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Old 22-09-2012, 09:23   #1
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Bad Weather And Low Presure

I have read books and articles on weather for the past 2yrs and most have a comment about the barometer dropping or low pressure, and somehow attribute that to bad weather. Now I get it with a tropical storm or hurricane, but other than those, I never understood how low pressure is a sign of bad weather.

I spent my first boat year on the east coast of the US, FL to NY. So the strong winds and storms "sever thunder storms" that I know come from a cold front (high pressure) passing through/under a warmer pocket (assuming that pocket is generally lower pressure too.

Now, in the SF bay there are no storms at all, and I'm pretty sure the strong winds we see here are also from high pressure cold fronts.

Can anyone explain the low pressure storm association or is it always referring to tropical storms or worse? If you have examples of NOAA maps or others weather charts, the would help.

Thank you,
austin
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Old 22-09-2012, 09:58   #2
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Re: Bad Weather And Low Presure

Hard to answer in forum post length, but the very short story is....

1. Wind strength is (primarily) driven by pressure gradient - that is how tight the pressure isobars are together. They can be tight/compressed at any pressure level - high or low pressure.

2. The wind can be strong either with a quickly dropping or raising barometer (when the weather macro systems are moving). However, the wind can also be strong even when the barometer is stable - in a 'stationary crush zone', usually when a low pressure system pushes up against a stationary high pressure system.

3. In coastal waters (like buzzards bay or SF bay) afternoon winds are typically driven by land/water temperature differential - which creates a pressure differential. During the afternoon, air pressure decreases over the land as temperature rises. The relatively cooler air over the sea rushes in to fill the gap. The result is a relatively cool onshore wind. This process usually reverses at night where the water temperature is higher relative to the landmass, leading to an offshore land breeze.

4. Typically cold fronts & warm fronts are both associated with low pressure systems and NOT high pressure systems - that is one end of the front is anchored on a low pressure system. There is typically compressed pressure isobars along these fronts (and thus stronger wind with a shift in wind direction), and typically a temperature change (and thus often precipitation, with the often associated thunder cells). Thus dropping pressure on the barometer can suggest the likelihood of an approaching frontal system with this wind and precipitation.
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Old 22-09-2012, 10:38   #3
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Re: Bad Weather And Low Presure

Check out

Northeast Pacific WX Briefing Package

Follow estarzinger's advise and watch the wind strengths associated with the various systems. The gradient is very important to watch. This summer I had 40 knot winds associated with a high because there were three low's nearby.
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Old 22-09-2012, 11:24   #4
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Re: Bad Weather And Low Presure

If you think of high and low pressure systems as Shop Vacs set to "blow" and "vacuum", respectively, only vast and comparatively weak and slower, you can begin to understand how pressure systems work: a cyclical turning creates either an "inward-sucking" or an "outward blowing" wind pattern along fairly predictable vectors.

Why this happens is related to rising and falling air heated near land and oceans, with spin imparted by the Earth's rotation, just as pool balls can have "English" imparted on them by offset rotation.

Where it goes a little deeper is when these systems interact, or how squalls can form rapidly, or "dangerous quandrants", or why if the low pressure system in the northern hemisphere is tracking, say, NNE, why is the wind coming from SW and is this to be anticipated?

I recommend all sailors who aren't "fair weather only" to take a basic weather course or pick up some books on the subject to understand and to anticipate when your nice 14 knot ride is liable to devolve in a couple of hours into a 45 knot gale, or conversely, when a slowly rising barometer and a stalled high is liable to cost you several gallons of fuel as you motor across a flat sea.

I agree that being able to read a synoptic weather map (the ones with isobars, tight or not) is a great skill to have. Few things will give you more insight into the next 24 hours aboard.

Nothing, however, is fixed: I have been out in plummeting barometer conditions ripe for squalls and seen only light, shifty winds that left my reefed down sails slatting. I've also found decent wind in Day 2 of a three-day high pressure system transition.

But both were pretty unusual. Typically, a rapidly falling barometer, tight isobars, "mackerel" clouds and lumpy seas portend finding where you put the tether and foulies. The key to developing a functional "nose for weather" is to understand the science and apply it to growing better instincts.
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Old 22-09-2012, 11:56   #5
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Viewing synoptic charts for your area will help to understand the wind. As stated earlier it is the pressure gradient that drives wind not the high or low pressure. When looking at synoptic charts you can see the gradient lines and with the lattitude legend in the corner you can use a set of dividers to see expected winds at your location.

