CLASSIFICATIONS OF CLOUDS

Clouds are classified into four families: high clouds, middle clouds, low clouds and clouds of vertical development.  As well, clouds are identified by the way in which they form.  There are two basic types: cumulus and stratus.  Cumulus clouds form in rising air currents and are evidence of unstable air conditions.  Stratus clouds form in horizontal layers and usually form as a layer of moist air is cooled below its saturation point.  Clouds from which precipitation falls are designated nimbus clouds.  The cloud heights referred to below are for the temperate regions.  In the polar regions, clouds tend to occur at lower heights and in the tropics at greater heights.

HIGH CLOUDS

The bases of high clouds range from 16,500 feet to 45,000 feet and average about 25,000 feet in the temperate regions.   They’re composed of ice crystals.

CLOUDS, PRECIPITATION AND FOG

Clouds form when the invisible water vapour that is present in the air changes into its visible form as water droplets or ice crystals.

The process by which water vapour changes into water droplets is called condensation and occurs when the relative humidity is high, when condensation nuclei are present in the air and when there is cooling of the air.

The level at which water vapour condenses and becomes visible is known as the re-condensation level. This level is in practice the base of the clouds. If the cloud forms at ground level, it is called fog rather than cloud.

Except at temperatures well below freezing, clouds are composed of very small droplets of water which collect on microscopic water absorbent particles of solid matter in the air (such as salt from evaporating sea spray, dust, and smoke particles). The abundance of these particles, called condensation nuclei, on which the droplets form, permits condensation to occur generally as soon as the air becomes saturated. If the condensation nuclei are particularly abundant, condensation may occur at less than 100% relative humidity.

Clouds which form at temperatures well below freezing are usually composed of small particles of ice known as ice crystals which form directly from water vapour through the process of sublimation. When the temperature is between freezing and about -15°C, clouds are composed largely of supercooled water droplets with some ice crystals as well.

Saturated warm air holds much more water vapour than does saturated cold air. Cooling saturated warm air will result in more water vapour condensing into visible water droplets than is the case when cooling saturated cold air. Denser, thicker cloud formations occur when condensation occurs in a warm air mass.

Clouds are formed in two ways. (1 ) Air, in which water vapour is present, is cooled to its saturation point and condensation occurs. The cooling process will occur as warm air comes in contact with a cold surface or with a surface that is cooling by radiation or as air is affected by adiabatic expansion. (2) Air, without a change in temperature taking place, may absorb additional water vapour until its saturation point is reached with the result that clouds are formed. Of these, the most common cause of cloud formation is adiabatic expansion, that is, cooling due to expansion brought about by lifting.

Stability of the air, of course, is one of the major factors which determines the strength and extent of vertical motion and therefore cloud formation. In stable air, the cloud forms in horizontal sheets of stratus cloud. In unstable air, cumulus clouds develop. The lifting process is initiated by a number of different phenomena.

OROGRAPHIC LIFT

Air blowing against a range of hills or mountains is forced upward, reaches a region of lower pressure, expands and cools. Condensation will occur when the dewpoint is reached. The type of cloud formed will depend on the moisture content and on the stability of the air. The slope and height of the terrain and the strength of the wind component that produces the upslope flow also have an effect.

If the air is dry, very little cloud will form. Stratus cloud will form if the air is moist and stable, cumulus and cumulonimbus, if the air is moist and unstable.

A long bank of cloud from which rain may fall forms on the windward side and on the upper parts of the hills or mountains. Due to the fact that the rising ground is always in the same place, orographic clouds and rain are typically persistent and usually widespread. The descending air on the leeward side of the mountains will be compressed and heated, causing dissipation of the clouds.

CONVECTION

Warm air rises. Owing to the heating of the ground by the sun, rising currents of air occur. The upward movement of air is known as convection. (The downward movement of air is known as subsidence. ) As currents of air rise due to convection, they expand. The expansion is accompanied by cooling. The cooling produces condensation' and a cumuliform cloud forms at the top of each rising column of air. The cloud will grow in height as long as the rising air within it remains warmer than the air surrounding it. The height of the cloud, however, is also dependent on the stability of the air in the mid levels of the troposphere. Convection also occurs when air moves over a surface that is warmer than itself. The air is heated by advection and convective currents develop. Warming of air by advection does not depend on daytime heating. Convection will, therefore, continue day or night so long as the airflow remains the same.

