleep to? Or Jed, what were those meterology primers that you waved at us in class?

--David.

http://www.wunderground.com/cgi-bin/wxStat...MEPEAKS1&type=3

[jeffcasey]

Most of us are not going to read, much less absorb, big meteorology texts. Most of us won't even carry barometers (or if we do, we won't look at them).

We also don't need to be able to extensively forecast the weather...we just need to know when to get the hell off the water.

Most of us **ARE** good at sniffing the winds. We are even pretty good at looking behind us for the big black wall of clouds, muttering "uh-oh", and heading to shore before it hits the fan.

I would encourage us to add one more rule of thumb: when the wind picks up, face into the wind, then LOOK RIGHT. If there appears to be anything ugly on the horizon, take heed. That is the "non-intuitive" thing, since it doesn't appear to be coming in with the wind...but all this science distills down to deceptive wind circulation.

Again: don't just look upwind for trouble, but face the wind and look right.

[Jed]

>"Generally", weather in the mid latitudes (most of the U.S.)

>comes from the west. A fairly regular exception is that of

>tropical storms which come up from the south.

OK this starts to answer the "where" question, but things are not so simple. The prevailing winds come from the (south)west in the mid-latitudes (30° to 60° north in the northern hemisphere). But this mid-latitude band only accounts for 1/6th of the range of latitudes. What about the other 5/6th's of latitudes? Although the (prevailing) winds come from the southwest, our "weather" is the sum of those winds and the High and Low pressure systems that come from other places. It is these "systems" that are actually much more important for the weather wary than the prevailing winds.

>Incoming storms are low pressure systems. Coriolis effects

>are deviations from Newton's laws due to our rotating frame

>of reference -- in the northern hemisphere, trajectories

>will be modified by coriolis effects to "bend" to the right.

> Air rushing into a low pressure zone will bend to the right

>of the center. Since air rushes in from all sides, this

>sets up counter-clockwise circulation.

Great explanation. Note that while most storms are low pressure systems (depressions), not all low pressure systems are storms. The difference between severe and benign low pressure systems now becomes an important point. If we were to stay off the water every time a low pressure system is close enough to feel we would rarely paddle at all.

>Thus a counter-clockwise storm approaching from the west

>will indicate its impending arrival with 1) dropping

>pressure and 2) winds out of the south. After the storm

>passes to the east, the winds will have shifted around to

>the north.

In you example the storm (depression) is approaching from due west. Unfortunately this is a condition that rarely occurs. Depressions don't move west to east here in the NE for the most part. There are of course exceptions but as a general rule lows in the NE do not move due east they track according to specific forces they you 've not added to the mix just yet.

There are also storms that are not associated with depressions per se. T Storms for example are not strictly associated with depressions.

>Similarly a counter-clockwise storm approaching from the

>south (such as hurricanes or wintertime nor'easters) will

>start with 1) falling pressure and 2) winds from the east.

>The typical nor'easter actually stays offshore, so the winds

>start from the east (when the storm is way south), then

>shift to winds from the north (when the storm is directly

>offshore). These bring in a lot of moisture and are where

>our big snow dumps in the winter usually come from.

Yes, this is the more typical thing for us to see here in the NE. Worthy of note is that besides the moisture that an offshore storm sends our way is the way those storms are actually fueled by the oceans itself. A hurricane acts as a mositure and heat pump taking both from the ocean and sending them up into the atmosphere to be deposited elsewhere.

>Jed wants certainties....let's see....weather is too

>complicated...two or more storm systems can combine and

>anything can happen (read "the perfect storm") - two storm

>systems can even spit out a third one like a pitching

>machine, and in nearly any direction. This sort of weird

>stuff doesn't happen too often, since the chances of big

>well developed low cells colliding is fairly low. Hence the

>"generally". Other wrinkles include local topography

>(mountains, lakes), which usually give different rules to

>the local weather.

While I do in fact seek certainties (not unlike other science bases thinkers, who shall remain unnamed) useful probabilities are of value as well. My post was really in response to David's question about how accurate the MWN barometer chart / observations was.

With regard to other, hard to predict conditions, don't forget our favorite cut-off lows that seem to have no regard what so ever for the rules that govern more organized systems.

