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THE ATMOSPHERE, PRESSURE, AND FORCES
Air, which is the material of which the atmosphere is composed, is a mixture of invisible gases. At altitudes up to 250,000 feet, the atmosphere consists of approximately 78% nitrogen and 21% oxygen. The remainder is made up of argon, carbon dioxide, several other gases, and water vapor. Water vapor acts as an independent gas mixed with air.
Water vapor is found only in the lower levels of the atmosphere. From the standpoint of weather, however, it is the most important component of the air. Because it can change into water droplets or ice crystals under atmospheric conditions of temperature and pressure, water vapor is responsible for the formation of clouds and fog. However, just when and under what conditions water vapor will change to a visible form is difficult to predict and is made more difficult by the fact that the amount of water vapor in the air is never constant, but varies from day to day and even from hour to hour.
The lower layer of the atmosphere contains an enormous number of microscopic impurities such as salt, dust and smoke particles. They are important to aviation for they are often present in sufficient quantities to reduce visibility. They also have a function in the condensation process.
The upper layers of the atmosphere do not contain dust particles or impurities for the sun's light to reflect off, and for this reason appear deep cobalt blue to black in color.
The atmosphere has weight. Although the weight of the atmosphere is only about one millionth the weight of the earth, it does exert a force or pressure on the surface of the earth. A square inch column of air weighs approximately 14.7 lbs. at sea level. This weight diminishes with height. At 20,000 feet. a square inch column weighs 6.75 lbs.
The characteristics of the atmosphere vary with time of day, season of the year and latitude. Consequently only average values are referred to in this section on flight environment.
PROPERTIES OF THE ATMOSPHERE
Mobility, capacity for expansion and capacity for compression are the principal properties of the atmosphere. These characteristics are of the utmost importance in a study of weather for they in combination are the cause of almost all atmospheric weather phenomena. The capacity for expansion is especially important. Air is often forced to rise by various lifting agents (thermal, frontal or mechanical means). Rising in areas of decreasing pressure, the parcel of air expands. In expanding, it cools. The cooling process may bring the temperature of the parcel of air to the degree where condensation occurs. Thus clouds form and precipitation may take place. Conversely, sinking air, as the external pressure increases, decreases in volume with an attendant rise in temperature.
DIVISIONS OF THE ATMOSPHERE
The atmosphere consists of four distinct layers surrounding the earth for a depth of many hundreds of miles. They are, in ascending order, the troposphere, the stratosphere, the mesosphere and the thermosphere.
THE TROPOSPHERE. This is the lowest layer of the atmosphere, and varies in height in different spans of the world from roughly 28,000 feet above sea level at the poles to 54,000 feet at the equator. Within the troposphere the pressure, density and temperature all decrease rapidly with height. Most of the weather occurs in the troposphere, because of the presence of water vapor and strong vertical currents. In the upper regions of the troposphere, winds are strong and the fast moving jet streams occur. The top layer of the troposphere is known as the tropopause. Here the temperature ceases to drop and remains substantially constant at about -56°C. The height of the tropopause varies, as already stated, from the poles to the equator, but also from summer to winter.
THE STRATOSPHERE. For a distance of about 50,000 feet above the tropopause, there is a layer known as the stratosphere in which the pressure continues to decrease but in which the temperature remains relatively constant in the vicinity of -56°C. This layer also varies in thickness, being quite deep over the poles and thinner over the equator. Water vapor is almost non-existent and air currents are minimal. The top layer of the stratosphere is called the stratopause.
THE MESOSPHERE. The mesosphere is characterized by a marked increase in temperature. At a height of about 150,000 feet, the temperature reaches 10°C. The rise in temperature is due to the presence of a layer of ozone which absorbs more of the sun's radiation. In the top layer of the mesosphere, called the mesopause, the temperature again drops rapidly reaching a level of about -100°C at 250,000 feet above the earth.