FRONTAL LIFT

When a mass of warm air is advancing on a colder mass, the warm air rises over the cold air on a long gradual slope. This slope is called a warm frontal surface. The ascent of the warm air causes it to cool, and clouds are formed, ranging from high cirrus through altostratus down to thick nimbostratus from which continuous steady rain may fall over a wide area.

When a mass of cold air is advancing on a mass of warm air. The cold air undercuts the warm air and forces the latter to rise. The slope of the advancing wedge of cold air is called a cold frontal surface. The clouds which form are heavy cumulus or cumulonimbus. Heavy rain, thunderstorms, turbulence and icing are associated with the latter.

TURBULENCE

When a strong wind blows over a rough surface, the friction between the ground and the air produces mechanical turbulence, or eddy motion. The intensity of the turbulence is dependent on the roughness of the underlying surface, the strength of the wind and the instability of the air. The eddy motion consists of irregular up and down currents. The air in the upward current cools, and if sufficient moisture is present and if the turbulence is vigorous, condensation may take place in the upper pad of the turbulent layer. The cloud layer has an undulating base which is lower in the rising eddies than in the sinking eddies. The top of the cloud marks the top of the turbulent layer and tends to be very flat. Very often an inversion exists above the turbulent layer and it is this which blocks further vertical motion. The cloud layers tend to be stratocumulus in form. Sometimes convection occurs in combination with the mechanical turbulence and then cumulus clouds will develop and will be embedded in the stratocumulus layer. If the air is very stable, the mechanical turbulence is dampened and confined to very low levels.

CONVERGENCE

When air piles up over a region as at the centre of a low pressure area, convergence is said to be occurring. The excess air is forced to rise; it expands and cools and, when the condensation level is reached, clouds form. Since all fronts lie in regions of low pressure, convergence is often a contributing factor to frontal weather.

Precipitation occurs when the water droplets (visible as a cloud) grow sufficiently in size and weight to fall due to gravity. In clouds with temperatures above freezing, vertical air currents cause the droplets to move about. As a result, they collide with other drops and gradually grow in size, as they absorb those drops with which they collide, and they gain momentum until they fall through the air as rain. A single water droplet must grow enormously in order for precipitation to take place. The average raindrop is about one million times larger than a cloud water droplet. This process is known as coalescence. Precipitation due to coalescence alone generally occurs only in warm climates.

In a stable cloud such as stratus, there is very little vertical motion, not even enough to sustain small water droplets. They frequently escape and drift slowly to the earth. This form of precipitation is called drizzle.

A second mechanism by which precipitation occurs requires that ice crystals and water droplets exist side by side in a cloud at temperatures below freezing. The ice crystals grow at the expense of the water droplets. The droplets tend to evaporate and the resulting water vapour sublimates on the ice crystals. The ice crystals grow in size and weight. They are sustained in the cloud until they grow large enough that their terminal velocity exceeds the updraft velocity in the cloud. They then fall as precipitation. If the temperature below the region of formation is above freezing, the crystals will melt, coalesces with other drops and will arrive at the earth as rain. If the temperatures are cold all the way to the ground, the ice crystals will aggregate into snowflakes. In the US and Canada, heavy rainfall usually occurs as a result of a combination of sublimation on ice crystals and coalescence.

Two facts are therefore significant. If the ice crystals are necessary for the occurrence of heavy precipitation, the cloud from which the rain is falling must have built up well above the freezing level. Since the size of a raindrop is a function of the turbulence in the parent cloud, large drops and heavy precipitation are an indication of strong vertical motion.

Steady precipitation falls from a layer of stratus cloud. A shower, a sudden heavy burst of precipitation, falls from a well developed cumulus or cumulonimbus cloud, which may be embedded in a stratus layer. Precipitation may take many forms.

DRIZZLE

Precipitation in the form of very small drops of water which appears to float is called drizzle. At temperatures at or below the freezing level, drizzle will freeze on impact with objects and is known as freezing drizzle.

RAIN

Precipitation in the form of large water droplets is called rain. Freezing rain is composed of supercooled water droplets that freeze immediately on striking an object which is itself at a temperature below freezing.

HAIL

Observations have revealed the fact that water drops, certainly in the liquid form, can exist with temperatures as low as -40°C. It is clear, then, that small drops can be supercooled a long way without freezing. Most big clouds formed as a result of an upward current of air are divisible into three well defined regions. First there is the lowest layer where the cloud particles are in the form of water drops.

