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HUMIDITY, TEMPERATURE, and STABILITY
Of all the elements which compose the lower atmosphere, water vapor is the most variable. Although it forms but a small proportion of the total mass of air at any time, its effects from a flying point of view are of great importance. It is the only gas which can change into a liquid or a solid under ordinary atmospheric conditions and it is because of this characteristic that most of the weather develops.
The moisture in the atmosphere originates principally from evaporation from the earth's water bodies, oceans, lakes, rivers, etc., and from transpiration from the earth's vegetation. It exists in the atmosphere in two forms. In its invisible form, it is water vapor. In its visible form, it is either water droplets or ice crystals. By a process called condensation, water vapor changes into water droplets. By a process called sublimation, water vapor changes into ice crystals. In its visible form, as either water droplets or ice crystals, moisture forms clouds and fog. Further developments within a cloud may lead to precipitation.
The amount of water vapor that a given volume of air can contain is governed by its temperature. Warm air can hold more moisture than cold air. When a mass of air contains the maximum amount of water vapor it can hold at a given temperature, it is said to be saturated. If the temperature falls any lower after the air is saturated, some of the invisible water vapor will condense out in the form of visible water droplets. By this process of condensation, clouds, fog and dew are formed. If the temperature is below freezing when the condensation occurs, the water vapor changes directly into ice crystals without passing through the visible water droplet stage. This process is known as sublimation. The process is reversible. If the visible water droplets or ice crystals are heated, they will turn back into the invisible gas, water vapor. Therefore, if fog or clouds are heated, they will disappear.
Any change of state, even though there is no change of temperature, involves a heat transaction. During the process of condensation, as water vapor changes to visible water droplets while remaining at the same temperature, energy is released to the atmosphere in the form of heat. This heat is known as the latent heat of vaporization. In the reverse process, heat is absorbed when liquid evaporates to its invisible state at the same temperature. Melting and freezing involve the exchange of the latent heat of fusion. In the process of sublimation, the change directly from water vapor to ice crystals, the release of energy in the form of heat is equal to the latent heat of vaporization plus the latent heat of fusion. Proof of the energy inherent in latent heat is seen in the thunderstorm and in the hurricane. The tremendous power of these two weather phenomena is derived from the energy released in the form of the latent heat of vaporization and of fusion as the water vapor changes to water droplets and to ice crystals. These violent forms of weather customarily occur in very warm, moist air since more energy is released during condensation and sublimation at warmer temperatures than at cooler ones.
The process of condensation or sublimation cannot take place unless there are microscopic panicles present in the air on which the water vapor can condense. The atmosphere contains a vast quantity of impurities, such as fine dust from desert, smoke from industrial regions, salts from the oceans, and seeds, pollen, etc. Since these originate from the earth, they exist only in the lower layers of the atmosphere. They act as condensation nuclei on which the condensation of water vapor takes place when air is cooled below its saturation temperature (dewpoint).
SUPERCOOLED WATER DROPLETS
One would expect that visible water droplets in the air would freeze as soon as the ambient temperature drops to 0°C. In fact, liquid water droplets often persist at temperatures well below 0°C and are then referred to as supercooled.
The reason for the existence of liquid water droplets at temperatures well below freezing is complicated. Briefly, it might be said that the freezing process is initiated by the nuclei on which the water droplets form. These nuclei are of different composition and because of their varying chemical characteristics, some of them do not initiate the freezing process until quite low temperatures. The temperature at which a supercooled water droplet freezes also depends on its size. Large droplets freeze at temperatures only slightly below freezing whereas very minute droplets may remain in liquid form until the temperature nears -40°C. Below -40°C, very few, if any, supercooled droplets exist. Supercooled water droplets are often found in abundance in clouds at temperatures between 0°C and -15°C.
The temperature to which unsaturated air must be cooled at constant pressure to become saturated (without the addition or removal of any water vapor) is called the dewpoint. When the spread between the temperature and dewpoint is very small, the air can be said to be nearly saturated and a slight drop in temperature may cause condensation in the form of clouds, fog, or precipitation.