>For more specifics, the weather geeks build big computer

>models (indeed, this is much of the incentive behind the

>development of some of the large supercomputers). Some of

>these models are nothing more than huge libraries of past

>weather....they match up conditions to previous conditions

>and predict based on what happened before. Some of them

>look at the regions of high & low pressure and predict where

>the large scale air transport will occur....thus predicting

>where existing storms will move to. All of this is very

>complicated at the detail level, and very expensive (you

>need a lot of local data to build the weather map).

All of this is of course true, but I find that workable models are attainable by even hobbyists once they start to approach meteorology as a science rather than as a dark art.

>Jed's last quote, "winds from the east and falling pressure

>indicate foul weather".... falling pressure indicates an

>approaching storm. With intact counter-clockwise

>circulation, an east wind means the storm is to the south.

>Since foul weather / warm-water-storms often travel south to

>north, a storm due south is probably aimed head-on for you.

>Similarly, winds out of the west imply a low to the north.

>Storms very rarely travel north to south, so the implication

>is that the storm is headed away (the west wind is probably

>associated with a rising barometer).

This "falling pressure indicate an approaching storm" really comes from falling pressure as an indication of higher wind speeds normally associated with the boundary between a High and adjacent Low pressure system.

Don't neglect to include the effects of high pressure systems on winds as well. Low pressure systems don't drive the winds all by themselves. This past summer we had an extended period of falling pressure and NE winds that were the result of a retreating High tracking ENE in eastern Canada.

>All this is from the hip, so any of it can be askew.

>Perhaps Scott can retrieve his soporiphic tome from under

>his pillow and correct any gotchas.

As you've said, the subject is much more complex than the simple models that most people consider. I enjoy the study but I while I have some limited understanding of the actual mechanisms I don't really think I understand the subject so much as I understand parts and pieces of the various mechanism involved.

Always fun to talk tech with a bona-fide geek.

(He said, defiantly calling the kettle black.)

Cheers,

Jed

jluby@teamnorthatlantic.com

Life is too short to own an ugly boat.

[jeffcasey]

I agree with all your points, Jed. I'll try not to prolong too much.

Regarding the global picture and all latitudes, this is perhaps getting too general -- most of us don't need to predict weather outside of New England.

Short answer: all this is a big engine driven by solar flux and confused by the earth's rotation. The solar flux is most intense at low latitudes (near the equator), and exacerbated over the oceans (moisture holds the heat and helps drive many weather effects). The earth's rotation has several effects:

1) global wind patterns are set by rotation -- east to west in the tropics are the main driving force. obstructions (continents) and coriolis forces cause rotation away from the equator, and the return winds (west to east) occur at mid latitudes....these dominate air flow over the U.S. similar lesser patters occur further away from the equator.

2) local highs and lows see rotation as the winds vector away or towards the center of the high or low. this gives the aforementioned circulation.

All the rest is details....which can go on forever, but which remain consistent with the big picture.

Extra geek points: if you understand coriolis forces, explain the effect of earth's rotation on tides. (but not over a beer....never drink and derive, alcohol and calculus don't mix).

[Guest guest]

Moran, Joseph M. and Morgan, Michael D., "Meteorology: The Atmosphere and the Science of Weather", Upper Saddle River, NJ: Prentice Hall), 1994.

As the sub-title indicates, it covers the science behind the weather in some detail. (For example, my favorite physical phenomonon is the enormous non-linear consumption and release of latent heat by water as it passes through phase changes ice-water and water-vapor. The textbook not only shows how central this effect is to a variety of weather phenomona, but also digs into the chemical bonding that cause it.)

That much said, since this is directed at college undergraduates, it is relatively readable. Once you get past a lame attempt at relevance to 20-year olds in the first chapter, it is pretty straight forward.

Besides, you learn cool things like the difference in physical properties that cause primary vs. secondary rainbows, or why halos around the sun only occur at 22 degrees and 46 degrees of arc. Or why freezing water onto threatened crops protects them from freeze damage. OK, not everybody gets off on this stuff....but I'd recommend it highly for those who want to understand the theory behind the weather.