Next there is a region where some of the water droplets are frozen into ice crystals (snow) but some are still liquid but supercooled. Third there is the highest region of the cloud where the water vapour sublimates into minute ice crystals. There is no sharp dividing line between the snow and supercooled water regions. For some distance the ice crystals and supercooled water drops are co-existing. When a supercooled water drop collides with an ice crystal, it at once freezes on the latter, imprisoning a little air which causes it to freeze in the form of soft ice. As it falls through the supercooled region, more soft ice is deposited on it, increasing its size. The ball of soft ice so formed then falls through the water region. Water freezes on it in the form of hard, transparent ice. Finally the ball falls out of the base of the cloud as a hailstone, a hard transparent layer of ice covering a soft, white core.

Sometimes gusts carry hailstones back up to the top of the cloud, in which case the whole process is repeated perhaps several times. In this way, very large hailstones are formed. The vertical gusts which produce very large hailstones may have speeds in excess of 85 knots. The conditions which produce hail are very similar to those in which thunderstorms originate. Hence hail is often encountered in a cumulonimbus thundercloud.

SNOW PELLETS (SOFT HAIL)

If the water region lying below the supercooled region of the cloud is not of great depth, the hailstone does not acquire the hard. transparent covering and arrives at the ground as the original soft white ice. It is then known as a snow pellet or soft hail.

SNOW

In the formation of snow, the invisible water vapour in the air sublimates directly into ice crystals, without passing through any intermediate water stage. Snow flakes are formed of an agglomeration of ice crystals and are usually of a hexagonal or star like shape. Snow grains are tiny snow crystals that have acquired a coating of rime. They fall from non-turbulent clouds.

ICE PRISMS

Ice prisms are tiny ice crystals in the form of needles. They may fall from cloud or from a cloudless sky. They exist in stable air masses and at very low temperatures.

ICE PELLETS

Ice pellets are formed by the freezing of raindrops. They are hard, transparent, globular, grains of ice about the size of raindrops. They generally rebound when striking the ground.

A thunderstorm is a weather phenomenon whose passage creates extremely serious hazards to flying. It has aptly been described as a cumulus cloud gone wild. It is always accompanied by thunder and lightning, strong vertical drafts, severe gusts and turbulence, heavy rain and sometimes hail.

The basic requirements for the formation of a thunderstorm are unstable air, some form of lifting action and a high moisture content. Since these are also the requirements for the formation of a harmless cumulus cloud, it follows that the intensity of the conditions is the key to development of a thunderstorm. These violent weather factories occur when an air mass becomes unstable to the point of violent overturning. Such unstable atmospheric conditions may be brought about when air is heated from below (convection), or forced to ascend the side of a mountain (orographic lift) or lifted up over a frontal surface (frontal lift). The resulting buoyancy causes air which is warmer than its environment to push up in the form of convection currents, like drafts up a chimney flue.

If a mass of superheated moist air rises rapidly, an equal amount of cooler air rushes down to replace it.

When these conditions lead to the development of a thunderstorm, the area in which the rising and descending currents are active is called a thunderstorm cell. A thunderstorm may be composed of a number of such cells. As a storm develops, each successive cell grows to a greater height than did the previous one.

There are three distinct stages in the life cycle of a thunderstorm. Every thunderstorm begins life as a cumulus cloud. The cloud starts growing upward, driven by the latent heat as water vapour condenses. Strong updrafts prevail throughout the cell and it rapidly builds up into a towering cumulonimbus cloud. Temperatures within the cell are higher than temperatures at the same level in the surrounding air, intensifying still more the convective currents within the cell. There is usually no precipitation from the storm at this stage of its development since the water droplets and ice crystals are being carried upwards or are kept suspended by the strong updrafts.

In its mature stage, the buildup of a towering cumulonimbus thunderstorm may reach heights as great as 60,000 feet. The updrafts may attain speeds of 6000 feet per minute. As the water droplets grow large enough to fall, they drag air down with them, starting a downdraft in the middle region of the cell that accelerates downward. The speed of the downdraft, although not as great as the updrafts, may nevertheless be as high as 2000 feet per minute. Violent turbulence is associated with the up and down drafts. The appearance of precipitation on the ground is evidence that the thunderstorm cell is in its mature stage. The mature stage lasts for about 15 to 20 minutes, although some thunderstorms have been known to last as long as an hour. Lightning, microbursts, gust front wind shear, hail and tornadoes are all phenomena associated with a thunderstorm in its mature stage. Then the cell begins to dissipate. The cool precipitation tends to cool the lower region of the cloud and the cell loses its energy. The downdraft spreads throughout the whole area of the cell with the exception of a small portion at the top where updrafts still occur. The rainfall gradually ceases. The top of the cell spreads out into the familiar anvil structure.

