Aviation is so dependent upon that category of fluids called gases and the effect of forces and pressures acting upon gases, that a discussion of the subject of the atmosphere is important to the persons maintaining and repairing aircraft.

Data available about the atmosphere may determine whether a flight will succeed, or whether it will even become airborne. The various components of the air around the earth, the changes in temperatures and pressures at different levels above the earth, the properties of weather encountered by aircraft in flight, and many other detailed data are considered in the preparation of flight plans.

Pascan and Torricelli have been credited with developing the barometer, and instrument for measuring atmospheric pressure. The results of their experiments are still used today with very little improvement in design or knowledge. They determined that air has weight which changes as altitude is changed with respect to sea level. Today scientists are also interested in how the atmosphere affects the performance of the aircraft and its equipment.

Composition of the Atmosphere

The atmosphere is a complex and ever changing mixture. Its ingredients vary from place to place and from day to day. In addition to a number of gases, it contains quantities of foreign matter such as pollen, dust, bacteria, soot, volcanic ash, spores, and dust from outer space.

The composition of the air remains almost constant from sea level up to its highest level, but its density diminishes rapidly with altitude. Six miles up, for example, it is too thin to support respiration, and 12 miles up there is not enough oxygen to support combustion. At a point several hundred miles above the earth, some gas particles spray out into space; some dragged by gravity fall back into the ocean of air below; others never return, but travel like satellites around the earth; and still others like hydrogen and helium escape forever from the earth's gravitational field. Physicists disagree as to the boundaries of the outer fringes of the atmosphere. Some think it begins 240 miles above the earth and extends to 400 miles; others place its lower edge at 600 miles and its upper boundary at 6,000 miles.

There are also certain nonconformities at various levels. Between 12 and 30 miles, high solar ultraviolet radiation reacts with oxygen molecules to produce a thin curtain of ozone, a very poisonous gas without which life on earth could not exist. This ozone filters out a portion of the sun's lethal ultraviolet rays, allowing only enough to come through to give man sunburn, kill bacteria, and prevent rickets. At 50 or 65 miles up, most of the oxygen molecules begin to break down under solar radiation into free atoms, and to form the incomplete molecule, hydroxyl (OH) from water vapor. Also in this region all the atoms become ionized.

Studies of the atmosphere have revealed that the temperature does not decrease uniformly with increasing altitude; instead it gets steadily colder up to a height of about 7 miles, where the rate of temperature change slows down abruptly and remains almost constant at -55° C (218° K) up to about 20 miles. Then the temperature begins to rise to a peak value of 77° C (350° K) at the 55 mile level. Thereafter it climbs steadily, reaching 2,270° C (2,543° K) at a height of 250 to 400 miles. From the 50 mile level upward, a man or any other living creature, without the protective cover of the atmosphere, would be broiled on the sunny side and frozen on the other.

The atmosphere is divided into concentric layers or levels. Transition through these layers is gradual and without sharply defined boundaries. However, one boundary, the tropopause, exists between the first and second layer. The tropopause is defined as the point in the atmosphere at which the decrease in temperature (with increasing altitude) abruptly ceases, between the troposphere and the stratosphere. The four atmosphere layers are the troposphere, stratosphere, ionosphere, and the exosphere. The upper portion of the stratosphere is often called the chemosphere or ozonosphere, and the exosphere is also known as the mesosphere.

The troposphere extends from the earth's surface to about 35,000 feet at middle latitudes; but varies from 28,000 feet at the poles to about 54,000 feet at the equator. The troposphere is characterized by large changes in temperature and humidity and by generally turbulent conditions. Nearly all cloud formations are within the troposphere. Approximately three-fourths of the total weight of the atmosphere is within the troposphere. The temperature and absolute pressure in the troposphere steadily decrease with increasing altitude to a point where the temperature is approximately -55° C (or 218° K), and the pressure is about 6.9 Hg on a standard day.

