Higher level clouds
Cirrus and cirrostratus

middle clouds
Altocumulus Castellanus

low clouds

clouds of vertical development
Cumulus (Cu)
Towering Cumulus (TCu)
Cumulonimbus (Cb)
cloud formation
frontal lift

It is essential that the pilot has a good knowledge of meteorology and that he/she understands what weather conditions are existing. Cloud formations will give a clear indication of this.

classifications of clouds

Clouds can occur at any level of the atmosphere wherever there is sufficient moisture to allow condensation to take place. The layer of the atmosphere where almost all cloud exists is the troposphere, although the tops of some severe thunderstorms occasionally pierce the tropopause.

Because of the large range in temperatures and air movement in the troposphere, clouds vary in structure and composition (a combination of ice crystal and water). Consequently, clouds are classified into three main groups: lower, middle and high level clouds.

Higher level clouds

Higher level clouds represent the cloud in the highest levels of the troposphere. They mostly appear brilliant white because of the ice crystals at that level. They tend to develop at or just above the top part of the troposphere. Higher level clouds can vary in shape, thickness and cover.

Sunlight can be observed passing through the higher level clouds most of the time. The amount of light that penetrates depends on the density and thickness of the layers. The thickness of such clouds are therefore relatively thin.

In most cases, the direction of movement of the higher level clouds do not necessarily represent the wind direction at the ground level. In fact, the wind at upper and ground levels often differ.

There are three main types of higher level clouds: cirrus, cirrostratus and cirrocumulus.

The bases of high clouds range from 16,500 feet to 45,000 feet and average about 25,000 feet in the temperate regions.

Cirrus and cirrostratus (Cl)(Cs)

Since the characteristics of cirrus and cirrostratus are similar, they can be discussed together including any differences.

Cirrus clouds are higher level clouds that develop in filaments or patches. They are virtually brilliant white attributed to their ice crystal composition. However, they lack in contrast between the top and base. They occur in flat sheets with a low height to base ratio and are usually isolated with large breaks of sky. Cirrus also vary dramatically in 'shape' or patterns they portray but these represent the fluctuating wind flow at that level both in the horizontal and vertical direction.

Cirrostratus represent clouds that are more widespread than cirrus but containing some similar features. Like cirrus, they are brilliant white and lack in contrast. Sunlight can pass through cirrostratus but this again depends on the varying thickness of the clouds.

Both cirrus and cirrostratus clouds vary in thickness. The sun can easily be observed through both types of clouds although the intensity of light that is observed depends on the thickness of their layers. In their thickest form, cirrus and cirrostratus will allow a similar intensity of light to pass through to that of thin altostratus. They do not only develop in one complete layer. It may be difficult to observe because of the lack of contrast but these clouds can consist of several thin layers.

Cirrus and cirrostratus tend to move in the direction of the wind at that level which differ to that at the surface. The most common direction of motion of these clouds are from a westerly direction. This varies with factors such as the latitude, weather conditions and time of the year. Their apparent velocities are relatively slow as compared to lower clouds.

Both cirrus and cirrostratus can occur in conjunction with any of the other cloud types. Obviously, all the lower and middle level clouds will obscure the view of the higher level clouds, appear to move faster and appear less defined. They can only be observed above other clouds when breaks in the clouds occur. Any type of higher level clouds can develop simultaneously.

Cirrus clouds tend to develop on days with fine weather and lighter winds at the surface. cirrostratus can develop on days with light winds but normally increasing in strength. Although both cirrus and cirrostratus tend to develop in fine weather conditions, they also acts as a sign of approaching changes in the weather conditions. Such changes could include any of the various types of cold front situations, thunderstorms or developing and advancing troughs of low pressure, normally with preceding cloud masses.

Except in the latter case, cirrus and cirrostratus will typically precede any other types of clouds as part of a cloud band. In fact cirrus normally precedes cirrostratus. Nevertheless, the higher level clouds will persist until the actual change in the weather occurs. The higher clouds can develop from a few hours up to a few days before an actual change in the weather conditions occurs. They may develop during one afternoon and dissipate but redevelop the next day and so on until the actual change occurs. If the amount of moisture in the lower layers of the atmosphere increases, other lower clouds may also develop changing the appearance of the cirrus or the cirrostratus clouds as well as partially or totally hiding them from view. The same situation occurs in the case where cirrus develop ahead of thunderstorms. Cirrus normally precede cirrostratus which are then followed by the anvil of the approaching thunderstorm. In fact, cirrus and cirrostratus in this case are the remnants downwind of the weakening anvil.

Both cirrus and cirrostratus can develop and persist after a change has passed through a certain location. In this situation, cloud will decrease within a few hours up to a few days following the change. If it persists for longer periods, a jet stream cloud mass may be involved.

Another situation where cirrus and cirrostratus can be observed is when lower cloud breaks or clears during days with showers or rain. This case is far less common but can indicate a few situations. The higher clouds may be the remnants of the cloud mass that produced the actual wet weather. They can also be developing ahead of other cloud masses associated with another system, leading to the situation already discussed above. It all depends on the weather situation at that time but the observation of the movement of the higher level clouds can be critical in determining what weather may follow.

Cirrus generally does not produce precipitation except when it results from dissipating thunderstorms. Precipitation from such cirrus usually consists of larger droplets and the cloud normally dissipates and vanishes completely. cirrostratus does not produce precipitation.