Individual thunderstorms are usually no more than 10 nautical miles in diameter. However, they do tend to develop in clusters of two or more. Such clusters, with individual thunderstorms in various stages of development, may cover vast areas and last for many hours, travelling great distances across the country during their life cycle. There are two main types of thunderstorm: air mass thunderstorms and frontal thunderstorms. Air mass thunderstorms usually form either singly or in clusters on hot summer days in warm moist air. They form either as a result of convection or orographic lift.

Frontal thunderstorms are associated most commonly with an advancing cold front, but do develop in warm fronts as well. They usually form a line that may extend for hundreds of miles. An advancing line of frontal thunderstorms should be avoided. Squall line thunderstorms rolling along in advance of a cold front are particularly violent.

Thunderstorms produce very complex wind patterns in their vicinity. Wind shear can be found on all sides of a thunderstorm cell, in the downdraft directly beneath the cell and especially at the gust front that can precede the actual storm by 15 nautical miles or more.

In the cumulus stage of a thunderstorm's development, there’s an inflow of air (updraft) into the base of the cloud. As the thunderstorm matures, strong downdrafts develop and the cold air rushing down out of the cloud spreads out along the surface of the ground well in advance of the thunderstorm itself undercutting the warm air in such a manner as to resemble a cold front. This is the gust front.  Turbulence within this lower level of spreading air is severe.

The severe downward rush of air and its outburst of damaging winds on or near the ground is commonly called a downburst. Downbursts can be classified as either macrobursts or microbursts. A macroburst is a large downburst with a diameter of 2 nautical miles or more when it reaches the earth’s surface and with damaging winds which last from 5 to 20 minutes. The most intense of these cause tornado like damage.

Smaller downbursts with a surface outflow diameter of less than 2 n. miles and peak winds that last less than 5 minutes are called microbursts. Wet microbursts occur in the presence of storm clouds with precipitation reaching the ground. Dry microbursts originate in moisture laden cumulus clouds. The downward flowing column of air contains precipitation (called virga) which evaporates before reaching the ground. The evaporation of the water appears to further cool the air, increasing the intensity of the microburst.

Downdrafts in microbursts can have vertical speeds as great as 6000 feet per minute. As they near the ground, these downdrafts spread out to become horizontal winds with speeds as high as 80 knots. They rapidly change direction and even reverse, causing very severe and dangerous wind shear. Because of the rapidly changing winds, both in speed and direction, an airplane encountering a microburst may lose so much lift that it cannot remain airborne. Depending upon the intensity of a thunderstorm, microbursts may occur anywhere up to 10 miles from the storm cell itself. Sometimes microbursts are concentrated into a line structure and, under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple microbursts in the same general area are common.

Lightning flashes are the visible manifestation of the discharge of electricity produced in a thunderstorm. Areas of positive and negative charge accumulate in different parts of the cloud until the difference in electrical potential reaches a critical value and the air breaks down electrically.

In an active thunderstorm, the positive charge usually collects in the upper portion of the cloud and the negative charge in the cloud base. Flashing in a single thunderstorm cell may reach a peak of five discharges in a minute. Besides the activity within the individual cell, lightning may travel from one cloud to another or from the cloud to the ground. Occasionally, it will travel from the ground to the cloud. It means a negative charge has accumulated in the ground.

Thunder is the noise which accompanies a lightning flash. It is attributed to the vibration set up by the sudden heating and expansion of the air along the path of the lightning flash.

Hail may be regarded as one of the worst hazards of thunderstorm flying. It usually occurs during the mature stage of cells having updrafts of more than average intensity. The formation of hail has been covered earlier in this section.

The barometric pressure ahead of a thunderstorm falls abruptly as the storm approaches then rises quickly when the rain comes, and returns to normal when the storm subsides. Occasionally after a storm, the pressure falls below normal. Then rises to near normal again. All this can happen in a matter of 10 to 15 minutes.