The other weather book I'd recommend is:

Crawford, William P., Mariner's Weather, New York: W.W. Norton & Co. Inc., 1974.

It is much more practical and, again as the title indicates, from the point of view of a mariner. For instance, lot of focus on fogs, winds and reading the weather from the water. There is some theory, but presented with a more intuative approach.

The author is oriented to sailing, not kayaking, but since wind and waves are also of concern to sailors, it applies to skinny boats just fine.

Any other references out there?

Scott

[Jed]

>Regarding the global picture and all latitudes, this is

>perhaps getting too general -- most of us don't need to

>predict weather outside of New England.

Fair enough, but since I sometimes teach this subject (to paddlers) my models extend to include these other areas as well.

>Short answer: << major snip>>

Again, you and I both know the limits of your statements above. I agree to stop here, but I will assert my right to agree to disagree with the winds being the main driving force at least with regard to the weather that we, as paddlers, care about. But at best, the differences are a matter of percentage points.

>Extra geek points: if you understand coriolis forces,

>explain the effect of earth's rotation on tides. (but not

>over a beer....never drink and derive, alcohol and calculus

>don't mix).

Extra Credit

OK, here goes. The coriolis force is pertinent for objects that are moving along the North South axis of the earth. But in the largest scale, the oceans themselves do not move during the tides. They do of course have currents which are affected by the coriolis force but even here is is arguable as to wether or not the coriolis force is of primary concern. My vote would be not.

The oceans exist as a loose envelope that seeks out the lowest points around the irregularly shaped land masses that are our continents. For now let's forget what's happening below the surface of the water but know that it is fundamentally the same as what happens around the dry land masses.

This loose envelope is large enough and loose enough that it is subject to the gravitational pull of heavenly objects as well as being pushed around by the forces associated with the accelerations of earth's orbit around the sun.

The gravitational pull of the moon and the gradient of this gravitational pull over the surface of the earth result in kind of elliptical shape for this jacket of water we call the oceans. With one bulge of the ellipsis oriented towards the moon and another bulge 180° opposed raised on the opposite side of the earth from the moon but still on the same line. Now add a second, less elliptical pair of bulges that track the sun. according to the same mechanism.

Add to this the inertial effects of the Earth/moon system rotation and the Earth/Sun system rotation which strengthen the bulges furthest away from the heavenly body and we can see a pair of ellipses of varying aspects superimposed over the earth. The sum of these ellipses defines the shape of the surface of the ocean at some point in time.

Now comes the fun part, the earth spins around it's axis but the ocean would prefer to stay stationary. So as the earth spins beneath this jacket of water, our location and the location of the land masses are constantly moving but the bulges stay oriented towards or away from the forces that create them. As we rotate towards a bulge, we sense that the water is rising but what really happening is we are approaching a region of relatively higher ocean surface. Similarly as we rotate away from a bulge we sense a lowering of water when really we are moving away from the previous bulge.

As the land masses slide under and around the oceans the water is pushed this way and that. This pushing around of the water is what we experience as tidal currents. Imagine a bulge approaching eastern Africa. There is no easy way for the bulge to slide through Africa so instead some of the water is pushed south and around the Cape of Good Hope. The same thing happens with South America and Cape Horn. All of this rotation of land masses and pushing and pulling of the oceans sets up a series of harmonic oscillations that we, quite simple refer to as, the tides.

There are many variations but the norm is that the combined ellipsis will pass our location on earth once every 24:50 hours (approx). This then leads us to expect a recession of tides where two high water events occur later each day, than they did the day before. In other more complicated situations the tides are much less regular and may exhibit only one cycle per day and/or one higher high water and one lower high water. We could talk about the variations for weeks but suffice to say that the variations are the exception to the more simple rule.

This has been fun, but now I need to stop not-working so I can go out and play on the water.

Cheers,

Jed

jluby@teamnorthatlantic.com

Life is too short to own an ugly boat.

[Jed]

>As an avid consumer of NWS forecasts, I must say that there

>is at least one good reason the "do it yourself", or at

>least, to know what is going on behind the NWS projections.

