Ask A Scientist© Weather Archive

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> > >   name        amber
> > >   status      student
> > >   age         12
>
Question -  Why are barometers so important to science. What is the  purpose of the barometers. Any input would help.

Barometers are instruments to measure the pressure of the atmosphere. We think of air as "very light", but in fact it weighs a lot when a column several kilometers high presses down upon the earth [and us]. That pressure [force per unit area] is measured in a number of unit but the one you usually hear on the TV weather is:" millimeters of mercury (mm Hg)." We will skip why that is so, but the point is:" The larger the number, the higher the atmospheric pressure.

The barametric pressure tell us the weight of air pushing down. This can be affected by several things: 1. How cold or hot the air is -- remember hot air rises because it is less dense than cold air. Cold air is denser and pushes down harder. 2. How high the column of air is -- the height of the atmosphere is not the same everywhere, and changes by the hour. If the readings from several locations are marked on a map, there is often one part of the map where all the readings are higher than on others. The line that separates the high readings and the low readings is traced on the map and is called a "front". It is a "cold front" if that line of demarcation moves approximately north to south. It is a "warm front" if the line of pressure differences moves south to north.

There are a number of ways to measure the pressure of the atmosphere. One way is easier to describe in words, without any pictures, so I'll describe that type:

Take two thin metal concave plates, seal them face to face, and seal them around the circumference of the rim. So you have a hollow chamber. Now you will have pumped out the hollow chamber with a vacuum pump so that there is no air in the chamber. How that is done doesn't concern us here, just accept that it can be done easily. So on the outside the atmospheric pressure pushing on the plates, trying to collapse them, and on the evacuated inside you have no air pressure [since there isn't any air in the chamber]. The
>thin metal plates are squeezed together more or less, depending on the atmospheric pushing on the outside surfaces. There are sensitive ways to observe and measure how much the metal plates move in and out. That is one type of barometer.

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>Dear Amber-
>
>   A barometer is an instrument to measure the pressure of the air...the
>atmospheric pressure. Changes in the atmospheric pressure indicate changes
>in weather. Weather stations around the world take atmospheric pressure
>readings each hour, along with other weather observations, such as
>temperature, wind speed and direction, humidity, and sky condition. Here is
>some more information about barometers...
>
>The following is from the Encyclopaedia Britannica...  here is the link..
>http://www.britannica.com/bcom/eb/article/6/0,5716,13596+1+13436,00.html
>
>barometer - a device used to measure atmospheric pressure. Because
>atmospheric pressure changes with distance above or below sea level, a
>barometer can also be used to measure altitude. There are two main types of
>barometers: mercury and aneroid.
>In the mercury barometer, atmospheric pressure balances a column of mercury,
>the height of which can be precisely measured. To increase their accuracy,
>mercury barometers are often corrected for ambient temperature and the local
>value of gravity. Common pressure units include pounds per square inch;
>dynes per square centimetre; newtons per square metre (the SI unit called
>the pascal); inches, centimetres, or millimetres of mercury; and millibars
>(1 millibar equals 1,000 dynes per square centimetre, or 0.75 millimetre of
>mercury). Normal atmospheric pressure is about 14.7 pounds per square inch,
>equivalent to 30 inches (760 millimetres) of mercury, 1,013.2 millibars, or
>101,320 pascals.
>Of the many different varieties of mercury barometers, most variations arise
>from different techniques for measuring the height of the mercury column.
>Though other liquids can be used in a barometer, mercury is the most common.
>Its density allows the vertical column of the barometer to be of manageable
>size. If water were used, for instance, the column would have to be 34 feet
>high.
>A nonliquid barometer called the aneroid barometer is widely used in
>portable instruments and in aircraft altimeters because of its smaller size
>and convenience. It contains a flexible-walled evacuated capsule, the wall
>of which deflects with changes in atmospheric pressure. This deflection is
>coupled mechanically to an indicating needle. A mercury barometer is used to
>calibrate and check aneroid barometers. Calibration can be, for example, in
>terms of atmospheric pressure or altitude above sea level.
>A barometer that mechanically records changes in barometric pressure over
>time is called a barograph. Though mercury barographs have been made,
>aneroid barographs are much more common. The motion of the aneroid capsule
>is magnified through levers to drive a recording pen. The pen traces a line
>on a graph that is usually wrapped around a cylinder driven by a clockwork
>mechanism.
>
>I hope this helps you with your question, Amber.
>
>Wendell Bechtold, meteorologist
>Forecaster, National Weather Service
>Weather Forecast Office, St. Louis, MO
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>Barometers serve several very important purposes. First,
>knowing the atmospheric pressure at different levels in
>the atmosphere allows us to track weather systems, such
>as high and low pressure, and therefore make good weather
>forecasts. Secondly, a form of barometer, called an altimeter,
>allows airplane pilots to know what height they are in the
>air; this is crucial to their ability to maintain the altitude
>that they need to fulfill their flight plan, and to land in
>poor visibility (by knowing their altitude in comparison to the
>altitude of the airport runway).
>
>David Cook
>Argonne National Laboratory

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