The stratosphere extends from the upper limits of the troposphere (and the tropopause) to an average altitude of 60 miles. In the stratosphere the temperature decline virtually stops; however, at 18 or 20 miles up, it often descends to -63° C (210° K).

The ionosphere ranges from the 50 mile level to a level of 300 to 600 miles. Little is known about the characteristics of the ionosphere, but it is thought that many electrical phenomena occur there. Basically, this layer is characterized by the presence of ions and free electrons, and the ionization seems to increase with altitude and in successive layers. The temperature increases from about 200° K at the lower limit to about 2,500° K at the upper limit. These extremely high temperatures in the upper altitudes do not have the same significance as would corresponding temperatures at sea level. A thermometer reading in this region would be determined more by solar radiation than by the temperature, because of the energy of the particles.

The exosphere (or mesosphere) is the outer layer of the atmosphere. It begins at an altitude of 600 miles and extends to the limits of the atmosphere. In this layer the temperature is fairly constant at 2,500° K, and propagation of sound is thought to be impossible due to lack of molecular substance.

Atmospheric Pressure

The human body is under pressure, since it exists at the bottom of a sea of air. This pressure is due to the weight of the atmosphere. The pressure which the atmosphere applies to a square inch of area is equal to the weight of a column of air one square inch in cross section which extends from that area to the "top" of the atmosphere.

Since atmospheric pressure at any altitude is due to the weight of air above it, pressure decreases with increased altitude. Obviously, the total weight of air above an area at 15,000 feet would be less than the total weight of the air above an area at 10,000 feet.

Atmospheric pressure is often measured by a mercury barometer. A glass tube somewhat over 30 inches in length is sealed at one end and filled with mercury (Hg). It is then inverted and the open end placed in a dish of mercury. Immediately, the mercury level in the inverted tube will drop a short distance, leaving a small volume of mercury vapor at nearly zero absolute pressure in the tube just above the top of the liquid mercury column. The pressure acting upward at the lower end of the tube above the level of mercury in the dish is atmospheric pressure. The pressure acting downward at the same point is the weight of the column of mercury. Thus, the height of the column of mercury, indicates the pressure exerted by the atmosphere.

This means of measuring atmospheric pressure gives rise to the practice of expressing atmospheric pressure in inches of mercury (in Hg) rather than in pounds per square inch (psi). It may be seen, however, that a simple relationship exists between pressure measurements in psi and in inches Hg. One cu in of mercury weighs 0.491 pound. Therefore, a pressure of 30 inches of mercury would be the equivalent of:

0.491 x 30 = 14.73 psi

A second means of measuring atmospheric pressure is with an aneroid barometer. This mechanical instrument lends itself to use on airplanes much more adequately than does the mercury barometer. Aneroid barometers (altimeters) are used to indicate altitude in flight. The calibrations are made in thousands of feet rather than in psi. For example, the standard pressure at sea level is 29.92 in Hg, or 14.69 psi. At 10,000 feet above sea level, standard pressure is 20.58 in Hg, or 10.10 psi. Altimeters are calibrated so that if the pressure exerted by the atmosphere were 20.58 in Hg, the altimeter would point to 10,000 feet. In other words, the altimeter is calibrated so that it indicates the altitude at which the prevailing atmospheric pressure would be considered standard pressure. Thus, the altitude read from the altimeter, being dependent upon atmospheric pressure, is called pressure altitude (Hp). Actually, an altimeter will read pressure altitude only when the altimeter adjustment is set at 29.92 inches Hg.

A third expression is occasionally used to indicate atmospheric pressure. Atmospheric pressure may be expressed in atmospheres. For example, a test may be conducted in a pressurized chamber under a pressure of six atmospheres. This merely means that the pressure is six times as great as standard sea level pressure.