Many GRIB viewers also show isobar lines and you can look at the gradients there as well.

Basically wind ia driven solely by geographic pressure differentials. These are created either by weather or by temperature...really totally by temperature on different scales.

As stated earlier a low pressure system creates a steep pressure gradient, visible on a synoptic chart with its isobars. Also you can have a localized temperature gradient like in SF in the summer where it blows 25kts+ in the bay by 2 every day during the summer.

Cold and warm fronts are generaly associated with low pressure systems and while there are moments where the barometer climbs with the passing of these fronts it should be generally lower than baseline. Again looking at a synoptic chart will show this in much more clarity.

I think many modern sailors have become so reliant on basic grib charts to give immediate local information that they have forgotten or lost the ability to use a synoptic chart to give big picture and preditictive weather info...
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Old 22-09-2012, 13:30   #6
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Re: Bad Weather And Low Presure

By the way, there is a vast library of free on-line weather seminars and training created by the national weather service (NWS) at:https://www.meted.ucar.edu/training_detail.php. The OP might want to start with the Basic Weather Processes seminar.
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Old 22-09-2012, 14:13   #7
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Re: Bad Weather And Low Presure

If you are in the SF Bay area sailing, then you need to be aware of what I learned it as "the central valley wind machine". I learned about it from one of the cruising guides... Basically, the Central Valley is large, and mostly ringed by high mountains. It gets hot there. Hot air rises. Cooler air rushes in to replace the rising air. The largest source of the replacement air comes in through the Carquinez straights, San Francisco Bay, and the Golden Gate. The mid-latitude westerlies give that air a shove, and you get the 20-30 knot breezes in the bay. Eventually all of that cold air brings clouds and fog into the central valley, which cools it down and the winds moderate. The clouds dissipate, and the cycle starts over again. Historically, it was a 7-9 day cycle.
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Old 22-09-2012, 18:06   #8
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Re: Bad Weather And Low Presure

You should also read up on land and sea breeze effect. These effects can be pretty significant around SF Bay and many other venues.
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Old 22-09-2012, 18:52   #9
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Re: Bad Weather And Low Presure

Quote:
Originally Posted by theway View Post
I have read books and articles on weather for the past 2yrs and most have a comment about the barometer dropping or low pressure, and somehow attribute that to bad weather. Now I get it with a tropical storm or hurricane, but other than those, I never understood how low pressure is a sign of bad weather.

I spent my first boat year on the east coast of the US, FL to NY. So the strong winds and storms "sever thunder storms" that I know come from a cold front (high pressure) passing through/under a warmer pocket (assuming that pocket is generally lower pressure too.

Now, in the SF bay there are no storms at all, and I'm pretty sure the strong winds we see here are also from high pressure cold fronts.

Can anyone explain the low pressure storm association or is it always referring to tropical storms or worse? If you have examples of NOAA maps or others weather charts, the would help.

Thank you,
austin
once you get into the tropics,your barometer is virtually useless any way,as your mean pressure gradient only varies 4-6 milibars daily unless there is a tropical storm,and even then it will give you very little warning.

in higher lattitudes it is a great too,coupled with cloud observations and wind shifts.

within 60 miles of land any weather is going to be more effected by the local topography to a lesser or greater extent than the prevailing ocean conditions.
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Old 22-09-2012, 20:59   #10
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Re: Bad Weather And Low Presure

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once you get into the tropics,your barometer is virtually useless any way,as your mean pressure gradient only varies 4-6 milibars daily
I believe the tropics was outside the OP's question . . . but the tropical trade winds are interesting - driven by a combination of a global scale pressure differential and the earths Coriolis effect.

Solar heating causes the air at the doldrums (near the equator) to heat and rise and create a low pressure band, and the air cools and falls around 30 N/S and creates a band of general higher pressure. This pressure differential creates a flow of air toward the doldrums. The Coriolis effect gives this air flow a westward component, so you have a general NE trade in the norther tropics and a general SE trade in the southern tropics.

During the winter season the pressure differential is greater, because while the tropics are warm all year it is much colder at 30 degrees during the winter, and so the trades are generally stronger during the winter.