Cirrus and cirrostratus can develop and persist at any time of the day despite the perception that it tends to occur during the day. This perception arises because it is much easier to observe cirrus during the day as compared to night time. The background darkness and the fact that the stars can easily be observed through cirrus and cirrostratus as thin layers allows them to camouflage from the view of the observer.


Cirrocumulus is a higher level cloud that is brilliant white but with a spotty appearance created by the many small turrets. The turrets indicate vertical turbulence within the cloud. Despite this spotty appearance, cirrocumulus contains many features associated with cirrostratus discussed above. It moves in directions similar to that of the other higher clouds.


This cloud can develop in conjunction with any other clouds as well as with cirrostratus clouds. In Sydney, cirrocumulus is not as common as the other high clouds and mainly develops during the winter times with west to south westerly air streams. The development of cirrocumulus sometimes occurs in conditions similar to those associated with the development lenticular altocumulus. cirrocumulus clouds do not produce precipitation and are normally associated with fine weather.

middle clouds

Middle level clouds are those clouds that develop in the middle layers of the atmosphere. These clouds are brighter and less fragmented in appearance due to their distance from the ground and the higher composition of ice crystals. Middle level clouds vary in thickness from relatively flat sheets of cloud to a more cumuliform appearance. In fact, the sun (and moon) can be observed through some thin middle level clouds.

Middle level clouds tend to have apparent speeds slower than the lower level clouds. (Recall the larger radius and associated arc length that the higher level clouds must undertake). They move in the direction of the wind at that level which does not necessarily be the same of that at the surface.

There are 3 basic types of middle level clouds: altocumulus, altostratus and nimbostratus. The bases of middle clouds range from 6500 feet to 23,000 feet.

Altocumulus (Ac)

As the name suggests, altocumulus refers to the middle level cloud that exhibit to some extent the features normally associated with cumulus. This includes cumuliform tops and bases that are usually relatively darker than the tops. This cloud type can be widespread or patchy depending on the conditions. It can vary in appearance from broken to smooth, and vary in thickness.

In its broken form, altocumulus can be confused with stratocumulus. To distinguish between them requires examining how defined the cloud appears, whether there other forms of middle level or upper level clouds present above the layer and the difference in brightness. Like stratocumulus, the breaks become more visible at a steeper angle of elevation.

If conditions are unstable in the middle level of the atmosphere, the air will tend to rise in currents allowing areas of cumuliform turrets to develop. In fact, altocumulus can develop from dissipating thunderstorms during the morning and then redevelop during the day if the air remains unstable. Altocumulus clouds therefore in this form indicate unstable or unsettled conditions.

Altocumulus can vary in its apparent movement (speed) depending on the wind and direction at that level. However, since altocumulus (like most other cloud types) represents an ever changing system, an observer must be careful in determining cloud motion. On some days, altocumulus continuously develop as it moves in the direction of the wind. Upstream, more altocumulus may develop giving the impression that the cloud is progressing slower than its actual speed. This process can occasionally create an illusion in terms of direction. Considering an example of altocumulus observed moving to the south east, because of development on the north and north-eastern side of the cloud band, the apparent direction may be more to the east.

Altocumulus can develop in more than one layer and also in conjunction with other cloud types. The lower layer will obscure part or all of the higher altocumulus cloud layer. This situation also applies to higher level clouds. Higher level clouds will be obscured by the altocumulus. Lower level clouds, however, will obscure part or all of the altocumulus cloud layer. In fact, it may be impossible to observe altocumulus above a full stratocumulus, stratus or lower level nimbostratus cover. If a break occurs, altocumulus can only be distinguished by its different (slower) speed and direction of movement.

Altocumulus also develop within the structure of cumulonimbus (thunderstorm producing) clouds. The appearance of altocumulus within thunderstorms vary depending on the structure, severity and the amount of moisture drawn into the thunderstorm. The altocumulus usually develops after the anvil (consisting of cirrostratus and altostratus) develops and becomes darker as the precipitation cascade approaches. However, on days where thunderstorms develop with widespread altocumulus conditions, the altocumulus obscures the thunderstorms and its development observed only through breaks in the cloud.

If altocumulus develops into thicker layers, precipitation can develop. The intensity of rainfall most often expected from altocumulus is light to moderate rainfall. If large cumulus develop amongst the rain bearing altocumulus, then heavier rainfall will develop. On days when precipitation from altocumulus becomes widespread and continuous, the cloud forms a smooth lighter-grey shaded sheet and becomes known as nimbostratus (at the middle layers).

Precipitation within altocumulus can develop rapidly at the rear even though the cloud may be moving fairly rapidly. This will obviously influence the duration of rainfall as well as the normally large cloud base. This situation often occurs before a cold front with unstable conditions. Thunderstorms can develop amongst the altocumulus band or they may develop after the cloud band clears well ahead of the actual change.

As discussed in the case of other clouds, lower clouds may be present below the altocumulus layer but not producing the rain. The observer again must consider which cloud is producing the rain to determine in which direction it is moving.

Another form of altocumulus is the lenticular type where the altocumulus appears in the form of a lens. They appear very smooth and flat, often displaying two or more layers. This occurs due to a wave effect in the air flow. This wave effect normally develops as a result of a mountain range on windy days. The wave effect forces air to rise above the condensation level and hence allows cloud to form. Due to the rise and fall effect (peaks and troughs), the cloud may only exist in areas of peaks and therefore appear patchy. The most striking feature of this cloud is that it tends to remain relatively stationary compared with the associated wind at that level. What is actually happening is that as the air begins to rise above the level of condensation, cloud forms. When the air falls below this level, dissipation occurs and the cloud disintegrates back to clear air. So long as the peak of the air wave remains stationary as compared to the ground, the cloud will develop and dissipate almost in the same position whilst the wind conditions persist.