Atmospheric Density

Since both temperature and pressure decrease with altitude, it might appear that the density of the atmosphere would remain fairly constant with increased altitude. This is not true, however, for pressure drops more rapidly with increased altitude than does the temperature. The result is that density decreases with increased altitude.

By use of the general gas law, studied earlier, it can be shown that for a particular gas, pressure and temperature determine the density. Since standard pressure and temperatures have been associated with each altitude, the density of the air at these standard temperatures and pressures must also be considered standard. Thus, a particular atmospheric density is associated with each altitude. This gives rise to the expression "density altitude," symbolized Hd. A density altitude of 15,000 feet is the altitude at which the density is the same as that considered standard for 15,000 feet. Remember, however, that density altitude is not necessarily true altitude. For example, on a day when the atmospheric pressure is higher than standard and the temperature is lower than standard, the density which is standard at 10,000 feet might occur at 12,000 feet. In this case, at an actual altitude of 12,000 feet, we have air which has the same density as standard air at 10,000 feet. Density altitude is a calculated altitude obtained by correcting pressure altitude for temperature.

The water content of the air has a slight effect on the density of the air. It should be remembered that humid air at a given temperature and pressure is lighter than dry air at the same temperature and pressure.

Water Content of the Atmosphere

In the troposphere the air is seldom completely dry. It contains water vapor in either of two forms: (1) Fog or (2) water vapor. Fog consists of minute droplets of water held in suspension by the air. Clouds are composed of fog. The height to which some clouds extend is a good indication of the presence of water in the atmosphere almost up to the stratosphere.

As a result of evaporation, the atmosphere always contains some moisture in the form of water vapor. The moisture in the air is called the humidity of the air. Moisture does not consist of tiny particles of liquid held in suspension in the air as in the case of fog, but is an invisible vapor truly as gaseous as the air with which it mixes.

Fog and humidity both affect the performance of an aircraft. In flight, at cruising power, the effects are small and receive no consideration. During takeoff, however, humidity has important effects. Two things are done to compensate for the effects of humidity on takeoff performance. Since humid air is less dense than dry air, the allowable takeoff gross weight of an aircraft is generally reduced for operation in areas that are consistently humid. Secondly, because the power output of reciprocating engines is decreased by humidity, the manifold pressure may have to be increased above that recommended for takeoff in dry air in order to obtain the same power output.

Engine power output is calculated on dry air. Since water vapor is incombustible, its pressure in the atmosphere is a total loss as far as contributing to power output. The mixture of water vapor and air is drawn through the carburetor and fuel is metered into it as though it were all air. This mixture of water vapor, air, and fuel enters the combustion chamber where it is ignited. Since the water vapor will not burn, the effective fuel/air ratio is enriched and the engine operates as though it were on an excessively rich mixture. The resulting horsepower loss under humid conditions can therefore be attributed to the loss in volumetric efficiency due to displaced air, and the incomplete combustion due to excessively rich fuel/air mixture.

The reduction in power that can be expected from humidity is usually given in charts in the flight manual. There are several types of charts in use. Some merely show the expected reduction in power due to humidity; others show the boost in manifold pressure necessary to restore full takeoff power.

The effect of fog on the performance of an engine is very noticeable, particularly on engines with high compression ratios. Normally, some detonation will occur during acceleration, due to the high BMEP (Brake Mean Effective Pressures) developed. However, on a foggy day it is difficult to cause detonation to occur. The explanation of this lies in the fact that fog consists of unvaporized particles of water. When these particles enter the cylinders, they absorb a tremendous amount of heat energy in the process of vaporizing. The temperature is thus lowered, and the decrease is sufficient to prevent detonation.

Fog will generally cause a decrease in horsepower output. However with a supercharged engine, it will be possible to use higher manifold pressures without danger of detonation.

Absolute Humidity

Absolute humidity is the actual amount of the water vapor in a mixture of air and water. It is sometimes expressed in g./cu m (grams per cubic meter), sometimes in lbs/cu ft. The amount of water vapor that can be present in the air is dependent upon the temperature and pressure. The higher the temperatures, the more water vapor the air is capable of holding, assuming constant pressure. When air has all the water vapor it can hold at the prevailing temperature and pressure, it is said to be saturated.