The relative humidity is the ratio of the actual water vapor present in the air to the amount which the same volume of air would hold if it were saturated (at the same atmospheric pressure and temperature). Saturated air has 100% relative humidity. Completely dry air has 0% relative humidity. When a given mass of air is heated and no new water vapor is added, the relative humidity of air decreases. If the mass of air is cooled, the relative humidity increases. If cooling continues long enough, the relative humidity will reach 100% and the air will be saturated. Hence, the smaller the spread between temperature and dewpoint, the higher will be the relative humidity. Fog or low clouds are likely to form when the temperature is within 2°C. of the dewpoint.
Absolute humidity expresses the weight of water vapor per unit volume of air. It is usually stated in grains of water vapor per cubic foot of air.
DEW AND FROST
On clear, still nights, vegetation often cools by radiation to a temperature below the dewpoint of the adjacent air. Moisture, known as dew, then collects on the leaves. Frost forms in the same way, when the dewpoint is colder than freezing. Water vapor sublimates directly as ice crystals and adheres to any object (such as a metal airplane) which has lost sufficient heat by radiation cooling to be cooler than the dewpoint. Sometimes dew forms and later freezes. Frozen dew is hard and transparent. Frost, however, is white and opaque.
The source of energy which warms the earth's surface and its atmosphere is the sun. The method by which the heat is transferred from the sun to the earth is known as solar radiation. Radiation itself is not heat. The temperature of a body is affected only if it can absorb radiation.
Some of the solar radiation that reaches earth is absorbed in the stratosphere and the ionosphere but the rest passes through the lower portions of the troposphere and is absorbed by the earth. The earth, in turn, radiates energy back into the atmosphere. This outgoing radiation is known as terrestrial radiation. On a worldwide basis, the average heat gained through incoming solar radiation is equal to the heat lost through terrestrial radiation in order to keep the earth from getting progressively hotter or cooler. However, regional and local imbalances between solar and terrestrial radiation cause temperature variations that have great significance in weather formation.
Some of the outgoing terrestrial radiation is absorbed by the lower levels of the atmosphere. The rest passes out into space. The lower levels of the atmosphere are therefore not heated directly by the sun. The sun heats the earth and the earth heats the atmosphere. This fact is of the greatest importance in an understanding of weather. The atmosphere is heated from below and not from above. The amount of solar energy received by any region varies with time of day, with seasons, with latitude and with surface topography. Temperatures can, therefore, vary widely.
Diurnal Variation. During the day, solar radiation exceeds terrestrial radiation and the surface of the earth becomes warmer. At night, solar radiation ceases but terrestrial radiation continues and cools the surface. Warming and cooling of the atmosphere occur as a result of this diurnal imbalance.
Seasonal Variation. Because the axis of the earth is tilted to the plane of its orbit, the angle at which the solar radiation strikes the each varies from season to season. The Northern Hemisphere receives more solar energy in June, July and August and is therefore warmer and receives less in December, January and February and is therefore cooler.
Latitude. The sun is more directly overhead in equatorial regions than at higher latitudes. The tropics consequently receive the most radiant energy and are warmer than the polar regions where the slanting rays of the sun deliver less energy over a given area.
Topography. Land surfaces absorb more solar radiation than do water surfaces and radiate it more readily. Land surfaces therefore warm up more rapidly during the day and cool more rapidly at night. All land surfaces do not, however, absorb radiation at a uniform rate. There is great variation in radiation absorption by varying types of land surface. Wet soil, such as is found in swamps and marshes, is almost as effective as water in suppressing temperature changes. Heavy vegetation acts as an insulation against heat transfer. The greatest temperature changes occur over arid, barren surfaces such as deserts and rocky plains.
Some of the solar radiation is reflected back out to space by the earth's surface and is not absorbed at all. Some of this reflection is due to the angle at which the radiation strikes the surface but the principal cause of reflection is the type of surface. A snow surface, for example, can reflect 90% of the radiation.
Clouds greatly affect temperature. A layer of clouds will reflect a high percentage of the incoming solar radiation back out to space, drastically reducing the amount of energy reaching the earth to warm it. On a cloudy night, the clouds absorb then outgoing terrestrial radiation and radiate a considerable pan of it back to earth, hindering the escape of heat.
HOW THE ATMOSPHERE IS HEATED
The lower layers of the atmosphere are heated by radiation, as we have learned, and to some small degree by conduction. Air is, however, a poor conductor of heat and thus conduction plays only a small role. The heat is distributed to the higher layers of the troposphere by several processes:
Convection: The air over a very warm surface becomes buoyant and rises rapidly through the atmosphere. A compensating flow of cold air descends to take its place. A vertical circulation is thereby created that distributes the heating through the upper layers.
