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Old 24-09-2012, 04:19   #16
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Sometimes it's best to think of rate of change versus direction of change. A quick rate of change either up or down is associated with steep pressure gradient and that means wind. That said i would rather have a rapidly climbing barometer than a plummeting barometer...
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Old 24-09-2012, 09:17   #17
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Quote:
Originally Posted by foolishsailor
Sometimes it's best to think of rate of change versus direction of change. A quick rate of change either up or down is associated with steep pressure gradient and that means wind. That said i would rather have a rapidly climbing barometer than a plummeting barometer...
I think this is the answer I was looking for. It clarifies and emphasizes the "rate of change" and that's probably the common factor with the things I've read. They speak generally about the bar dropping and either explicitly mention the faster it drops the worse, or it's probably implied.

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Old 03-10-2012, 10:28   #18
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Re: Bad Weather And Low Presure

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I think this is the answer I was looking for. It clarifies and emphasizes the "rate of change" and that's probably the common factor with the things I've read. They speak generally about the bar dropping and either explicitly mention the faster it drops the worse, or it's probably implied.

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Except it's not true in a crush zone - when you have high winds and no rate of change in the pressure. You simply have very tight but essentially stationary isobars.
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Old 04-10-2012, 07:49   #19
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Re: Bad Weather And Low Presure

The verbage in older days was "The glass is falling" or "the glass is rising". If you look at a barometer, with a round scale, you will see that on the left side of the scale (there where the reading will be lowest) it will say "storm" then as the glass rises (millibar increase), the needle passes through rain, change, fair and sunny.

This is why almost everyone except meteorologists (and experienced sailors) associate a rising barometer with better weather, and falling barometers with inclement weather.

I'll agree with Foolish sailor, generally I prefer to see a rising barometer than a falling one (and yes evans starzinger you are perfectly correct in your observation)

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Old 04-10-2012, 08:02   #20
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Re: Bad Weather And Low Presure

Part of the problem is the question of what is considered "bad weather"? High pressure systems can (and frequently do) have high wind speeds associated with them, particularly in the crush zones around the edges. But they rarely have high winds internally or rain/clouds/waterspouts/etc. Most of these aspects are associated with low pressure systems and fronts tied to low pressure systems.

So, if you consider strong winds alone to be "bad weather" then a high-low correlation isn't accurate. But if you consider bad weather to include rain, cold, etc. in addition to wind then the correlation is much higher.

With regard to San Francisco Bay, I was always taught as a younger man that "The Bay doesn't blow, the Valley sucks"
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Old 04-10-2012, 12:39   #21
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Re: Bad Weather And Low Presure

I consider that strong winds (not storm winds, mind) make the boat go. Your decision as skipper is to determine appropriate area of sail, appropriate direction of boat, and appropriate precautions securing provisions, gear and crew.

Also to have snacks pre-made if it looks like a blow hard enough to inhibit galley duty.

One of the reasons not really discussed is that the ability to cultivate a weather forecasting sense is not to avoid "bad weather", but to seek it out, or at least the edge of it, in order to get somewhere.

A sunny-day gale is a glorious thing to experience offshore, and is very much the sort of thing you get on the edge of grimmer conditions and systems.

Much of the diesel burnt on sailboats is, in my view, due to an unfamiliarity with synoptic charts, an unfamiliarity with route planning and pilot charts, and a lack of experience deliberately getting used to a steady diet of 25-40 knot conditions.

I'm not trying to, uh, "swing the lead" here, but rather to emphasize that light air and not plenty of air is the enemy of the sailor on passage, and it should be understood, anticipated and welcomed, unless you really like motoring.

Which I do not.
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Old 04-10-2012, 13:16   #22
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Re: Bad Weather And Low Presure

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A sunny-day gale is a glorious thing to experience offshore...
Couldn't agree more, unless I have to tack into it and the spray coming up is cold
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Old 04-10-2012, 13:32   #23
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Quote:
Originally Posted by S/V Alchemy
I consider that strong winds (not storm winds, mind) make the boat go. Your decision as skipper is to determine appropriate area of sail, appropriate direction of boat, and appropriate precautions securing provisions, gear and crew.

Also to have snacks pre-made if it looks like a blow hard enough to inhibit galley duty.

One of the reasons not really discussed is that the ability to cultivate a weather forecasting sense is not to avoid "bad weather", but to seek it out, or at least the edge of it, in order to get somewhere.

A sunny-day gale is a glorious thing to experience offshore, and is very much the sort of thing you get on the edge of grimmer conditions and systems.