The direction of the wind associated with lenticular altocumulus can be determined by considering the sharpest edge as the end of the cloud where the air is flowing in and the opposite end where the air is flowing out. Sometimes this will be the longest span of cloud. The most efficient method of determining the direction of the wind is by closely examining the direction that the patterns and ripples within the cloud base move. The cloud will also be moving in the direction of the wind within the cloud region.

Lenticular altocumulus is generally not associated with precipitation. The conditions associated with the development of this cloud involves more horizontal rather than vertical flow. The air masses are also more stable and drier.

Lenticular altocumulus mostly develop during the day when the atmosphere is most lively in terms of strong winds at that level. The wind conditions at the surface are often very similar to the direction of wind at the cloud level. In the case of Sydney, lenticular altocumulus tend to develop during the morning period and clear off the coast during the evening. Almost all lenticular altocumulus in Sydney develop under the influence of south westerly, westerly or north westerly air streams associated with cold fronts.

Altocumulus can also develop in the form of ripples. In this case, the altocumulus cloud appears broken but lined as a result of minor wind wave ripples. In fact it develops in conditions associated with the development of lenticular altocumulus. This type of cloud obviously does not produce precipitation.

Altocumulus can develop from the spreading out of the tops of cumulus. The spreading out occurs as the tops of the cumulus grows until it reaches an inversion layer (or stronger winds that cause divergence) situated in the middle levels of the atmosphere. Because the cumulus updraughts are not strong enough to pierce this layer, the tops begin to spread in the form similar to that of an anvil facing in the direction of the wind at that level. Occasionally, this situation may further develop into thunderstorm or thundery shower conditions.

Altocumulus Castellanus (Acc)

Altocumulus with a turreted appearance.  Instability is a characteristic.   Altocumulus castellanus may develop into cumulonimbus. (below)

Altostratus (As)

Altostratus refers to middle level cloud that appears as a flat, smooth dark grey sheet. These clouds are most often observed as large sheets rather than isolated areas. However, in the process of development, altostratus may develop in smaller filaments and rapidly develop to larger sheets. These types of clouds in certain conditions normally indicate an approaching cloud mass associated with a cold front, a trough system or a jet stream.

Altostratus can develop into a thick or thin layer. As a thin layer, the sun can be observed through the cloud. In its thinner form, the developing altostratus can sometimes be confused with approaching cirrostratus. In its thicker form, the sun can only occasionally be observed through the thinner sections if not at all. Obviously, the thicker the altostratus, the darker it becomes. When observed closely, it becomes apparent that altostratus is not just one complete layer but a composition of several thin layers.

Altostratus can produce precipitation. It will normally develop and then thicken. The precipitation is observed as relatively thick dark sections since precipitation cascades are very difficult to observe with the same colour in the background. In this situation, rain will develop as a light shower and gradually increase to showers, light rain or moderate rain. If the precipitation becomes persistent, the cloud then becomes known as nimbostratus. The duration of the precipitation is influenced by factors similar to those discussed with other types of clouds.

In certain conditions, altostratus will develop during the afternoon period and increase to cover most or all of the sky. By late afternoon, evening or during the night, precipitation will develop. This situation is the most common observation that occurs in Sydney. However, altostratus can develop at any time as well as the associated precipitation.

As discussed above, altostratus can develop in conjunction with other clouds such as cirrostratus, altocumulus and stratocumulus. Obviously, the lower clouds will obscure the view of altostratus, appear to move faster and appear less defined. Although altocumulus is a middle level cloud, it will develop below altostratus. Sometimes, altocumulus can be observed developing from dissipating altostratus. cirrostratus can often be observed above altostratus when it does not cover the sky. On days where altostratus is observed above a stratocumulus cover, it may indicate a trough with possible rain or even thunderstorms either during the afternoon or within the next few days.

Like altocumulus, altostratus also forms part of thunderstorms normally within or below the lower part of the anvil region. Of course this depends on the height of the thunderstorm anvil. Different structures of thunderstorms display various forms of altostratus. As the anvil of the thunderstorm passes overhead, the altostratus begins to appear normally with a grey base but becoming increasingly dark.

Some altostratus develop in situations similar to the development of lenticular altocumulus. Altostratus in this form develops in large sheets and has a patchy base appearance. The cloud seems to be moving rapidly but because of its development at the rear actually progresses very slowly in the direction of the wind at that level. This type of cloud does not produce any rain.


Nimbostratus can be described as a widespread light grey or white sheet of cloud that produces persistent rain or showers. Because of its light colours, nimbostratus lacks contrast and in fact is quite difficult to photograph. Being sufficiently thick to produce precipitation, the sun or moon can rarely be observed through nimbostratus. The cloud may be more than 15,000 feet thick.  It is generally associated with warm fronts.

Because of its lack of contrast, it is difficult to determine the apparent speed and direction of nimbostratus. This speed can sometimes be determined by observing the movement of a break in the cloud or observing the cloud's motion against the occasional glimpse of the sun or the moon that is relatively motionless. Another method involves the observation of approaching intermittent showers although patterns of precipitation can sometimes change dramatically.

Generally, precipitation associated with nimbostratus is long in duration. The intensity can vary from light to heavy depending on the associated conditions. Normally, light to moderate rain is associated with nimbostratus. However, the passage of strong lows and cold fronts can produce moderate to heavy precipitation. In Sydney, weather associated with flooding rains often contains thick nimbostratus layers.