>That's the problem that NWS forecasts are a best guess, and

>apart from rain variations, do not discuss discontinuous

>alternate scenarios, even when they have some significant

>non-zero cance of happening.

>

>Here is an illustration. A bunch of us were out on -- guess

>where -- Casco on October 12, with a NWS prediction for a

>sunny afternoon. As we ate lunch, the weather grew

>increasingly unsettled, until it became apparent that our

>predicted sunny day was not going to materialize. In fact,

>we were getting at least a mild nor'easter. We then

>confirmed that by noting -- surprise, surprise -- that the

>wind was coming from the NE rather than the predicted S/SE.

The NWS forecasts weather over very large regions. You may have been caught in a local disturbance that was not significant to their larger picture. S/SE winds in this area are an idication of either a Low to the S or SW (common) or a North/South oriented front (less likely) or a NE High (least likely). But again, even these things are large in scale and not local effects.

>I checked the weather history for Peaks when I got back (see

>below, from the wonderful wunderground.com) and it showed

>barometer drop all day (30.07 at 1 am, 29.93 at noon, 28.78

>at 6 pm), and the wind shift from SSE to N-NE at 6:45 AM.

>Well, we didn't need a barometer to note the wind direction,

>but the combination certainly would have triggered at least

>suspicion of the official forecat even as we set out.

Another, cheaper and available technique would be to watch the sky. If the sky was partly cloudy you would have seen thinkening cloud cover to the south. If clouds approach from the south then it's either a depression or a warm front. The depression will bring the counter-clockwise (SE-E-NE) winds that Jeff defined so well and will bring with it some specific cloud types while a front may or may not bring westerly winds with it but will bring a differnt type of cloud. Note, it is not the surface winds that are of interest but rather the mid-level winds that we cannot feel.

>Now, did the NWS say anything like "sunny in the afternoon

>with 5 kts from the S, but with a 20% chance of a mild

>noreaster with rain and 10-15 kts from the NE". Of course

>not! That would just confuse most people.

True enough, but the NWS would have provided a synopsis that would indentify the dominant weather system that was in control of the local weather for that day. So I guess my recommendation would be to listen for the synopsis, hear their actual forcast and adjust as neccessary during the day based on actual real-time observations.

The effect you experienced is exactly why for some of us understanding the weather is very important. Since I prefer high winds I need to take the large scale model that the NWS provides and look for opportunties for local effects that will provide me the opportunties that I seek. This is like flying closer to the flame, the margin of error is reduced, encouraging one to have a significant model from which to plan.

>But the erratic front, or something like it, was most likely

>what really happened. The 20% (or whatever) chance of a

>noreaster depended on exactly where you were and how the

>storm decided to track north. So, the prediction for sun and

>S winds was based on the *most likely* scenario. In this

>case, however, there was a big discontinuity between the

>most likely event and the second most likely event.

The most likely scenario and that for the larger region that the forecast serviced. For more detailed / relevent info consider the dominant weather system in place over your location.

>So, finally a conclusion. It seems to this sub-novice

>meterologist that we mariners should be aware of what's

>really going on so we can identify those variables and

>possibilities that just don't appear in the NWS best-guess

>forecasts. We need to be alert to signs that the NWS

>best-guess is not actually happening.

Ah, the awakening .. . .

>Of course, it would helped a lot in this case just to note

>the wind direction, and even more to have had that barometer

>along. At the very least, we could have tuned into the

>weather channels on our VHFs, but with such a sunny

>prediction, who needed it?!

Be careful of local and surface effects. It is the mid-level winds that are the true indicator of the coming weather.

>>I say this all with some personal trepidation since I really

>was trying to avoid upgrading my status from sub-novice

>meterologist to at least novice, already having more than

>enough in my life to keep me busy. But I'm now thinking I

>need to. Scott, what's that book you fall asleep to? Or

>Jed, what were those meterology primers that you waved at us

Wind, Weatther and Waves by Environment Canada

Instant Weather Forecasting by Alan Watts

Have fun!

Jed

jluby@teamnorthatlantic.com

Life is too short to own an ugly boat.

[Guest guest]

Just got through the Coriolis Effect earlier this week in said textbook....and had the same nagging question as I've had before. Perhaps someone can explain it in a way that makes intuitive sense.