Much of the diesel burnt on sailboats is, in my view, due to an unfamiliarity with synoptic charts, an unfamiliarity with route planning and pilot charts, and a lack of experience deliberately getting used to a steady diet of 25-40 knot conditions.

I'm not trying to, uh, "swing the lead" here, but rather to emphasize that light air and not plenty of air is the enemy of the sailor on passage, and it should be understood, anticipated and welcomed, unless you really like motoring.

Which I do not.
Very good points here. Which makes me think more toward regional weather and its relatively consistent patterns and storm types and associated wind strengths and direction. Maybe you all can help me map this out.

----------
Northern hemisphere:
Above 30 degrees
- Dominant: W-NW
- Storms: NW (cold fronts)

Below 30 degrees
- Dominant: E-NE (trades)
- Storms: ? (Low pressure squalls) ?

-----------
Southern hemisphere:
Above 30 degrees
- Dominant: W-SW
- Storms: SW (cold fronts) ?

Below 30 degrees
- Dominant: E-SE (trades)
- Storms: (Low pressure squalls) ?

Some of this is straight from a world wind pattern map (below) but it would be nice to know which types of storms happen in different regions too. Like the tornados and thunder storms on the east coast of the US. Anyone know of a world map that shows local storm types, this kind of thing may already been plotted.

http://apesnature.homestead.com/files/fg09_05c.jpg


Thanks,
austin
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Old 04-10-2012, 19:00   #24
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Re: Bad Weather And Low Presure

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Couldn't agree more, unless I have to tack into it and the spray coming up is cold
Well, I generally hold that gentlemen do not sail to weather...

On the other hand, it does sluice the bird poop off the side decks.
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Old 04-10-2012, 19:09   #25
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Re: Bad Weather And Low Presure

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Originally Posted by theway View Post
Anyone know of a world map that shows local storm types, this kind of thing may already been plotted.

http://apesnature.homestead.com/files/fg09_05c.jpg


Thanks,
austin
By definition, a world map would not show local storm types...they are too transient and variable, unlike the monsoon winds or the Trades, which are generally very predictable as they are a function of the earth's rotation, the seasons and esoterica like Hadley cells and other "big picture" machinations.

There's a very good and "user-friendly" guide the Canadian government issues here: Wind, Weather and Waves: En56-125/2-1998E - Government of Canada Publications

I'm sure there's a U.S. equivalent. It explains Great Lakes weather, which is predictably unpredictable in that you can't say when something will happen, but you can say when it's likely and what form it will take. For instance, in Lake Ontario, we are at the northern edge of "Tornado Alley". A hot, still summer day with weather coming from the WSW, cumulus clouds building over land, and high cirrus with a plunging baro means look towards the west end of the lake for heavy squalls up to 60 knots that last maybe 15 minutes as they race over the lake end to end, dumping inches of warm rain, hail and craziness.

The same could be said for the entire U.S. east coast, except that Gulf action and the sub-tropics of Georgia, Florida, etc. bring more cycling afternoon storms.

Basically, I can predict with reasonable accuracy how the day will play out in my locale. You can, too: it's just a matter of combining observation with basic weather knowledge.

And if you are going with the wind aft of the beam, remember to look over the stern once in a while.
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Old 05-10-2012, 00:59   #26
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Re: Bad Weather And Low Presure

Global wind pilot charts will give you a reasonable idea of where and how strong the wind will be during the various parts of the year. There are 12 charts, one for each month

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Old 05-10-2012, 04:20   #27
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Re: Bad Weather And Low Presure

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Originally Posted by atoll View Post
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.
(-/+), +, +, ;-)

To be more exact: it is not actually 'useless' but one does have to start using it in a new way there. As you said, daily gradients are pretty set for any specific part of the world and time of the year. So a definitive departure from such values will indicate either the ruling high to be gaining strength (too bad, if you happen to be sailing close to its center!) or else an approaching tropical wave or any other low-associated phenomena.

Since for gradient we need an associated high-low pair, and since the worst of the weather is associated with low systems (tropics or otherwise) then to say that the low pressure is an indication of possibly bad wx holds universally true.

But one must see the whole picture too - in some cases the wx can dramatically deteriorate on a fixed (and not necessarily low) pressure level too - for example when a low is pressing into the high and none of them is willing to give way. Then a boat can be caught in the s.c. compression zone - the pressure will remain the same but the isobars will dramatically compress and so we will get the gradient and the wind and associated sea state - while the glass remains stable.

So to say, stick to the old glass, improve your wx reading/interpreting skills and use the reasoning applicable for the part of the world you are living, sailing, in.

Cheers,
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