Life exists at the bottom of an ocean of air called the atmosphere. This ocean extends upward from the Earth’s surface for many miles, gradually thinning as it nears the top. Near the surface, the air is relatively warm from contact with the Earth. The temperature in the United States averages about 15° C (59° F) the year round. As altitude increases, the temperature decreases by about 2° C (3.5° F) for every 1,000 feet (normal lapse rate) until air temperature reaches about –55° C (–67° F) at 7 miles above the Earth.

For flight purposes, the atmosphere is divided into two layers: the upper layer, where temperature remains practically constant, is the “stratosphere;” the lower layer, where the temperature changes, is the “troposphere.” Although jets routinely fly in the stratosphere, the private pilot usually has no occasion to go that high, but usually remains in the lower layer-the troposphere. This is the region where all weather occurs and practically all light airplane flying is done. The top of the troposphere lies 5 to 10 miles above the Earth’s surface. [Figure 5-1] Obviously, a body of air as deep as the atmosphere has tremendous weight. It is difficult to realize that the normal sea level pressure upon the body is about 15 pounds per square inch, or about 20 tons on the average person. The body does not collapse because this pressure is equalized by an equal pressure within the body. In fact, if the pressure were suddenly released, the human body would explode. As altitude is gained, the temperature of the air not only decreases (it is usually freezing above 18,000 feet) but the air density also decreases; therefore there is less pressure. Pressure is rapidly reduced up to 18,000 feet where the pressure is only half as great as at sea level.

Oxygen and the Human Body

 The atmosphere is composed of about four-fifths nitrogen and one-fifth oxygen, with approximately one percent of various other gases. Oxygen is essential to human life. At 18,000 feet, with only half the normal atmospheric pressure, the body intake of oxygen would be only half the normal amount. Body reactions would be definitely below normal, and unconsciousness might result. In fact, the average person’s reactions become affected at 10,000 feet and may be affected at altitudes as low as 5,000 feet.
 To overcome these unfavorable conditions at high altitudes, pilots use oxygen-breathing equipment and wear protective clothing, or fly in pressurized cabins in which temperature, pressure, and oxygen content of the air can be maintained within proper range.

Significance of Atmospheric Pressure

 The average pressure exerted by the atmosphere is approximately 15 pounds per square inch at sea level. This means that a column of air 1 inch square extending from sea level to the top of the atmosphere would weight about 15 pounds. The actual pressure at a given place and time, however, depends upon several factors. These are altitude, temperature, and density of the air. These conditions definitely affect flight.

Measurement of Atmospheric Pressure

 A barometer is generally used to measure the height of a column of mercury in a glass tube. It is sealed at one end and calibrated in inches. An increase in pressure forces the mercury higher in the tube; a decrease allows some of the mercury to drain out, reducing the height of the column. In this way, changes of pressure are registered in inches of mercury (in. Hg.). The standard sea level pressure expressed in these terms is 29.92 inches at a standard temperature of 15° C (59° F).

 The mercury barometer is cumbersome to move and difficult to read. A more compact, more easily read, and more mobile barometer is the aneroid, although it is not so accurate as the mercurial. The aneroid barometer is a partially evacuated cell sensitive to pressure changes. The cell is linked to an indicator which moves across a scale graduated in pressure units.
 

If all weather stations were at sea level, the barometer readings would give a correct record of the distribution of atmospheric pressure at a common level. To achieve a common level, each station translates its barometer reading into terms of sea level pressure. A change of 1,000 feet of elevation makes a change of about 1 inch on the barometer reading. Thus, if a station located 5,000 feet above sea level found the mercury to be 25 inches high in the barometer tube, it would translate and report this reading as 30 inches. [Figure 5-2] Since the rate of decrease in atmospheric pressure is fairly constant in the lower layers of the atmosphere, the approximate altitude can be determined by finding the difference between pressure at sea level and pressure at the given atmospheric level. In fact, the aircraft altimeter is an aneroid barometer with its scale in units of altitude instead of pressure.

Figure 5-2.—Barometric pressure at a weather station is expressed as pressure at sea level.

Effect of Altitude on Atmospheric Pressure

 It can be concluded that atmospheric pressure decreases as altitude increases. It can also be stated that pressure at a given point is a measure of the weight of the column of air above that point. As altitude increases, pressure diminishes as the weight of the air column decreases. This decrease in pressure (increase in density altitude) has a pronounced effect on flight.

Effect of Altitude on Flight

 The most noticeable effect of a decrease in pressure, due to an altitude increase, becomes evident in takeoffs, rate of climb, and landings. An airplane that requires a 1,000-foot run for takeoff at a sea level airport will require a run almost twice as long to take off at an airport which is approximately 5,000 feet above sea level. The purpose of a takeoff run is to gain enough speed to generate lift from the passage of air over the wings.

If the air is thin, more speed is required to obtain enough lift for takeoff—hence, a longer ground run. It is also true that the engine is less efficient in thin air, and the thrust of the propeller is less effective. The rate of climb is also slower at the higher elevation, requiring a greater distance to gain the altitude necessary to clear any obstructions. In landing, the difference is not so noticeable except that the plane has greater groundspeed when it touches the ground. [Figures 5-3 and 5-4]

Figure 5-3.—Atmospheric density at sea level enables an airplane to take off in a relatively short distance.
 
 

Effect of Differences in Air Density  Differences in air density caused by changes in temperature result in changes in pressure. This, in turn, creates motion in the atmosphere, both vertically and horizontally (current and winds). This action, when mixed with moisture, produces clouds and precipitation—in fact, these are the phenomena called “weather.”

Pressure Recorded in “Millibars”

 The mercury barometer reading at the individual weather stations is converted to the equivalent sea level pressure and then translated from terms of inches of mercury to a measure of pressure called millibars. One inch of mercury is equivalent to approximately 34 millibars; hence, the normal atmospheric pressure at sea level (29.92), expressed in millibars, is 1,013.2 or roughly 1,000 millibars. The usual pressure readings range from 950.0 to 1,040.0.
 Individually these pressure readings are of no particular value to the pilot; but when pressures at different stations are compared, or when pressures at the same station show changes in successive readings, it is possible to determine many symptoms indicating the trend of weather conditions. In general, a falling pressure indicates the approach of bad weather and a rising pressure indicates a clearing of the weather.

Wind

 The pressure and temperature changes discussed in the previous section produce two kinds of motion in the atmosphere—vertical movement of ascending and descending currents, and horizontal flow known as “wind.” Both of these motions are of primary interest to the pilot because they affect the flight of aircraft during takeoff, landing, climbing, and cruising flight. These motions also bring about changes in weather, which require a pilot to determine if a flight can be made safely.

 Conditions of wind and weather occurring at any specific place and time are the result of the general circulation in the atmosphere. This will be discussed briefly in the following pages.

 The atmosphere tends to maintain an equal pressure over the entire Earth, just as the ocean tends to maintain a constant level. When the equilibrium is disturbed, air begins to flow from areas of higher pressure to areas of lower pressure.