THE THERMOSPHERE. Temperature again begins to rise in the thermosphere and increases for an indefinite distance into space, rising as high as 3000°C at 400 miles. This does not mean that a space ship, if it was cruising at this height, would experience a temperature of 3000°C by contact with the atmosphere. The temperature in these rarified layers is based on the kinetic theory of gases. The only heat the space ship would experience would be what it would receive from the radiation of the sun. The spectacular auroras form in the upper regions of the thermosphere. Within the thermosphere is a region known as the ionosphere, which extends from about 50 miles to 250 miles above the earth's surface. Within this region, free electron density is very great and distance from Sea Level affects radio communication. The ionosphere is the layer which reflects low and medium frequency radio waves back to the earth. Very high frequency (VHF) radio waves, however, penetrate this layer. Beyond the ionosphere lies a layer so thin that the pressure drops to little more than a vacuum. Under these conditions, the concept of temperature has little meaning and is usually replaced by definitions of the energy states of individual molecules. This thin upper layer is known as the exosphere.
SPACE. Since air becomes gradually thinner with increasing altitude, the upper limit of the atmosphere is, for all practical purposes, difficult to define. Now that man has invaded the cosmic realm of outer space, the question arises as to just where space actually begins. At an altitude of 90 to 100 miles, one enters the realm of satellites and aerodynamic lift is no longer a requirement for maintaining height above the earth. This region has been accepted by some authorities as the boundary of outer space. Ninety miles is recognized as the limit of national sovereignty.
The decrease with height of pressure, density and temperature which occurs in the lower layers of the atmosphere is not constant but varies with local conditions. However, for aeronautical purposes it is necessary to have a standard atmosphere. There are several different standard atmospheres in use, but they vary only slightly. The ICAO standard atmosphere for the continent of North America, based on summer and winter averages at latitude 40° assumes the following conditions:
1. The air is a perfectly dry gas.
2. A mean sea level pressure of 29.92 inches of mercury.
3. A mean sea level temperature of 15°C.
4. The rate of decrease of temperature with height is 1.98°C per 1000 feet.
The pressure of the atmosphere at any point is due to the weight of air above it. Pressure at the surface of the earth is usually measured by the mercury barometer and is expressed in inches of mercury. The mercury barometer consists of an open dish of mercury into which the open end of an evacuated glass tube is placed. Atmospheric pressure forces mercury to rise in the tube. The greater the pressure, the higher the column rises. A measurement expressed in inches is, in effect, the length of the column of mercury, the weight of which will balance a column of air extending from the ground to the top of the atmosphere.
Violent vertical currents exist in the cloud. Hail is frequently present within the cloud and may occasionally fall from it. A line of cumulonimbus is often an indication of a cold front. The cloud should be avoided because of its turbulence. The danger of heavy icing and violent electrical activity also exists. Cumulonimbus clouds may be embedded in stratiform clouds.
One hectopascal (hPa) is equal to one millibar (mb), the term in common usage prior to the adoption of metric terminology.
A pressure expressed as 29.92 inches of mercury is equivalent to 1013.2 hectopascals (millibars).
One kilopascal (kPa) equals 10 hectopascals. A pressure expressed as 103.32 kPa is equivalent to 1033.2 hectopascals (millibars).
STATION PRESSURE, SEA LEVEL PRESSURE AND ALTIMETER SETTING
Station pressure is the actual atmospheric pressure at the elevation of the observing station. It is, in other words, the actual weight of a column of air extending up from the station level to the outer limit of the atmosphere. The value is determined directly from the mercury barometer at the observing station. Since the weight of the atmosphere decreases with altitude, it follows that the atmospheric pressure reading at a station at 5000 feet elevation will be less than that at a station at 1000 feet and still less than that at a station at sea level.
To have a consistent record of the distribution of atmospheric pressure, it is necessary to reduce the station pressures to a common level. This standard is called mean sea level (MSL) pressure. The reduction to MSL pressure involves adding to the station pressure the weight of an imaginary column of air extending from the station level down to mean sea level. In determining MSL pressure, local temperature must be taken into account. The temperature value is based on the average of the surface temperatures at the time of observation and for 12 hours before the time of observation. Mean sea level pressure is expressed in hectopascals (millibars).
To make an altimeter in an airplane correctly read the true height above mean sea level, it must be set to a standard atmospheric pressure. This pressure reading is called the altimeter setting. It differs slightly from the mean sea level pressure. In reducing station pressure to sea level pressure for altimeter seeing purposes, the standard sea level temperature of 15°C and the standard lapse rate of 1.98°C per 1000 feet is used in computing the equivalents. When correctly set, the altimeter will then read the true elevation of the airport at which the airplane is parked. (Conversely, if a pilot does not know the altimeter setting but does know the elevation of the airport at which the airplane is parked, he can dial in the correct elevation and get also the correct altimeter setting.) Altimeter setting is reported in inches of mercury.