You are correct that the barometer is not so much use with all this in the tropics because the pressure differential is spread out over a planetary scale (thousands of miles), as opposed to a couple hundred miles for low and high pressure systems in the mid/high latitudes. Also the tropical systems are essential like a large fixed crush zone because the low and high pressure locations are relatively stationary. So, the pressure gradient per mile or per hour is low, but it is over a huge area so sucks a huge about of air.

This solar warming/cooling effect also explains why the worst tropics squalls are usually around 2am . . . but that's for another post.

This is of course all complicated by over laying moving 'regular' low and high pressure systems (and tropical 'waves') within the tropics, and in the mid-latitudes which crush or relax the pressure differential. You can see some of this overlay with a barograph if you follow it closely, but it is still diluted by the being spread out over large areas.

Look up 'hadley cells' if anyone wants more info.
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Old 23-09-2012, 16:06   #11
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All good stuff thank you, I have a pretty good understanding of weather, have done weather fax on occasion, and look at NOAA daily because I enjoy it. I have read numerous books on weather, some specific to mariners. So I think I understand most of it and well past the basics.

I mainly was wondering why most books relate the bar dropping to bad weather, when it doesn't seem to be the case in most situations other than tropical depressions/storms or worse which are severe lows.

thanks again
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Old 23-09-2012, 16:14   #12
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Re: Bad Weather And Low Presure

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I mainly was wondering why most books relate the bar dropping to bad weather, when it doesn't seem to be the case in most situations other than tropical depressions/storms or worse which are severe lows.
"bad weather" means something quite different to an experience (professional) mariner than to a recreational boater. They don't consider rain and 30 kts "bad weather". What they do consider bad weather (lets say +50kts) is in fact most often associated with intense low pressure systems.
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Old 23-09-2012, 16:45   #13
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Re: Bad Weather And Low Presure

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I mainly was wondering why most books relate the bar dropping to bad weather, when it doesn't seem to be the case in most situations other than tropical depressions/storms or worse which are severe lows.

thanks again
In The Pacific Northwest anything below 1000 mb usually indicates a big blow, especially when the cold front passes through. The cold front is associated with a sudden wind shift and dramatic increase in wind speed.
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Old 23-09-2012, 17:50   #14
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Re: Bad Weather And Low Presure

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...
I mainly was wondering why most books relate the bar dropping to bad weather, when it doesn't seem to be the case in most situations other than tropical depressions/storms or worse which are severe lows.
...
I think associating "bad" weather with low pressure is an over simplification to some extent (easier to write books and trite sayings about the weather that way). Deteriorating weather conditions are of course typically associated with well defined areas of low pressure (tropical development, closed areas of low pressure, trofs, tropical waves, frontal boundaries, etc...), but as pointed out what really matters for wind conditions is pressure gradient -- not just whether the nearby pressure is relatively low or high. Ultimately, it is wind conditions, which are driven primarily by pressure gradients, that we as sailors are most concerned about because that drives everything we do in terms of boat handling and sea state.

An accurate barometer is also important in verifying conditions at your location relative to forecast or current synoptic conditions. For example, if the pressure at your plotted position agrees with the most recent weather fax then you have a higher degree of confidence in the forecast. If it does not agree, then that might reduce your confidence in the forecast....or your navigation. If you have a reasonable degree of confidence in the forecast, then the local baro reading can be useful for determining your relative position to a low or high pressure area at that point in time and what route decisions you want to make next.

Whether it rains a bit more or less is not as significant of a consideration...and you can get rain with either approaching high or low pressure systems.
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Old 23-09-2012, 19:03   #15
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Re: Bad Weather And Low Presure

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All good stuff thank you, I have a pretty good understanding of weather, have done weather fax on occasion, and look at NOAA daily because I enjoy it. I have read numerous books on weather, some specific to mariners. So I think I understand most of it and well past the basics.

I mainly was wondering why most books relate the bar dropping to bad weather, when it doesn't seem to be the case in most situations other than tropical depressions/storms or worse which are severe lows.

thanks again
I think as Belizesailor says it is an oversimplification.

High pressure vs. low pressure is the first step in building a "picture" of what is happening and what might happen. Temperatures of the masses is next. Location (land mass effects) must be considered. Saturation of the air may give an idea of what might happen with cloud formation and rain when the two masses meet.

High pressure gradient, warm saturated air mass meeting a cold one is a recipe for high winds and lots of rain.

Of course the definition of "bad weather" and everyone's different yardstick is important. Are we going golfing or extreme sailing - LOL
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