Although light and elastic, air does have weight. Because of its weight, the atmosphere has a

pressure of approximately 14.7 pounds per square inch at sea level. Put another way, imagine a square 4

inches on one side and 5 inches on the longer side. This is a space 20 inches square, and at sea level, the

atmospheric pressure on the square is 294 pounds (14.7 x 20). Standard air is 15C., and normal

barometric pressure at sea level is 29.92 inches of mercury. The three figures (14.7 pounds, 15C., an

29.92 inches) make up "standard conditions," a term often used instead of the three figures. Assuming a

constant temperature, the density of a volume of air varies directly with the pressure. If the pressure is

doubled, the density is doubled.

The letter C in 15 stands for Centigrade, a scale in the metric system for measuring temperature.

The temperature of 15 C is equal to 59 Fahrenheit. The conversion formula to change Centigrade to

Fahrenheit is: 9/5 x (15) + 32 = 59F. To convert from Fahrenheit to Centigrade, the formula is: C =

59 - 32 x 5/9).

Figure 1.2 shows the atmosphere divided into layers circling the earth. The layers, starting with the

one next to the earth, are the troposphere: most of our weather occurs in this layer. Next are the

stratosphere, ionosphere, and exosphere. The tropopause is a narrow area between the troposphere and

the stratosphere. Notice in figure 1.2 that the various cloud formations appear to straddle the line

identifying the tropopause location. The jet stream is near the upper portion of the troposphere. The

troposphere varies in height from 60,000 feet above sea level over the Equator to 25,000 feet over the

poles. Its height varies with the seasons; it is higher in summer than in winter. In the temperate zones, it

is about 35,000 feet above sea level.

The temperature in the troposphere decreases as an aircraft gains altitude, because the air closest to

the earth is warmer and receives the largest amount of the sun's energy. The variation in temperature

with altitude is called the temperature lapse rate and is usually expressed in degrees per thousand feet. If

observations taken day after day at thousands of locations around the world were averaged, the average

temperature lapse rate would be about 2C or 3 1/2F per thousand feet.