Advection: A flow of air that moves from a cold area over a warm area will be heated in its lowest layers by the warm earth over which it is flowing. Warming of the air in this manner is known as advection heating.
Turbulence: Mechanical turbulence, which is the result of friction between the air and the ground, causes a mixing process that spreads the surface heat into the air aloft.
Compression: There are some weather systems which are favorable for the development of sinking air. This occurs in anticyclonic pressure systems or in air flowing down the side of a mountain range. As the air descends, it reaches regions of increased atmospheric pressure and is compressed and its temperature rises. This phenomenon is called subsidence.
The scales commonly used in meteorology to measure the temperature are the Celsius, Fahrenheit, and Absolute Scales such as Kelvin and Rankine. The Celsius Scale is used in Canada and Europe for all meteorological temperature readings. In the US, temperatures are given in Fahrenheit and Celsius scales.
On meteorological maps, lines joining places of equal temperature are known as isotherms.
DENSITY AND TEMPERATURE
The density of air means its mass per unit of volume. Cold air is dense because the molecules, which compose it, are moving relatively slowly and are packed closely together. Warm air is less dense because the molecules, which compose it, are moving rapidly about. Hence, they take up more space and consequently there are fewer molecules in a given volume. Since cold air is denser, it is therefore heavier and tends to sink due to the force of gravity. Warm air, being lighter, is pushed up by the denser cold air and tends to rise.
HOW THE ATMOSPHERE IS COOLED
Since the atmosphere is heated from below, the temperature usually decreases with height through the troposphere. The rate of decrease with height is called the lapse rate. The average lapse rate is about 2°C per thousand feet. This figure is based on the lapse rate of I.C.A.O. Standard Air is defined as 1.98°C per thousand feet. It is this figure which is universally used in the calibration of altimeters.
This lapse rate, however, can really be only a theoretical figure. In practice, it seldom exists since there are such wide variations in air masses and the cooling process. Sometimes, the temperature remains unchanged through several thousand feet. Sometimes, the temperature will rise with height.
The atmosphere is cooled by several processes. At night, solar radiation is cut off but terrestrial radiation continues. The temperature of the earth, therefore, gradually decreases. Air in contact with the cooling earth will in turn be cooled. This cooling process is known as radiation cooling. It rarely affects more than the lower few thousand feet of the atmosphere. Radiation cooling will be reduced if a blanket of clouds is present. The clouds absorb the terrestrial radiation and reflect it back to earth, slowing down the rate at which the earth cools. If the circulation is such that air from a warm region moves over a colder region, the air will be cooled. Cooling due to this process is known as advection cooling.
The most important cooling process in the atmosphere is cooling resulting from expansion as air is forced to rise. As the air rises, it encounters lower pressure and expands. As it expands, the temperature of the air decreases. In a rising current of air, the temperature decreases at a rate that is entirely independent of the lapse rate in the surrounding non-rising air. Such a temperature change results from the adiabatic process, the word adiabatic meaning that the temperature change takes place without adding or taking away heat from outside the parcel of air. Conversely, if air should sink, it will be compressed by the increasing pressure and its temperature will rise (adiabatic heating).
Adiabatic Lapse Rates
Saturated air cools by expansion at a different rate than does unsaturated air. The dry adiabatic lapse rate is considered to be 3°C per 1000 feet. If the air rises and cools until the temperature reaches the dewpoint, condensation will occur, since the air has now become saturated. The change of water vapor to water droplets involves a heat transaction, the latent heat of vaporization, which causes the air to cool at a slower rate than that at which unsaturated air cools. The saturated adiabatic lapse rate is about 1.5°C per 1000 feet. The saturated adiabatic lapse rate, however, shows considerable variation because it is dependent on the rate at which the water vapor is condensing and the figure of 1.5°C must be regarded as an average value only. In actuality, the saturated adiabatic lapse rate can vary from 1°C to almost 3°C.
If saturated air aloft begins to sink, it will be compressed and its temperature will rise. As it warms, it becomes unsaturated and will heat at the dry adiabatic lapse rate. Adiabatic heating of air flowing down the eastern slope of the Rocky Mountains produces the warm, dry Chinook wind. Understanding and applying the adiabatic lapse rate is important to you as a pilot. Knowing surface temperature and dewpoint, you are able to determine at what height you might expect the bases of the clouds to be. You are also able to determine at what altitude you might expect to encounter icing conditions. You are able to calculate the temperature at various altitudes.