As discussed in earlier cases, nimbostratus can develop or occur with most other types of clouds. Stratus and stratocumulus will often develop below nimbostratus in its middle level form and obscure the view of the whole cloud base. With approaching precipitation regions, the lower clouds may appear darker or lighter than the nimbostratus creating some contrast. This depends on the intensity of the background nimbostratus. The movement of the lower clouds do not necessarily have to be the same as the nimbostratus.

Although stratocumulus clouds can develop below nimbostratus, they can also thicken to develop into a nimbostratus layer with precipitation. This refers to nimbostratus in its lower levels of the atmosphere. It can be difficult to distinguish this from nimbostratus in the middle levels of the atmosphere. It often requires observation of the initial cloud (stratocumulus or altostratus) or the cloud that follow. Another useful method is measuring the apparent speed of the cloud if it can be observed. Of course, the lower the cloud, the less likelihood that lower clouds will be observed below the nimbostratus.

Nimbostratus can develop from altostratus if it becomes sufficiently thick to produce precipitation. In fact, increasing altostratus cloud tends to lead to nimbostratus. Generally, the altostratus will become darker and gradually rain will develop. This sometimes leads to a lighter appearance of the cloud base although the cloud still remains reasonably thick.

Lower level nimbostratus can develop below altostratus and partially or completely obscure it from view. However, if the altostratus layer develops into nimbostratus itself, the lower level nimbostratus will most probably become difficult to see especially if precipitation begins to fall.

The weather conditions that produce middle level (and sometimes lower level) nimbostratus also lead to the development of higher level clouds. Nimbostratus developing or occurring below higher level clouds will obscure most or all of it from view. The higher clouds can only be observed through breaks of the nimbostratus if and when they occur. These breaks often occur when the cloud is decreasing in intensity and conditions are beginning to clear.

low clouds

Lower level clouds consist of those clouds in the lower layers of the atmosphere. Because of the relatively low temperatures at this level of the atmosphere, lower level clouds usually reflect lower amounts of light and therefore usually exhibit low contrast. The clouds at this level also appear not as well defined. When observed closely, it is easy to observe the turbulent motions and hence the ever-changing structure.

Being closer to the ground, lower level clouds appear to move or progress faster than other clouds. The clouds generally move in the direction of the wind very similar to the direction of the wind on the ground.

The most efficient method used to recognise lower clouds is when observed in conjunction with other clouds. The lower clouds will obscure part or all the view of the upper level clouds if they pass in between the observer's line of sight. In other words, all the observer can see is the lower clouds as well as parts of the higher level clouds through breaks of the lower clouds. What is observed will vary due to the different directions and relative wind speeds associated with the different layers of clouds.

There are 3 main types of lower level clouds: cumulus, stratocumulus and stratus.

The bases of low clouds range from surface height to about 6500 feet.

Stratus (St)

Stratus is defined as low cloud that appears fragmented and thin. It can also occur in the form of a layer or sheet. The sun, moon and generally the sky can usually be observed through stratus clouds, especially at a steep angle of elevation. Stratus lacks the vertical growth of cumulus and stratocumulus, and therefore lacks the contrast. This is more evident when observed as one layer as compared to patchy stratus. Being closest to the ground, stratus clouds normally move fairly rapidly in the direction of the wind depending of course on the wind speed.

Like stratocumulus, stratus develops under several conditions or weather situations. Stratus mostly develop under the influence of wind streams where moisture condenses in the lower layers of the atmosphere. Wind changes during the summer months often lead to the development of stratus as the wind evaporates moisture from the ocean and condensing as turbulence mixes the surface air with the cooler air above. In these conditions, stratus develop in patches and gradually may become widespread forming into stratocumulus.

On days with nimbostratus and rain, stratus cloud develop simply due to the amount of moisture in the air. With light winds, stratus are normally observed in sheets. In stronger wind conditions, stratus develops in patches, similar in appearance to stratocumulus. Both the direction and appearance of stratus can change rapidly with changing weather conditions. It can clear and redevelop several times during certain conditions usually appearing when rain approaches, and clearing as the rain clears. Being the lowest cloud layer, it obscures at least partially the view of stratocumulus or other types of clouds above.

Stratus, like stratocumulus, can develop in weather conditions associated with thunderstorms and thunderstorm development. In this case, stratus is observed moving rapidly towards the storms and thickening in the region of the updraughts, especially those of severe thunderstorms. The stratus is only the visible condensed water vapour feeding into the thunderstorm. One good example of a thunderstorm illustrating this behaviour is the violent hailstorm that occurred on the 18th of March, 1990 in Sydney (This storm is not illustrated here). Earlier in the day, stratus had developed with a south to south-easterly change and was moving rapidly with the air stream. As the thunderstorms developed and approached, the stratus thickened to form stratocumulus. As the storm (which was a supercell) with the updraught region moved almost overhead, the stratocumulus cleared rapidly. The major rain band then moved through with strong winds, heavy rain and medium to large hail in some areas.

Stratus can develop in the various types of weather conditions associated with stratocumulus discussed above. However, the characteristics of stratus do not vary as much as stratocumulus and therefore they are easily distinguishable. Therefore, there is no real need to discuss further the weather conditions associated with stratus clouds.

Stratocumulus (Sc)

Stratocumulus are low clouds that generally move faster than cumulus and are not as well defined in appearance (recall the techniques of observing clouds). They tend to spread more horizontally rather than vertically. Like cumulus, the bases of stratocumulus are normally darker than the tops. However, they can vary in terms of characteristics.