I can recite the rules of the effect. I understand the frames of reference explanation that underlies the effect, and even learned from my textbook why the effect is strongest at the poles and zero at the equator (hint: the rotation of the earth translates into 100% rotation around a vertical axis at the poles, but no rotation around a vertical axis at the equator).

Here's the question: doesn't the atmosphere rotate WITH the earth due to surface friction? If so, while the path of a weather system would curve viewed from space (the relative framework effect), why does it appear to curve when viewed from the frame of the earth (say longitude and latitude)? Wouldn't the whole kit and kaboodle (earth, atmosphere and storm) all rotate together so the track of the weather system would appear to be straight VIEWED FROM EARTH (which is where we are). I know I'm missing something here....

In other words, is this only a relative reference effect, or does the storm track (or wind into a low pressure center) physically curve RELATIVE TO the pressure gradient between the high and the low? It seems to, which would suggest a physical effect, but without some physical link to the frame of reference (e.g. space) from which the path appears to curve, how does it curve? Again, this assumes the atmosphere is stuck to the ground, relatively speaking.

Scott

[Guest guest]

Jed:

OK, you can respond when you've got your work done and been paddling:

Your explanation is lucid, but inadvertantly suggests that the envelope of water rotates relative to the earth and that currents are the consequence of this lateral movement of water. I believe what is happening is that a tidal wave appears to pass a fixed observer. We know waves are energy moving through a stationary medium; the only movement of the water molecules is vertical (plus some small component of horizontal movement as part of a circular path within the wave itself). I've always understood that tidal currents are the result of differences in tidal heights as the water runs, in effect, downhill to areas of lower water heights by force of gravity. This would include Cape Horn, etc. not to mention local gulfs, bays and enclosured harbors. (This ignores currents due to other forces such wind, thermal differentials, etc.).

Underlying this scenario is the assumption that while the oceans bulge and can re-level themselves locally due to differential heights, the envelope of water itself does not experience a net 'slip' relative to the earth because of rotation. After all, it's had a few billion years to get up to speed. Or to put it differently, what is the 'friction' or other force that slows the oceans down relative to the earth's rotation?

Now I have to get back to work...

Scott

[djlewis]

>The NWS forecasts weather over very large regions. You may

>have been caught in a local disturbance that was not

>significant to their larger picture. S/SE winds in this area

>are an idication of either a Low to the S or SW (common) or

>a North/South oriented front (less likely) or a NE High

>(least likely). But again, even these things are large in

>scale and not local effects.

Yes, certainly -- local variations. In this case, however, ex post facto observations and VHS weather channel monitoring revealed that there was indeed a storm that came up from Boston. In fact, my wife got caught out on the road in the metro area in a much worse downpour than we had on Casco. So I think this particular instance was a discontinuous second-most-likely forecast that came to pass over a large area, rather than a local variation.

>Another, cheaper and available technique would be to watch

>the sky. ...

Yes, I know there are more things in heaven and earth than heretore dreamt in my philosophy. Thank you for the hints.

>True enough, but the NWS would have provided a synopsis that

>would indentify the dominant weather system that was in

>control of the local weather for that day. So I guess my

>recommendation would be to listen for the synopsis, hear

>their actual forcast and adjust as neccessary during the day

>based on actual real-time observations.

Yes... good points all. I will starting watching the NWS synopses as well as the most-likely-guesses.

Thanks. --David.

[Jed]

Scott,

>Your explanation is lucid, but inadvertantly suggests that

>the envelope of water rotates relative to the earth and that

>currents are the consequence of this lateral movement of

>water.

Agreed, certainly individual water molecules in the oceans must rotate with the earth and yet there is an undeniable gravitational pull on the water that is dependent on the distance from the moon.

>I believe what is happening is that a tidal wave

>appears to pass a fixed observer. We know waves are energy

>moving through a stationary medium; the only movement of the

>water molecules is vertical (plus some small component of

>horizontal movement as part of a circular path within the

>wave itself).