Let us consider the following situation. The surface temperature is 15°C and the dewpoint is 5°C. The surface elevation is 1500 feet.
Question: At what height might the bases of the convective type clouds be expected to be?
Answer: Knowing that the temperature in a rising column of unsaturated air decreases 3°C per 1000 feet, it is necessary to determine the spread between the surface temperature and the dewpoint and divide that figure by 3. The spread is 10°. Ten divided by 3 is 3.3. The bases of the cloud can be expected to be at a height of approximately 3300 feet AGL, or 4800 feet ASL.
Question: At what height might icing conditions be encountered in the cloud?
Answer: In saturated rising air, the lapse rate averages about 1.5°C per 1000 feet. The base of the cloud is at 4800 feet ASL at a temperature of 5°C. The spread between the dewpoint and freezing is 5°C. Five, divided by 1.5 is 3.3. The freezing level, therefore, could be expected at about 3300 feet above the bases of the clouds, or at 8100 feet ASL.
Question: What is the temperature at 10,000 feet ASL?
Answer: The temperature at 4800 feet is 5°C. For the remaining 5200 feet, the temperature decreases 1.5°C per 1000 feet, giving a cooling of 5.2 x 1.5 or 7.8 degrees. The temperature at 10,000 feet would, therefore, be 5° less 8°, or -3°C. If must be remembered that the temperature within a rising column of air is quite different than the temperature in the surrounding non-rising air.
INVERSIONS AND ISOTHERMAL LAYERS
Normally, the temperature of the atmosphere decreases with height. However, this is not invariably the case. Sometimes, warmer air may be found at higher altitudes. Such a reversal of normal conditions is known as an inversion in that the temperature is actually increasing with height. In an isothermal layer, the temperature remains constant throughout a layer of some depth. Inversions and isothermal layers can occur on a clear, still night when the cold ground cools the air above it in the lower levels. The temperature at the top of an inversion so formed may be 15° to 20°C warmer than the temperature at the surface. If there is a wind, the turbulence will mix the air in the lower few thousand feet of the atmosphere and distribute the cooling effect. In this case, the inversion will be much weaker, or the temperature may be so uniform as to produce an isothermal layer.
Inversions may also occur as cold air, which is denser than warm air, flows into a low flying area such as a valley and becomes trapped there. Warm air, lifted above colder air over a frontal surface (see Fronts), is another cause of an inversion. The intensity of an inversion is weak during windy conditions, under a cloud cover and in maritime areas and it is strong under calm conditions and in valleys. A ground based inversion favors poor visibility by trapping fog, smoke and other obstructions in the lower levels of the atmosphere.
The normal flow of air tends to be horizontal. But disturbances may cause vertical updrafts or downdrafts to develop. Air that will resist upward or downward displacement and tends to return to its original horizontal level is said to be stable. Air, which tends to move further away when disturbed, is unstable. The vertical currents associated with an unstable condition may cause turbulence and, if intensive enough, thunder shower activity. When air rises, it expands and cools adiabatically. If a mass of rising air (cooling by expansion) is still warmer than the air surrounding it, it is unstable. If disturbed, it will tend to rise further. If a mass of rising air is cooler than the air around it, it is stable. If disturbed, it will tend to sink back to its original level.
The relationship between lapse rate and stability, therefore, depends on the steepness of the lapse rate. Unstable air is indicated by a steep lapse rate: stable air by a shallow lapse rate, an isothermal layer or an inversion. It follows, also, that any modification of the lapse rate results in modification of stability. If the lapse rate becomes steeper, the air becomes unstable. It the lapse rate becomes less steep, the air becomes more stable.
A lapse rate can be steepened by increasing the lower level temperature (as happens with daytime heating of the earth) or by decreasing the temperature aloft. It can be made shallower by decreasing the temperature in the lower levels (as happens with nighttime cooling of the earth) or by increasing the temperature aloft (as in an inversion).
Since the lapse rate is seldom uniform with height, there is considerable variation in the degree of stability or instability. In some situations, the rising currents may reach 20,000 feet or more; in other situations, the rising air will rise only a few thousand feet. It is possible for air to be stable near the surface and unstable aloft, but more commonly, it is unstable in the lower few thousand feet and stable aloft.