Depending on the weather conditions, stratocumulus can appear like cumulus since stratocumulus can develop from cumulus. They may also appear as large flat areas of low, grey cloud. Sometimes stratocumulus appear in the form of rolling patches of cloud aligned parallel to each other. Stratocumulus can also appear in the form of broken clouds or globules. The sun, moon and generally the sky can be observed through the breaks in broken stratocumulus clouds. Of course, this depends on how large the breaks are, how high the clouds rise and the angle of elevation of the breaks with respect to the observer. This generally applies to all clouds but is more notable with clouds in broken form.

Stratocumulus mostly develop in wind streams moving in the direction of the wind similar to the direction of the wind at the surface. The friction created by the earth causes turbulence in the form of eddies. With sufficient moisture, condensation will occur in the lower layers of the atmosphere visible as clouds. The amount of stratocumulus covering the sky depends on the amount of moisture concentrated at that level of the atmosphere. The speed that the cloud moves varies according to the wind speed at that level.

Stratocumulus cloud also can develop in the form of lenticularis. The only method that can be used to distinguish between these clouds is that stratocumulus will not appear as well defined, will tend to move more quickly. Sometimes they develop below cumulus or cumulonimbus which means that it must be low cloud.

Nimbostratus (Ns)

A low layer of uniform, dark grey cloud. When it gives precipitation, it is in the form of continuous rain or snow. The cloud may be more than 15,000 feet thick.  It is generally associated with warm fronts.

Little turbulence occurs in stratus.   The low cloud bases and poor visibility make VFR operations difficult to impossible.

clouds of vertical development

The bases of this type of cloud may form as low as 1500 feet. They are composed of water droplets when the temperature is above freezing and of ice crystals and supercooled water droplets when the temperature is below freezing.

Cumulus (Cu)

Cumulus are cauliflower-shaped low level clouds with dark bases and bright tops. When observing cumulus, you are actually observing the condensation process of rising thermals or air bubbles at a certain level in the atmosphere known as the condensation level.

The air rising above this level condenses and cloud is observed. Since the height of this level is fairly constant at any particular time, then the bases of cumulus are usually flat.

The appearance of cumulus like other clouds can be illusive. If stratus formed at the same level as cumulus, the cumulus will appear different observed from different perspectives with respect to the sun's position. (If light from the sun must reflect to get to the observer, then the cloud will tend to appear brighter and display more contrast than cloud reflecting very little direct sunlight. In fact, the latter case indicates that the shadow area of the cloud is facing the observer). A similar situation may occur when observing cumulus below a much darker background such as a thunderstorm. The cumulus clouds appear as a uniform white or at least much lighter with little or no contrast. The same cumulus clouds observed away from this cloud band will appear darker, with more contrast.

With practice, an observer can easily determine the size of cumulus clouds (or any clouds in general) by considering the following factors; their apparent distances, coverage of the sky (density), their angle of elevation (how much of their base can be observed), how much overlapping occurs, and their base to height ratio. Cumulus often occurs in conjunction with other clouds and may vary in appearance. If cumulus is observed below other clouds, the shadow effect of other clouds can decrease contrast of the cumulus.

Towering Cumulus (TCu) 

Cumulus clouds that build up into high towering masses.  They are likely to develop into cumulonimbus.  Rough air will be encountered underneath this cloud. Heavy icing may occur in this cloud type. (below)

Cumulonimbus (Cb)

 Heavy masses of cumulus clouds that extend well above the freezing level.  The summits often spread out to form an anvil shaped top that is characteristic of thunderstorm. (below)

cloud formation

Generally upward motion of moist air is a prerequisite for cloud formation, downward motion dissipates it. Ascending air expands, cools adiabatically and, if sufficiently moist, some of the water vapour condenses to form cloud droplets. Fog is likely when moist air is cooled, not by expansion but by contact with a colder surface.

The diameter of the condensation nuclei is typically 0.02 microns but a relatively small number may have a diameter up to 10 microns. Maritime air contains about one billion nuclei per cubic metre, polluted city air contains many more. The diameter of a cloud droplet is typically 10 to 25 microns and the spacing between them is about 50 times diameter, perhaps one mm, with maybe 100 million droplets per cubic metre of cloud. The mass of liquid in an average density cloud approximates 0.5 gram per cubic metre.

Above the freezing level in the cloud some of the droplets will freeze if disturbed by contact with suitable freezing nuclei, or an aircraft. Freezing nuclei are mainly natural clay mineral particles, bacteria and volcanic dust, perhaps 0.1 microns in diameter. There are rarely more than one million freezing particles per cubic metre thus there are only sufficient to act as a freezing catalyst for a small fraction of the cloud droplets. Most freezing occurs at temperatures between –10 °C and –15 °C.

The balance of the droplets above freezing level remains in a supercooled liquid state, possibly down to temperatures colder than –20 °C, but eventually, at some temperature warmer than –40 °C, all droplets will freeze by self-nucleation into ice crystals, forming the high level cirrus clouds. In some cases fractured or splintered ice crystals will act as freezing nuclei. The ice crystals are usually shaped as columnar hexagons or flat plate hexagons, refer 3.5.2 below and 12.2.2.

Condensation of atmospheric moisture occurs when:

the volume of air remains constant but temperature is reduced to dewpoint, e.g. contact cooling, mixing of different layers
the volume of an air parcel is increased through adiabatic expansion
evaporation increases the vapour partial pressure beyond the saturation point
a change of both temperature and volume reduces the saturation vapour partial pressure.


