The Atmosphere and the Weather

The Behavior of Air

Overview of Pressure and Density

The atmosphere contains a tremendous number of molecules being pulled toward earth by the force of gravity.

These molecules excert a force on all surfaces with which they are in contact, and the amount of force per unit of surface area is pressure.

Another way to explain the concept of pressure follows: Each time an air molecule bounces against an object, it gives a tiny push. This small force (push) divided by the area on which it pushes is called pressure:

Pressure

=

force area

Air pressure at a given location on the Earth's surface is described as the weight per unit area of the column of air above that location.

At sea level, the molecules of air exert an average force of about 14.7 pounds on each square inch of area.

Due to gravity air molecules are held near the ground. Nearly 26.5 x 10¹⁸ (a bit less than 27,000,000,000,000,000,000) molecules occupy a cubic centimeter of air at 0°C near sea level.

This implies that air density (its mass of air in a given volume) must decrease with height.

The pressure at any level in the atmosphere may be measured in terms of the total height of the air above that level --> climb in elevation, fever molecules are above --> atmospheric pressure always decreases with height.

The gas law

The relationship among the pressure, temperature, and density of air can be expressed by:

pressure

=

temperature

x

density

x

constant

Therefore, a change in one variable causes a corresponding change in the other two variables. Thus, it would be easier to understand the behavior of a gas if we keep one variable from changing and observe the behavior of the other two.

If we hold temperature constant:

pressure

~

density (temperature constant)

That is, if we temperature of a gas is held constant, as the pressure increases the density increases, and as the pressure decreases the density decreases. On in other words: at the same temperature, air at a higher pressure is more dense than air at a lower pressure.

If we hold the pressure constant:

(constant pressure)

x

constant

~

density

x

temperature

That is, when the pressure of a gas is held constant, the gas becomes less dense as the temperature goes up, and more dense as the temperature goes down. Therefore, at a given atmospheric pressure, air that is cold is more dense than air that is warm.

Pressure units

Numerous units for measuring pressure have been introduced and, at times, are confused in the literature.

Conversion factors for units of air pressure

1 bar	=	1000 millibars (mb)
1 mb	=	0.02953 in. of mercury
1 inch of mercury	=	33.8639 mb
1 kilopascal (kPa)	=	1000 Pascals (Pa)
1 hectopascal (hPa)	=	100 Pa
1 mb	=	1 hPa
1 inch of mercury	=	35.4 mm of mercury

Links to weather plots

• Arizona current weather
• US Surface Weather, pressure, fronts
• U. of A. current pressure, corrected to sea level.

Have you ever wandered why your ears "pop" when ascending or descending? Find out more about this in a special Ear-Popping web page.

Barometers:

Atmospheric pressure is measured with an instrument called the barometer.

Diagram showing the construction of Torricelli's barometer. The height of the mercury column is a measure of atmospheric pressure.

Two main types: the mercury barometer and the aneroid barometer.

Evangelista Torricelli, a student of Galileo, invented the mercury barometer in 1643. In his experiment, Torricelli immersed a tube, sealed at one end, into a container of mercury (see Figure below).

Aneroid barometer.

Atmospheric pressure then forced the mercury up into the tube to a level that was considerably higher than the mercury in the container. Torricelli determined from this experiment that the pressure of the atmosphere is approximately 30 inches or 76 centimeters.

Why mercury?

Mercury seldom rises to a height above 79 cm (31 inches);
Water is 13.6 less dense than water --> atmospheric pressure of 76 cm (30 in.) of mercury would be equivalent to 1034 (408 in.) or water.

The aneroid barometer contains a metal box called and aneroid cell, which is tightly sealed. External changes in pressure cause the cell to expand or contract. Any change in its size is amplified by levers and transmitted to an indicating arm.

A barograph is a recording aneroid barometer.