So far, we have considered masses, or hunks, of air, which have, for any reason, been displaced upwards. If a layer of air becomes warmer in the lower levels or cooler in the upper levels, it will have a steep temperature lapse rate. It will therefore become unstable. A layer of air which has a small temperature lapse rate will be stable. There will be little tendency for vertical currents to develop.
FLIGHT CHARACTERISTICS OF STABLE AIR
Poor low level visibility, fog may occur, stratus type cloud, steady precipitation, steady winds which can change markedly with height. Smooth flying conditions.
FLIGHT CHARACTERISTICS OF UNSTABLE AIR
Good visibility (except in precipitation) heap type cloud (cumulus or cumulonimbus), showery precipitation, gusty winds, turbulence may be moderate to severe.
There are five principal conditions that provide the lift to initiate rising currents of air.
1. Convection: The air is heated through contact with the earth's surface. The rising columns are usually local and separated by areas of sinking air. They result from unequal heating of different types of land surface and especially from the different surface temperatures of land and water areas.
2. Orographic Lift. Air moving up a sloping terrain, such as a mountainside, will continue its upward movement, especially if it is unstable.
3. Frontal Lift. When different air masses meet, warm air is forced aloft by the advancing or receding wedge of cold air.
In the past, the weatherman based his predictions of the weather mainly upon the existence and movement of high and low pressure areas and the wind and weather systems which are associated with them. Today, the whole system of weather forecasting is based upon the properties of air masses (of which pressure is only one factor), the changes which occur as an air mass moves away from its source, and the weather phenomena which can be predicted along the front where two air masses of different properties come in contact.
An air mass may be defined as a large section of the troposphere with uniform properties of temperature and moisture in the horizontal. An air mass may be several thousands of miles across.
It takes on its original properties from the surface over which it has formed. An air mass which has formed over the ice and snow surfaces of the Arctic would be cold and dry. An air mass lying over the South Pacific would be warm and moist.
An air mass which has formed over a large body of water and is therefore moist is referred to as maritime air. One which originates over a large land area and is therefore dry is referred to as continental air.
The three main sources of the air masses of North America are:
a. The Arctic Region, which extends from the north pole south to the permafrost line.
b. The Polar Region, which extends south from the permafrost line to where the mean temperature is 10°C.
c. The Tropical Region, which lies below Latitude 30°N.
The principal air masses of North America are as follows:
Continental Arctic (cA) and Continental Polar (cP): Cold dry air masses which originate over the intensely cold ice and snow
4. Mechanical Turbulence. Friction between the air and the ground disrupts the lower levels of the air into a sense of eddies. These eddies are usually confined to the lower few thousand feet of the atmosphere but may extend higher if the air is unstable and surface winds are strong.
5. Convergence. In a low-pressure area, the winds blow across the isobars into the center of the low. Air accumulates in the center of the low and the excess air is forced to use.
VAPOUR TRAILS (CONTRAILS)
The white vapor or condensation trails (contrails) you see high up in the blue in this age of jet airplanes owe their origin to two different causes.
1. EXHAUST TRAILS. When the hydrogen and carbon in aviation fuel are burned, the carbon produces a colorless gas and the hydrogen produces water vapor, both of which are invisible. The latter product of the combustion process, which comes out of the exhaust, will remain invisible as long as the humidity of the surrounding air has not reached its saturation point. As stated in the section above, warm air can contain much more invisible water vapor than can cold air before it becomes saturated. In the extreme low temperatures encountered at very high altitudes, the cold air is incapable of absorbing the excess water vapor coming out of the exhaust. The water vapor therefore condenses into a visible cloud of water droplets or ice crystals. This is known as an exhaust trail.
2. WING TIP TRAILS. As we learned in Theory of Flight (Drag), vortices, in the form of eddies rotating with a corkscrew motion, are formed off the tips of an airplane wing in flight. These rapidly rotating vortices have considerable centrifugal force acting outwards, which causes a rarefaction and therefore an expansion of the air in the middle of the vortex. Air, which expands, cools. If the vortex is strong enough and the humidity of the air high enough, this cooling will cause condensation. The white cloud-like trails which form off the wing tips are known as wing' tip trails.
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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