The pressure readings that are taken at various weather reporting stations all over North America are transmitted to forecast offices and are plotted on specially prepared maps. Areas of like pressure are joined by lines, wind direction arrows are entered and the result is a weather map that gives the weatherman a symbolic picture of the weather over the whole continent.
The lines that join, on a weather map, areas of equal barometric pressure are called isobars. These lines are drawn on the map at intervals of four hectopascals (millibars), above and below the value of 1000 hectopascals (millibars). When the isobars are drawn in, they form definite patterns. They never cross, but form roughly concentric circles and form themselves into distinct areas of high and low pressure.
The various types of pressure systems are lows, secondary lows, troughs, highs, ridges and cols.
The pressure patterns on the weather map are very like contour lines on a topographic map; the high-pressure areas correspond to hills and the low-pressure areas to valleys. It is important to recognize that the pressure systems are relative to the pressure around them. A high pressure area, in the center of which the pressure reading is, for example, 1000 hectopascals (millibars), is classified a high because the surrounding pressure is less than 1000 hectopascals. A pressure area with the same pressure reading of 1000 hectopascals (millibars) at the center would be classified as a low if the surrounding pressure is higher than 1000 hectopascals.
These pressure systems are constantly moving or changing in appearance. Lows may deepen or till and highs may build or weaken. Most systems move in a general west to east direction.
If we put down on a map, beside the barometer readings and wind arrows, the state of the weather, we shall see that, in the high pressure areas, it is usually fine and clear, probably cooler, while in the low pressure areas, it is generally rainy and cloudy on the east side, and probably fine on the west side. The weather, in fact, is connected with the shape of the isobars.
LOW PRESSURE AREAS
Areas of low pressure are called cyclones, depressions or simply lows. A low is a region of relatively low pressure with the lowest pressure at the center.
A low may cover a small region such as a county, or it may extend across half a continent. Some are much deeper than others are. A tornado, for example, is a very deep, small but concentrated, low. A deep low is one where the barometer is very low in the center and the isobars are rather close together. A shallow depression is low in the center, but not much lower than the surrounding areas.
Depressions seldom stay long in one place but generally move in an easterly direction. Their average rate of movement is 500 miles a day in summer (about 800 kilometers), and 700 miles a day in winter (about 1100 kilometers). Their drift is generally to the northeast or southeast. Only rarely is there an exception to this pattern of easterly drift of low-pressure areas.
This is a smaller disturbance of a cyclonic nature which forms within the area dominated by the main depression. The secondary center revolves around the main center in an anti-clockwise direction. Secondaries are frequently associated with thunderstorms in summer and gales or heavy precipitation in winter.
TROUGH OF LOW PRESSURE
This is an elongated U-shaped area of low pressure with higher pressure on either side, which may bring about a gradual windshift.
The term trough is also applied to the V-shape formed by the sharp bending or kinking of the isobars along a frontal surface, sometimes referred to as a V-shaped depression. Sudden windshifts may be expected, accompanied by the type of weather generally associated with fronts.
A col is a neutral region between two highs and two lows. Weather conditions are apt to be unsettled. In winter, the mixing of air of dissimilar air masses frequently produces fog. In summer, showers or thunderstorms may occur. While it is quite possible for weather conditions to be fair, generally speaking, cols may be regarded as regions of undependable weather.
HIGH PRESSURE AREAS
A high, or anti-cyclone, is an area of relatively high pressure, higher than the surrounding regions, with the highest pressure in the center and decreasing towards the outside. The accompanying weather is usually fine to fair, clear and bright, with light, moderately cool breezes, becoming cooler at night with possibly a little frost. In winter, a high very often brings clear, cold weather.
Occasionally, however, an anti-cyclone occurs in which a persistent cloud sheet develops, causing dull, gloomy weather.
The winds associated with high pressure are usually light and rather variable. They circulate in a clockwise direction around the center, blowing outwards into the lows.
Highs move much more slowly across the country than depressions and occasionally remain almost stationary for days at a time.