Relative Humidity

Relative humidity is the ratio of the amount of water vapor actually present in the atmosphere to the amount that would be present if the air were saturated at the prevailing temperature and pressure. This ratio is usually multiplied by 100 and expressed as a percentage. Suppose, for example, that a weather report includes the information that the temperature is 75° F and the relative humidity is 56 percent. This indicates that the air holds 56 percent of the water vapor required to saturate it at 75° F. If the temperature drops and the absolute humidity remains constant, the relative humidity will increase. This is because less water vapor is required to saturate the air at the lower temperature.

Dew Point

The dew point is the temperature to which humid air must be cooled at constant pressure to become saturated. If the temperature drops below the dew point, condensation occurs. People who wear glasses have had the experience of going from cold outside air into a warm room and having moisture collect quickly on their glasses. This happened because the glasses were below the dew point temperature of the air in the room. The air immediately in contact with the glasses was cooled below its dew point temperature, and some of the water vapor was condensed out. This principle is applied in determining the dew point. A vessel is cooled until water vapor begins to condense on its surface. The temperature at which this occurs is the dew point.

Vapor Pressure

Vapor pressure is the portion of atmospheric pressure that is exerted by the moisture in the air (expressed in tenths of an inch Hg). The dew point for a given condition depends on the amount of water pressure present; thus a direct relationship exists between the vapor pressure and the dew point.

Wet and Dry Bulb Temperatures

Vapor pressure and humidity may be determined from charts based on the wet and dry bulb temperatures (figure 7-20). The dry bulb temperature is obtained from an ordinary thermometer.

The wet bulb temperature is obtained from a thermometer which has its bulb covered with a thin piece of wet cloth.

Because of moisture evaporation from the wet cloth, the wet bulb will read lower than the dry bulb. The more rapid the evaporation, the greater will be the difference in readings. The rate of evaporation is dependent upon the degree of saturation of the air. In using the wet bulb thermometer, it must be moved through the air at a rate of about 1,200 f.p.m. to give a worthwhile reading. This is accomplished by mounting both the wet and dry bulb thermometers on a frame which can be hand rotated around a pivot and the desirable rate of 1,200 f.p.m. attained.

If the air is saturated, no evaporation will take place and the wet and dry bulb temperatures will be the same. Thus, these two temperatures coincide at the dew point.

Physical Laws Related to the Atmosphere

Although air is composed of various gases and must be treated as a mixture for certain purposes, it is treated as a uniform gas in aerodynamic calculations. Air is a fluid since it has the fluid property to flow, and it is also a gas, since its density is readily varied. As is usual in engineering work, certain simplifying assumptions are made. One standard assumption is that in dry air there is no water vapor present. Ordinary flight - takeoff charts may be corrected for vapor pressure, but subsonic flight does not consider vapor pressure as an appreciable factor.

Another standard assumption is that friction or "viscosity effect" may be neglected when dealing with airflow. The air is then considered to be a perfect fluid. However, some exceptions must be made, particularly in the case of thin boundary layers of slow moving air directly adjacent to a moving body.

The Kinetic Theory of Gases Applied to Air

The kinetic theory states that a gas is composed of small, distinct particles called molecules. The size of the molecules is small compared to the average distance between them. Further, the molecules are moving about at a high rate of speed in random directions so that they are constantly colliding with one another and with the walls of any container that may surround them. The pressure produced by a gas is the result of these continuous impacts against a surface, and since the impacts are essentially infinite in number, a steady pressure is effected.