Rain [RA] and drizzle [DZ]

Cloud droplets tend to fall but their terminal velocity is so low, about 0.01 metres/sec, that they are kept aloft by the vertical currents associated with the cloud construction process, but will evaporate when coming into contact with the drier air outside the cloud. Some of the droplets are larger than others and consequently their falling speed is greater. Larger droplets catch up with smaller and merge or coalesce with them eventually forming raindrops. Raindrops grow with the coalescence process reaching maximum diameters, in tropical conditions, of 4 – 7 mm before air resistance disintegrates them into smaller raindrops, which start a self perpetuating process. It takes about one million cloud droplets to form one raindrop.

The terminal velocity of a 4 mm raindrop is about 9 metres/sec. Only clouds with extensive depth, 3000 feet plus, will produce rain (rather than drizzle) but very small high clouds, generating heads, may produce trails of snow crystals which evaporate at lower levels – fall streaks or virga.

Drizzle forms by coalescence in stratiform clouds with depths possibly less than 1000 feet and with only weak vertical motion, otherwise the small ( 0.2 – 0.5 mm) drops would be unable to fall. It also requires only a short distance or a high relative humidity between the cloud base and the surface, otherwise the drops will evaporate before reaching the surface. Terminal velocity approximates 1 – 2 metres/sec.

Light drizzle [–DZ]: visibility greater than 1000 metres
Moderate drizzle [DZ]: 500 to 1000 metres
Heavy drizzle [+DZ]: less than 500 metres

Light rain showers: precipitation rate under 2.0 mm/hour
Moderate rain showers: 2.0 to 10 mm/hour
Heavy rain showers: more than 10 mm/hour

Light rain [–RA]: under 0.5 mm/hour, individual drops easily seen
Moderate rain [RA]: 0.5 to 4 mm/hour, drops not easily seen
Heavy rain [+RA]: more than 4 mm/hour, rain falls in sheets

Weather radar reports precipitation into six reflectivity levels:

light precipitation
light to moderate rain
moderate to heavy rain
heavy rain
very heavy rain, hail possible
very heavy rain and hail, large hail possible

Scotch mist is a mixture of thick cloud and heavy drizzle on rising ground, formed in conditions of weak uplift of almost saturated stable air.

Snow [SN]

At cloud temperatures colder than –10 °C where both ice and supercooled liquid water exist, the saturation vapour pressure over water is greater than that over ice. Air that is just saturated with respect to the supercooled water droplets will be supersaturated with respect to the ice crystals, resulting in vapour being deposited onto the crystal. The reduction in the amount of water vapour means that the air is no longer saturated with respect to the water droplets and, to achieve saturation equilibrium again, the water droplets begin to evaporate. Thus ice crystals grow by sublimation and water droplets lessen, i.e. in mixed cloud the ice crystals grow more rapidly than the water droplets. Snow is frozen precipitation resulting from ice crystal growth and falls in any form between small crystals and large flakes. This is known as the Bergeron-Findeison theory and probably accounts for most precipitation outside the tropics. Snow may fall to the surface or, more often, melt below the freezing level and fall as rain.

Snowflakes are built by snow crystals colliding and sticking together in clusters of several hundred – aggregation. Most aggregation occurs at temperatures just below freezing, the snow crystals tending to remain separate at colder temperatures.

Hail and other ice forms

The growing snow crystals acquire a fall velocity relative to the supercooled droplets and growth also continues by collision and coalescence with supercooled droplets forming ice pellets [PE], the process being termed accretion,or opaque riming if the freezing is instantaneous incorporating trapped air, glazing if the supercooled water freezes more slowly as a clear layer. The ice pellets in turn grow by coalescence with other pellets and further accretion and are termed hail [GR] when the diameter exceeds 5 mm. The size reached by hailstones before falling out of the cloud depends on the velocity and frequency of updraughts within the cloud. Hail is of course an hazard to aviation, particularly when it is unexpected, for example hail falling from a Cb anvil can appear to fall from a clear sky. Snow grains [SG] are very small, flattened, opaque ice grains, less than 1 mm and equivalent to drizzle. Snowflakes that, due to accretion, become opaque, rounded and brittle pellets, 2 – 5 mm diameter, are called snow pellets or graupel [GS]. Sleet is transparent ice pellets less than 5 mm diameter that bounce on impact with the ground. Sleet starts as snow, partially melting into rain on descent through a warm layer, then refreezing in a cold near-surface layer. The term is sometimes applied to a snow/rain mixture or just wet snow. Diamond dust [IC] is minute airborne ice crystals that only occur under very cold (Antarctic) conditions.

When raindrops form in cloud top temperatures warmer than –10 °C the rain falls as supercooled drops. Such freezing rain or drizzle striking a frozen surface, or an aircraft flying in OAT at or below zero, will rapidly freeze into glaze ice. Freezing rain is responsible for the ice storms of North America and northern Europe, but the formative conditions differ from the preceding.

The seeder – feeder mechanism

Any large scale air flow over mountain areas produces, by orographic effect, ice crystals in cold cloud tops. By themselves the falling crystals would cause only light drizzle at the ground. However as the crystals fall through the low level mountain top clouds they act as seed particles for raindrops formed by cloud droplet coalescence with the falling crystals, producing substantial orographic rainfall in mountain areas.

Aerial cloud seeding involves introducing freezing nuclei (silver-oxide crystals with a similar structure to ice crystals) into parts of the cloud where few naturally exist, in order to initiate the Bergeron-Findeison process.