Ok, I'm with you but as the wave strikes a land form it must either reflect, sending a wave backwards or it must dissipate sending the water off in different directions. Even if we ignore the problems at the end of the wave's journey there is still the circular motion that is no longer insignificant and now results in actual movement of water back and forth. Is this a sloshing type of effect?

>I've always understood that tidal currents

>are the result of differences in tidal heights as the water

>runs, in effect, downhill to areas of lower water heights

>by force of gravity. This would include Cape Horn, etc. not

>to mention local gulfs, bays and enclosured harbors. (This

>ignores currents due to other forces such wind, thermal

>differentials, etc.).

Agreed, I guess I'm saying that the differences in heights are caused by localized welling up of the water as a result of some obstruction (either below or at the water's surface) that forces the bulge or wave to deform.

>Underlying this scenario is the assumption that while the

>oceans bulge and can re-level themselves locally due to

>differential heights, the envelope of water itself does not

>experience a net 'slip' relative to the earth because of

>rotation. After all, it's had a few billion years to get

>up to speed. Or to put it differently, what is the

>'friction' or other force that slows the oceans down

>relative to the earth's rotation?

Gravitational attraction towards the moon.

I don't think the water actually slips relative to the earth, but wether there is this bulge or a wave there are certainly land-forms and underwater features that they will inevitably collide with. I think both versions support localized deformations of the bulge / wave and hence support the resultant flow of water required to normalize the surface height. I'm not completely comfortable with the scale of water movement that either theory would seem to require.

This whole work thing just has to stop.

Jed

jluby@teamnorthatlantic.com

Life is too short to own an ugly boat.

[jeffcasey]

Scott -

Intuition comes very hard when studying rotating frames of reference. Your puzzlement is exactly backwards -- you expect a moving object to appear to curve from space (fixed frame of reference) but appear to move straight from earth (rotating frame of reference).....in fact, it will be the opposite. Perhaps the best you can do is to imagine sitting on a merry-go-round and play catch with somebody. From your local frame of reference, the ball certainly won't obey Newton's Laws. Movement on the earth's surface is no different.

One example: sit near the equator and fire an object (cannonball, cloud, whatever) due north. From your frame of reference, it should continue due north. Similarly from space, you expect it to go north with the velocity vector imparted from sitting on a rotating planet. It will obviously follow the curve of the earth, since gravity will act on it, but that much we accept. As it moves north, however, it is moving from a region with large rotating velocity towards a region with smaller rotating velocity. Since it keeps the tangential velocity component it had when you tossed it northward, it appears to curve to the right, since it now moves more eastward with a higher velocity than the land under it. Examples in other directions are a bit harder to come up with simple models for, but they work just as well. Vertical motion is displaced as well...but you have to get into the numbers to make that work out.

This effect is a real physical effect, not just a fiction of our frames of reference. Winds really do curve "to the right" in the northern hemisphere. In WW2, they generated big tables of corrections for naval guns, since these guns shot far enough that the aim would be spoiled by coriolis effects. Again, it all comes from the fact that we are sitting on a "merry go round"...the rotating earth.

Jed -

Full credit....you are correct, the coriolis effect plays no role in tides. I'm not sure what you meant, however, when you said coriolis forces were pertinent for objects moving along the North-South axis of the earth.....there is no such limitation. Your explanation of the water envelope wiggling around to accommodate perturbations in gravity is perfect. Thus, the water envelope would like to be a perfect sphere except for:

- the earth is pear shaped (but who cares),

- the earth is spinning, so there is a centripetal effect which makes gravity a bit less at the equator, so the oceans bulge out a bit there,

- the moon overhead reduces gravity a bit, so the oceans bulge under the moon (one of the high tides),

- the earth-moon system rotates (once/month) around a common center of mass which is NOT at the center of the earth...it is offset towards the moon by about 1/3 the diameter of the earth....thus there is a second centripetal effect which makes gravity a bit less on the side away from the moon, and the oceans bulge out a bit there (the other high tide),

- the sun overhead reduces gravity a bit, so the oceans bulge under the sun (this is weaker than the moon's effects, and tends to just confuse the issue, but it does reinforce the tides at new & full moons, and cancel them out a bit at quarter moons).