RIDGE OF HIGH PRESSURE
An anti-cyclone ridge is a neck or ridge of high pressure with lower pressure lying on either side. The weather in a ridge is generally fine to fair.
Pressure readings are taken at regular intervals (usually hourly) at weather stations. Weather maps are prepared four times a day at six-hour intervals. From these readings and maps, the changes in pressure can be observed and approaching weather forecast. If a low, for example, is approaching a station, the pressure will steadily fall. Once the center of the low has passed by, the pressure will begin to rise. This pattern of changing pressure is called pressure tendency.
If some of the air were removed from a room, the pressure would be reduced and the pressure outside would force air in through the doors and windows until the room was again filled with the normal amount of air. Similarly, in the case of low and high pressure areas, there is a tendency for the higher pressure air in a high to flow towards the area of lower pressure.
The speed at which this movement of air occurs depends on the pressure gradient. The pressure gradient is defined as the rate of change of pressure over a given distance measured at right angles to the isobars.
The steepness of the pressure gradient is measured by the nearness of the isobars. Where the isobars are spaced widely apart, the pressure gradient is shallow and the movement of air (wind) is slow or light. Where the isobars are very close together, the gradient is steep and the wind is strong.
The air moving from a high-pressure area to a low-pressure area does not, however, flow directly from the one to the other. It is deflected to the right in the Northern Hemisphere by a force called the coriolis force and, as a result, flows parallel to the isobars.
Anything moving above the surface of the earth will continue to move in a straight line if no force acts on it, but the earth in its rotation moves under the moving body. The moving body is, therefore, apparently deflected to the right in the Northern Hemisphere. This is known as Fennell's Law. The apparent deflecting force is called the coriolis force.
Hence, the wind does not blow straight into an area of low pressure from all sides but is deflected to the right and blows around the area of low-pressure counterclockwise in the Northern Hemisphere.
The coriolis force causes water to swirl counterclockwise in a wash basin when the plug has been removed.
The horizontal movement of air, called wind, is a factor of great importance to a pilot as he plans a flight. Upper winds encountered enroute will affect groundspeed either favorably or detrimentally and thus have a bearing on time enroute and fuel consumption. Surface winds are important in landing and take-off.
In the case of a high-pressure area, the air flows out from the area of high pressure, but is deflected to the right as in the former case. As a result, the wind blows clockwise round an area of high pressure in the Northern Hemisphere.
In either case, if you stand with your back to the wind, the low-pressure area will be on your left side. This is known as Buys Ballot's Law.
In the lower levels of the atmosphere, a third force acts on the direction and speed of the air moving from areas of high pressure to areas of low pressure. This force is surface friction. Friction between the air masses and the surface of the earth tends to slow down the movement of the air, thereby reducing the wind speed. This in turn retards the coriolis force. As a result, air tends to move across the isobars at a slight angle inwards towards the center of the lows and outwards from the centers of the highs.
The more surface friction, the greater the angle at which the flow of air is deflected from a flow parallel to the isobars. Over water, it is less than over land where the roughness of the terrain may cause a deflection of as much as 40°. Over water, the deflection is rarely more than 10°. The friction effect is greatest near the surface of the earth, but may be carried along by turbulence to heights as great as 2000 feet. Above this altitude, it is practically negligible and the winds tend to flow parallel to the isobars as a result of the coriolis force.
Centrifugal force, acting on the circulating flow of air around the high and low pressure areas, tends to increase wind speed in the high pressure areas and decrease it in the low pressure areas.
CONVERGENCE AND DIVERGENCE
Convergence. The inflow of air into an area is called convergence. It is usually accompanied by an upward movement of air to permit the excess accumulation of air to escape. Areas of convergent winds are favorable to the occurrence of precipitation in the form of thunderstorms, rain, hail or snow.
Divergence. When there is a flow of air outwards from a region, the condition is known as divergence. The outflow is compensated by a downward movement of air from aloft. Areas of divergent winds are not favorable to the occurrence of precipitation.
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The basis for this section is the Flight Training Manual by Transport Canada. However, the text was modified for US users and readers by Dr. Claudius Carnegie of the ALLSTAR website. Any questions should be directed to Dr. Carnegie at email@example.com directly.
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Updated: 12 March, 2004