Fog is defined as an obscurity in the surface layers of the atmosphere which is caused by a suspension of water droplets, with or without smoke particles, and which is defined by international agreement as being associated with visibility less than 1000 metres. If the visibility exceeds 1000 metres then the obscurity is mist – met. code BR.

Radiation fogs are the prevalent fogs in Australia, with occurrence peaking in winter; caused by lowering of ground temperature by re-radiation into space of absorbed solar radiation from the earth’s surface. Radiation fogs mainly occur in moist air on cloudless nights within a high pressure system, particularly after rainfall. The moist air closest to the colder surface will quickly cool to dewpoint with condensation occurring. As air is a poor conductor a light wind, 2 – 6 knots, will best facilitate the mixing of the cold air throughout the surface layer, creating fog. The fog itself becomes the radiating surface in turn, encouraging further cooling and deepening of the fog. An increase in atmospheric pollution products supplies extra condensation nuclei to enhance the formation of fog or smog.

A low level inversion forms containing the fog which may vary from scattered pools in surface depressions to a general layer 1000 feet in depth. Calm conditions will result in a very shallow fog layer or just dew or frost. Surface winds greater than 10 knots may prevent formation of the inversion, the cooled air is mixed with the warmer air above and not cooling to dewpoint. If the forecast wind at 3000 feet is 25 knots plus the low level inversion may not form and fog is unlikely. In winter radiation fog may start to form in the evening and persist to mid-day, or later if the sun is cut off by higher level cloud and/or the wind does not pick up sufficiently to break up the low level inversion.

Advection fog may occur when warm, moist air is carried over a surface which is cooler than the dewpoint of the air. Cooling and some turbulence in the lower layer lowers temperature to dewpoint and fog forms. Sea fogs drifting into coastal areas are advection fogs forming when the sea surface temperature is lower than the dewpoint but with a steady breeze to promote air mixing. Dewpoint can be reached both by temperature reduction and by increased water vapour content through evaporation. Advection fogs will form in valleys open to the sea when temperature falls in the evening combined with a sea breeze of 5 to 15 knots to force the air upslope. Thick advection fogs may be persistent in winter, particularly under a mid-level cloud layer.

Shallow evaporation fogs or steaming fogs result from the immediate condensation of water vapour that has just evaporated from the surface into near saturated air. Steaming from a sun warmed road surface after a rain shower demonstrates the process. Sea smoke or frost smoke is an evaporation fog occurring in frigid Antarctic air moving over relatively warm waters and prompting evaporation into the cold air which, in turn, quickly produces saturation.

Freezing fog is a fog composed of supercooled water droplets which freeze on contact with solid objects, e.g. parked aircraft. When near saturated air is very cold, below –24 °C at sea level to –45 °C at 50 000 feet, the addition of only a little moisture will produce saturation. Normally little evaporation takes place in very cold conditions but release of water vapour from engine exhausts, for instance, can quickly saturate calm air, even though the engine exhaust heat tends to lower the relative humidity, and will produce ice fog at the surface or contrails at altitude. If the temperature is below –40 °C ice crystals form directly on saturation. Contrails persist if relative humidity is high but evaporate quickly if low. Distrails occur when the engine exhaust heat of an aircraft flying through a thin cloud layer dissipates a trail.

Frontal fog or rain-induced fog occurs when warm rain evaporates at surface level in light wind conditions and then condenses forming fog.

orographic lift

An airstream reaching a mountain barrier is forced to rise, both at the surface and the upper levels, and cools adiabatically. If the lift and the moisture content are adequate condensation occurs at the lifting condensation level and cloud is formed on or above the barrier. Stratus is formed if the air is stable, cumulus if the air is slightly unstable. If there is instability in depth, coupled with high moisture, CB may develop. Refer 3.6 below. Solar heating of mountain ridges causes the adjacent air to be warmer than air at the same level over the valleys, thus the ridge acts as a high level heat source, increasing buoyancy and accentuating the mechanical lifting.

Orographic cloud – cap cloud – in stable conditions tends to form continuously on the windward side, clearing on the lee side. Lenticular cloud may also form a high cap above a hill when there is a layer of near saturated air aloft, orographic lifting causing condensation, descent causing evaporation. A mountain wave may form, particularly in a sandwiched stable layer resulting in the formation of a series of lenticular clouds.


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.


Synoptic scale atmospheric vertical motion is found in cyclones and anticyclones, mainly caused by air mass convergence or divergence from horizontal motion. Meteorological convergence indicates retardation in air flow with increase in air mass in a given volume due to net three dimensional inflow. Meteorological divergence, or negative convergence, indicates acceleration with decrease in air mass. Convergence is the contraction and divergence is the spreading of a field of flow.

If, for example, the front end of moving air mass layer slows down, the air in the rear will catch up – converge, and the air must move vertically to avoid local compression. If the lower boundary of the moving air mass is at surface level all the vertical movement must be upward. If the moving air mass is just below the tropopause all the vertical movement will be downward because the tropopause inhibits vertical motion. Conversely if the front end of a moving air mass layer speeds up then the flow diverges. If the air mass is at the surface then downward motion will occur above it to satisfy mass conservation principles, if the divergence is aloft then upward motion takes place.

Rising air must diverge before it reaches the tropopause and sinking air must diverge before it reaches the surface. As the surface pressure is the weight per unit area of the overlaying column of air, and even though divergences in one part of the column are largely balanced by convergences in another, the slight change in mass content (thickness) of the over-riding air changes the pressure at the surface.