The coriolis effects are stronger (geeks: this is first order in rotational velocity of the earth, centripetal effects are second order), but the coriolis effects are also proportional to the velocity of the affected object.....tides concern water which is essentially not moving, so no effect.

Scott -

A couple more good questions you had:

tidal currents: your comments on movement of water is correct for wave motion, but the local effect of tides really is that as the apparent sea level goes up and down, the local bays empty and fill. This represents real displacement of water, thus the currents are real honest-to-god currents.

friction: there is enormous friction in the tides. all that water must pour into and out of twisty turny bays and channels all over the world, and there is enormous energy released to heat, noise, evaporation, etc that is directly and indirectly resulting. The tidal engine is driven mostly by the moon's orbit around the earth (and to a much lesser extent the earth's orbit around the sun). This friction means that the moon is slowing down, and falling into closer and faster orbits around the earth. Eventually, it will fall into an orbit close enough that the tidal forces on the moon will break it up (the orbital mechanics of rocks on the near and far side of the moon will pull it apart with a greater force than the moon's gravity has to hold it together). Something like this is thought to have broken up a closely orbiting moon of Saturn a short time ago, leading to some spectacular rings.

Work? What is this work stuff that everybody keeps talking about?

[Jed]

>Jed -

> Full credit....you are correct, the coriolis effect plays

>no role in tides. I'm not sure what you meant, however,

>when you said coriolis forces were pertinent for objects

>moving along the North-South axis of the earth.....there is

>no such limitation.

Jeff,

Prior to this evening I thought the coriolis effect was limited to ojects moving towards or away from the poles / equator. I believe I understand the angular speed vs the translational (?) speed differences along the latitudes. While waiting for your response to Scott's coriolis question I started to write a response and thought I'd check my model one last time.

I found this link from a google search ( http://www.physics.ohio-state.edu/~dvandom/Edu/newcor.html ) after the Bad Science site said that the effect was not restricted to a north/south trajectory. After I read through the explaination a few times, I finally caught a glimpse of how the effect works moving east or west from a point north or south of the equator.

Please confirm that there is no coriolis effect at the equator for an object traveling due east or due west. As long as the above assumption is true then I can finally rest. If it's wrong I'll be up all night trying to figure out why.

>Your explanation of the water envelope

>wiggling around to accommodate perturbations in gravity is

>perfect. <>

Thanks for the explanation, I like your's much better than mine.

One last request, please help me understand with what happens when the continents slam into the tidal bulges. Is there some current forced as a result of thes collisions?

> The coriolis effects are stronger (geeks: this is first

>order in rotational velocity of the earth, centripetal

>effects are second order), but the coriolis effects are also

>proportional to the velocity of the affected

>object.....tides concern water which is essentially not

>moving, so no effect.

OK, completely lost on the first order versus second order thing.

Cheers and thanks,

Jed

jluby@teamnorthatlantic.com

Life is too short to own an ugly boat.

[Guest guest]

Jed:

I'm still confused by much of this (a lot to digest) but one thing I'm pretty clear about is where the coriolis effect is in play.

It's all over each hemisphere, in any direction, except exactly on the equator. The strength of the effect varies with latitude. Why? Because the coriolis effect only has effect when there is rotation around a vertical axis. Imagine it this way: At the north pole, an Eiffel Tower rotates like a spinning top. A few miles from the pole, the tip of the tower traces a circle of a few miles in diameter, but overall it's pretty much rotating on a vertical axis. In Greenland, the tower is still more or less still rotating in a vertical axis, but it it leans out a bit more since there is a larger component of horizontal movement. Down here in the 40's latitude, the Eiffel Tower sweeps out a cone approximating the shape of the mouth of a funnel, but still ends up completing a vertical rotation. Down in the islands, you'd have to hang onto the Eiffel Tower for dear life, but you'd still get to look in all directions around the globe (similar to standing on the rim of a merry-go-round) in the course of 24 hours. Only at the equator would you experience a simple sweep during the 24 hour cycle facing the same direction, without any component of vertical rotation.

