Page 13 - Atmospheric Pressure
What is Atmospheric Pressure?
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.
|How is Pressure
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.
Example 2) Convert 29.72 inches to millibars.
|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
1) Elevation: As elevation increases, pressure decreases.
2) Relative humidity (water vapor content of the air):
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