The following diagrams illustrate some examples of convergence and divergence:

Note: referring to the field of flow diagrams above, the spreading apart (diffluence) and the closing together (confluence) of streamlines alone do not imply existence of divergence or convergence as there is no change in air mass if there is no cross isobar flow or vertical flow. (An isobar is a curve along which pressure is constant and is usually drawn on a constant height surface such as mean sea level.)

Divergence or convergence may be induced by a change in surface drag, for instance when an airstream crosses a coastline. An airstream being forced up by a front will also induce convergence. For convergence / divergence in upper level waves. Some divergence / convergence effects may cancel each other out e.g. deceleration associated with diverging streamlines.

Developing anti-cyclones – “highs” and high pressure ridges, are associated with converging air aloft and consequent wide area subsidence with diverging air below . This subsidence usually occurs between 20 000 and 5000 feet typically at the rate of 100 – 200 feet per hour. The subsiding air is compressed and warmed adiabatically at the DALR, or an SALR, and there is a net gain of mass within the developing high. Some of the converging air aloft rises and, if sufficiently moist, forms the cirrus cloud often associated with anti-cyclones.

As the pressure lapse rate is exponential and the DALR is linear the upper section of a block of subsiding air usually sinks for a greater distance and hence warms more than the lower section and if the bottom section also contains layer cloud the sinking air will only warm at a SALR until the cloud evaporates. Also when the lower section is nearing the surface it must diverge rather than descend and thus adiabatic warming stops. With these circumstances it is very common for a subsidence inversion to consolidate at an altitude between 3000 and 6000 feet. The weather associated with large scale subsidence is almost always dry, but in winter persistent low cloud and fog can readily form in the stagnant air due to low thermal activity below the inversion, producing ‘anti-cyclonic gloom’. In summer there may be a haze layer at the inversion level which reduces horizontal visibility at that level although the atmosphere above will be bright and clear. Aircraft climbing through the inversion layer will usually experience a wind velocity change.

Developing cyclones, “lows” or "depressions", and low pressure troughs are associated with diverging air aloft and uplift of air leading to convergence below. There is a net loss of mass within an intensifying low as the rate of vertical outflow is greater than the horizontal inflow, but if the winds continue to blow into a low for a number of days, exceeding the vertical outflow, the low will fill and disappear. The same does not happen with anti-cyclones which are much more persistent.

A trough may move with pressure falling ahead of it and rising behind it giving a system of pressure tendencies due to the motion but with no overall change in pressure, i.e. no development, no deepening and no increase in convergence.


Like CU, surface heating, may provide the initial trigger to create isolated CB within an air mass but the initial lift is more likely to be provided by orographic ascent or convergence effects.

In the formative stages of a CB the cloud may have an updraught pulse of 1000 – 2000 feet/min, the rising parcel of air reaches altitudes where it is much warmer than the surrounding air, by as much as 10 °C, and buoyancy forces accelerate the parcel aloft possibly reaching speeds of 3000 – 7000 feet/min. Precipitation particles grow with the cloud growth, the upper levels of the cloud gaining additional energy from the latent heat released from the freezing of droplets and the growth of snow crystals and hailstones. When the growth of the particles is such that they can no longer be suspended in the updraught, precipitation, and its associated drag downdraught, occurs.

If the updraught path is tilted, by wind shear or veer, rather than vertical, then the precipitation and its downdraught will fall away from the updraught rather than back down through it (consequently weakening, or stopping, the updraught) and a co-existing updraught/downdraught may become established. An organised cell system controlling its environment and lasting several hours may evolve.

Middle level dry air from outside the cloud is entrained into the downdraught of an organised cell. The downdraught is further cooled by the dry inflow air evaporating some of its water and ice crystals and tends to accelerate downwards in vertical gusts and, at the same time, maintaining the higher horizontal momentum it gained at upper levels from the higher forward speed of the storm at that height. When the cold plunging air nears the surface the downburst spreads out in all directions as a cold gust front or squall, strongest at the leading edge of the storm, weakest towards the trailing edge.

Anvils may take several forms:

Cumuliform: forms when a very strong updraught spreads rapidly and without restriction.

Incus: a severe storm attains maximum vertical development when the updraught reaches a stable layer which it is unable to break through, often the tropopause, and the cloud top spreads horizontally in all directions forming an overhanging anvil.

Back-sheared: the cloud top spreads upwind, against the high level flow and indicating a very strong updraught.

Mushroom: a rollover or lip around the underside of an overhanging anvil indicating rapid expansion.

Overshooting top: a dome-like protusion through the top of an anvil indicating a very strong updraught pulse. The overshooting top in large tropical storms has been known to develop into a 'chimney' towering maybe 10 000 feet into the stratosphere with an extensive plume cloud extending downwind from its top. Such clouds transfer moisture to the stratosphere.

Each organised cell system contains an updraught / downdraught core beneath which is the outflow region containing the rain shield and bounded by the downdraught gust front, a flanking line with a dark flat base underneath which is the inflow region of warm moist air and a spreading anvil. The CU and TCU generated by the inflow within the flanking line are the genesis of new cells. Within the core the condensation of moisture from the inflow region produces rain, hail and snow and the associated downdraught to the outflow region. When the cool air outflow exceeds and finally smothers ,or undercuts and chokes off, the inflow the storm dissipates.

High moisture content in the low level air with dry mid level air plus atmospheric instability are required to maintain CB development. The amount of precipitation from a large storm is typically 200 000 tonnes but severe storms have produced 2 million tonnes.