How does this relate to the coriolis effect? Even when a wind or a weather system is tracking west to east at, say, 42 degrees north latitude, the vertical component of the rotation of the earth still causes the storm system to rotate on its vertical axis (more or less depending on latitude). As a result, it will curve, to the right in the northern hemisphere.

Scott

[Guest guest]

Jeff said, the earth-moon system rotates (once/month) around a common center of mass which is NOT at the center of the earth...it is offset towards the moon by about 1/3 the diameter of the earth....thus there is a second centripetal effect which makes gravity a bit less on the side away from the moon, and the oceans bulge out a bit there (the other high tide).

I agree in principle with Jeff’s common center of gravity influence . However, not his ratio of that influence, or the ultimate formation and sustaining force of the bulges. Here is a useful site that might be interesting in this discussion.

http://www.co-ops.nos.noaa.gov/restles1.html#Intro

“The Tractive Force. It is significant that the influence of the moon's gravitational attraction superimposes its effect upon, but does not overcome, the effect's of the earth's own gravity. Earth-gravity, although always present, plays no direct part in the tide-producing action. The tide-raising force exerted at a point on the earth's surface by the moon at its average distance from the earth (238,855 miles) is only about one 9-millionth part of the force of earth-gravity exerted toward its center (3,963 miles from the surface). The tide raising force of the moon, is, therefore, entirely insufficient to "lift" the waters of the earth physically against this far greater pull of earth's gravity. Instead, the tides are produced by that component of the tide-raising force of the moon which acts to draw the waters of the earth horizontally over its surface toward the sublunar and antipodal points. Since the horizontal component is not opposes in any way to gravity and can, therefore, act to draw particles of water freely over the earth's surface, it becomes the effective force in generating tides.

1. At any point on the earth's surface, the tidal force produced by the moon's gravitational attraction may be separated or "resolved" into two components of force - one in the vertical, or perpendicular to the earth's surface - the other horizontal or tangent to the earth's surface. This second component, know as the tractive ("drawing") component of force is the actual mechanism for producing the tides. The force is zero at the points on the earth's surface directly beneath and on the opposite side of the earth from the moon (since in these positions, the lunar gravitational force is exerted in the vertical - i.e., opposed to, and in the direction of the earth-gravity, respectively). Any water accumulated in these locations by tractive flow from other points on the earth's surface tends to remain in a stable configuration, or tidal "bulge".”

Good discussion. Rick

[Guest guest]

Lightbulbs going off here on the tractive force (tangental moment) vs. the lifting theory (vertical moment), Rick. Great reference on the NOAA site.

Scott

[jeffcasey]

Jed -

Sorry I didn't respond last night...I hope you got some sleep. You may not now, since you still miss it. You want me to confirm that there is no coriolis effect at the equator for an object travelling due east or west.....sorry.

When you get quantitative, the formula for the coriolis force gives you what is called a cross-product. It is proportional to the velocity of the object in motion, and is perpendicular to BOTH the velocity of the object and the spin axis of the earth. The only time you get zero coriolis force is if your motion is exactly parallel with the spin axis of the earth. Examples: moving N or S while exactly on the equator, moving straight up or down at the poles.

Your example of moving east or west at the equator won't provide a coriolis deflection to the "right" or "left" from the point of view of a flat map, but it will provide a deflection vertically. If you are at the equator travelling east, you will be deflected upwards; if travelling west, you will be deflected downwards. Confused yet?

The simple tidal picture has a water world. Add continents, and you want to "go local". The local effect is that the sea rises in Gloucester, so water pours into the harbor, then into the little nooks and crannies. All pretty intuitive. It only gets complicated when the resonant period of the harbor/bay is similar to the tidal period (or a multiple). Then you get a bathtub sloshing effect, where the waves get bigger each time.....go see the 50+ foot tides in the Bay of Fundy.

First order, second order....don't worry about it.

Rick -

I agree completely. There are no real differences....when you try to explain quantitative stuff with analogies, the results are always imperfect. I like the tractive force explanation too. The quantitative way to do this is to calculate equipotential surfaces, which are the vector sum of all the forces. The water will want to flow around until it is at constant equipotential...which is not necessarily at constant height. Since some of the small perturbations follow the moon around the globe, these equipotentials shift up and down at any particular location.