Page 13 - Atmospheric Pressure


What is Atmospheric Pressure?

If someone placed a weight on your head you would feel the force of that weight pressing down on you. Atmospheric pressure is the force of all the atmosphere above your head, more than 75 miles of it, pressing down on your head and on everything else on the surface of the earth.

Reading the Pressure Scales:

Notice that each division (small mark) on the millibar scale is 1 (1.0) millibar. When you are reading the millibar scale you must count by whole millibars.

On the inches scale each division is only 1/100 (0.01) of an inch. When you are reading the inch scale you must count by 0.01 inches (see illustration above).

How is Pressure Measured?

Atmospheric pressure is measured with a barometer. Barometers may measure pressure using 2 different scales: Millibars or Inches (of mercury).
Inches (of mercury):

A mercury barometer is just an upside-down tube with the top closed and the bottom open. All the air is removed from the tube. The open bottom is placed in a dish of mercury. Air pressure forces the mercury up the tube. The more the pressure, the higher the mercury goes. Normal pressure at sea level is enough to make the mercury rise 29.92 inches up the tube.

29.92 inches = 1 atmosphere (see graph from page 14 ESRT below)

Converting from one scale to the other:

Example 1) Convert 1006.0 millibars to inches.
         Find 1006.0 millibars on the left side of the illustration above and then carefully and accurately find the same value on the inch scale on the right side. 1006.0 millibars = 29.71 inches.

Example 2) Convert 29.72 inches to millibars.
Find 29.72 inches on the right side of the illustration above and then carefully and accurately find the same value on the millibar scale on the left side. But 29.72 is in the middle between 1006.0 and 1007.0 millibars. What now?

Re-read the question! Does it say 'answer to the nearest tenth (0.1) millibar'? Then say 1006.5 mb. Does it say 'answer to the nearest whole millibar'? Then choose either 1006 or 1007 mb. If no accuracy is specified use your best judgment.

Most barometers today replace the mercury (highly toxic) with a can from which all the air has been removed. These are called aneroid barometers. The can expands or contracts as pressure changes and this causes a pointer on a dial to move. The dial may be marked in either millibars or inches.

What Variables Affect Air Pressure?

1) Elevation: As elevation increases, pressure decreases.

2) Relative humidity (water vapor content of the air):
    As humidity increases, pressure decreases.

3) Temperature: As temperature increases, pressure

Notice that all of these relationships are inverse. As one value increases, the other decreases.

Highest pressure: Cool/Dry air

Lowest pressure: Warm/Moist air

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