What's your altitude?

 

from the midwest, a flatlander, I think around 800ft.

 

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

 

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.

 

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

 

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-line/heavy-weather/page2.asp

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

 

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.

 

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.

 

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

 

Or how about hopeless.

I set the needle to indicate change either up or down, but that was about a week ago. I didn't pay any attention until last night.
I promise to be more vigilant in my duties.


Thanks for all of the replies.

 

If you take readings every 12 hours or so (at a minimum) it will give you an idea of the barometric pressure trends, and as you get experience with seeing the the pressure change and begin to get an idea of what you can expect with a given change in pressure. BTW, the faster the pressure drops, the shorter the duration, but greater the intensity, of the storm generally.

 

The ship's log should have barometric pressure entered in it every two hours. This will allow you to monitor the trend of the barometer. Without a record the barometer is useless. Recording barometers, or barographs, are available but pricey.
As CD vividly explained, extra low readings are the only spot readings of interest except that you probably will be amply occupied with other matters anywhere below 920mb!<G> The lowest I've ever experienced at sea was 890mb and it's exactly as CD advertised. On a barograph, as the center approaches, the needle will drop virtually vertically and you'll wonder if the drum is still turning (for the time component)! This is also where you'll see "pumping" of the barometer-the weather system is so violent and changing so rapidly that the needle will pump up and down 5-7mb as you watch it.
There is a diurnal component to barometric pressure readings. The barometer will have two highs and two lows for a given day. They are not radical departures and will not obscure the trend, and vary with location. And, as Dog stated, it is not only the trend, but the rate of trend that is significant. This can best be illustrated by observing a cross-sectional of warm and low front air masses in any weather guide.

 

sailaway-

My recommendation of 12 hours at a minimum was assuming that you are not at sea... but trying to learn how to use a barometer for weather prediction. If you are at sea, I would agree that every two (2) hours is a good minimum.

 

sailaway-

My recommendation of 12 hours at a minimum was assuming that you are not at sea... but trying to learn how to use a barometer for weather prediction. If you are at sea, I would agree that every two (2) hours is a good minimum.

One cruiser told me they take a reading about every 12 hours until something "interesting" starts to happen, and then they start making log entries much more often.

 

One interesting feature of the official NOAA weather stations is that they now include a chart plotting wind speed, wind gusts and barometric pressure. you can check them online to see what is happening within the last few minutes for the raw data, and also check that one feature. Here is the link to the weather station closest to my marina. Click on the link for Combined Plots of Wind Speed and Pressure (or however it is worded.)
Always interesting.

http://www.ndbc.noaa.gov/station_page.php?station=FSKM2

 

Magic,
As I'm sure Dog would agree, if you're only glancing at it every twelve hours you are probably twelve hours behind in responding to whatever "interesting" things are happening. This, again, illustrates the benefit of a barograph-a simple glance is enough to see the latest trend and precipitousness of same.

Of course, I agree with him about in port versus at sea observations. A sharp downward trend at sea, observed in real time, may mean the ability to navigate to the navigable side of an approaching low instead of taking a needless and unsafe beating.

Absent a barograph, recording periodic barometric readings is a necessity.

 

As part of my regular log book is a line for the barometric pressure, which is to be taken at the hourly position record entry... that gives a pretty good idea of the pressure trend at a glance, by looking at the log for the past day or so—but this is only done on longer trips. When I'm daysailing, and in range of VHF weather, I use that instead.