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  #1  
Old 01-16-2007
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Barometric Pressure

A question about pressure. (When were all done you guys can tell me how stupid I am.)

Here it is.

In my part of the world, today we got our first substantial snowfall of the season. We probably got around 4 - 5 inches, not a whole lot but enough that I had to shovel the driveway.

After shoveling and after dinner, I checked the barometer and it reads 31.5 inches and is most of the way in the FAIR.

I though that possibly I had a bad reading on my Barometer so I checked it against local readings from the net. My 31.5 is very close to observations in the area.

My question is, is there a difference when reading pressure in the summer season as opposed to winter? Why am I reading FAIR when we just received snow?

Just curious, I know you guys will be able to set me straight.


I just thought of a possible explanation, our snow was concidered lake effect snow. Cold winds blowing over the warmer lake produced the snow. Would that not show as a low but rather as norm or fair?
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Old 01-16-2007
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What's your altitude?
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Old 01-16-2007
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from the midwest, a flatlander, I think around 800ft.
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Old 01-16-2007
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just for calibration purposes, that would make you're standard pressure around 29 in, instead of 29.92. Also, depending on the type of front, sometimes the weather is ahead of the front, and sometimes behind it. My Wx classes were 18 years ago, so I'm a bit rusty on that.

Charlie
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Old 01-16-2007
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If the skies are blue and the barometric pressure is steady at 31.5", it is very likely that you are in a high pressure system—which features colder, drier air... and that the snow was the result of the front that passed through as the high pressure system moved into the region.

The actual pressure isn't as important as what the pressure trend is... if it is rising, then the weather will generally get better... and if it is falling, then it will generally get worse.
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Old 01-16-2007
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By the time you are out shoveling snow the centre of the low pressure system has pasted you bye.

A low pressure system works the same in winter as in summer. A low rotates in a CCW direction and starts with a SW wind around here. The leading edge will bring clouds and then precipitation, in winter snow.

As the low moves through the sky will lighten and the rain/snow let up. By this time the wind has backed around to the north and the high pressure following the low is moving in. You now head out to shovel and find the pressure has already risen. You are left with a strong cold wind out of the North or NW that is bringing in cold dry artic air. Just like today was with a high pressure system and bright sun.

My skiing friends think it is great, I am of to the boat show to dream.

Gary

Last edited by Gary M; 01-16-2007 at 10:30 PM.
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Old 01-16-2007
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The only usable information that one can derive from observing a barometer is its present pressure and the trend, as has been said here before.

A pressure in the "Fair" range means nothing unless it has been steadily preceded by rising pressure.

A good reference is Fitzroy's Barometer Instructions, which have been around for over a hundred years and probably can't be much improved. These used to be published in the annual Eldridge Tide and Pilot Book, but at least excerpts are shown at:
http://www.sciencemuseum.org.uk/on-l...ther/page2.asp

Also:
See Fitzroys "remarks" at:
http://www.queenswood.com/barometer/admiral.htm

Last edited by Goodnewsboy; 01-16-2007 at 11:53 PM.
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Old 01-17-2007
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I carry a barometer watch in my pocket and check the pressure often and here is what I can tell you about it ... not much! Like other people said, the absolute reading don't really tell you much. The pressure rises and falls over time, sometimes steady for a while, but normally it changes a little bit each day. Even if it is steady it is only the resolution of the readings that makes it appear steady, if you use a sensitive vario like the people who fly paragliders you can see the pressure is always changing.

I found it useful to watch the barometer while also studying cloud formations and looking at daily weather fax charts for my area. Doing all those things you can get a good idea what is apt to happen next. Also using a small compass and looking at the direction the wind is coming from can tell you a lot about what direction the low pressure center is in and over time you can kind of sense where it is and where it is moving to, and even how big it is based on how long and hard the wind is blowing from different directions. There is a lot more you can do but just those few things really helps in my opinion because after a few weeks you start to get some idea what is likely to happen next.

I have a huge amount to learn about this and continue to read weather books and watch my instruments.
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Old 01-17-2007
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Someone please correct me if I am wrong, but wouldn’t the average air pressure be higher during the winter months than during the summer months (in higher latitudes) due to the density of the cold air?

Also, as far as I know, the words on wall mounted barometers are really just decoration.
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Old 01-17-2007
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Well, let's get a couple of things straight here:

1) It is the CHANGE on a barometer that is of interest. The above poster was correct.

2) The words on the Barometer are for decoration, as is the actual pressure.

HOWEVER!!.... like all rules, there are exceptions to both of the two above, as I can attest to:

IF THE BAROMETER HITS THE LOW 900S OR DROPS INTO THE 800S, THE WORDS ARE NO LONGER DECORATION. It will rain. It will blow really, really hard. The seas will be enormous and confused. The island you were dumb enough to tie up to will be underwater as the swells come across. Boats will sink around you. You cannot get off your boat, and would not be much better if you could.

The problem is that you cannot write all of that stuff on a barometer and have it fit. Thus, they just put RAIN. And guess what, it will rain. Maybe in the 800's, they should have a fourth word: SCREWED.

- CD
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