Just as pressure is produced by molecular impact against a surface, it is also transmitted by molecular impact. Assuming that molecules are perfectly elastic (that no friction exists between them), a pressure wave once started will continue indefinitely. For most purposes this assumption is adequate; however, it is not completely correct. For instance, sound represents a series of weak pressure waves to which the ear is sensitive. If the energy that the sound represents were not lost, the sound would continue indefinitely. It follows then that this imperfect elasticity must in some way be associated with fluid friction or viscosity, since the presence of viscosity is also a source of energy loss.

On the basis of the kinetic theory, pressure may be increased in two ways: First, by increasing the number of molecules in a given space, which is the same as increasing the density; and secondly, by increasing the speed of the molecules, which can be done by increasing the temperature, since the temperature increase produces an increase in the molecular speed.

Analysis of the kinetic theory leads to one definite relationship between the temperature, pressure, and density of a gas when the gas is subjected to a given set of conditions. This relationship is known as the equation of state.

Equation of State

Provided that the temperature and pressure of a gas are not excessively different from those normally experienced on the earth's surface, the following equation holds true:


where: P = pressure in lbs/sq ft
           V = specific volume.
           R = a constant for a given gas (for air R = 53.345).
           T = absolute temperature (Rankine = °F + 459.4).

If the temperature and pressure are such that the gas becomes a liquid, or if the pressure falls to such a value that uniform pressure no longer exists, the equation will be invalid. In practical aeronautical work, these extremes are only encountered in a supersonic wind tunnel or in the outer portions of the atmosphere. This formula must be further clarified for practical engineering, by the introduction of air density.

Standard Atmosphere

If the performance of an aircraft is computed, either through flight tests or wind tunnel tests, some standard reference condition must be determined first in order to compare results with those of similar tests. The conditions in the atmosphere vary continuously, and it is generally not possible to obtain exactly the same set of conditions on two different days or even on two successive flights. Accordingly, there must be set up a group of standard conditions that may be used arbitrarily for reference. The set of standard conditions presently used in the United States is known as the U. S. Standard Atmosphere.

The standard atmosphere approximates the average conditions existing at 40° latitude, and is determined on the basis of the following assumptions. The standard sea level conditions are:

Pressure at 0 altitude (P0) = 29.92 inches of mercury.
Temperature at 0 altitude (T0) = 15° C 59° F.
Gravity at 0 altitude (g0) = 32.174 ft/sec2

The U.S. Standard Atmosphere is in agreement with the International Civil Aviation Organization (ICAO) Standard Atmosphere over their common altitude range. The ICAO Standard Atmosphere has been adopted as standard by most of the principal nations of the world.

Variations from Standard Days

As may be expected, the temperature, pressure, density, and water vapor content of the air varies considerably in the troposphere. The temperature at 40° latitude may range from 50° C at low altitudes during the summer to -70° C at high altitudes during the winter. As previously stated, temperature usually decreases with an increase in altitude. Exceptions to this rule occur when cooler air is trapped near the earth by a warmer layer. This is called a temperature inversion, commonly associated with frontal movement of air masses.

Pressure likewise varies at any given point in the atmosphere. On a standard day, at sea level, pressure will be 29.92 in Hg. On nonstandard days, pressure at sea level will vary considerably above or below this figure.

Density of the air is determined by the pressure and temperature acting upon it. Since the atmosphere can never be assumed to be "standard," a convenient method of calculating density has been devised. Since air pressure is measured in inconvenient terms, it is expedient to utilize the aneroid altimeter as a pressure gauge and refer to the term "pressure altitude" instead of atmospheric pressure.

Pressure Altitude

Pressure altitude is the altitude in the standard atmosphere corresponding to a particular value of air pressure. The aircraft altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere.

With the altimeter of an aircraft set at 29.92 in Hg, the dial will indicate the number of feet above or below a level where 29.92 in Hg exists, not necessarily above or below sea level, unless standard day conditions exist. In general, the altimeter will indicate the altitude at which the existing pressure would be considered standard pressure. The symbol Hp is used to indicate pressure altitude.