Recognising cap, banner, rotor, lenticular and mammatus clouds — and what they tell you about wind, lift, turbulence and coming rain.
There are only two main types of clouds. These are stratus and cumulus.
Stratus clouds are flat, layered clouds (think of flat or stratified) caused by the slow rise of widespread areas of air. These clouds cover broad areas of the sky and make the day gray. They often are found in stable conditions and are normally caused by frontal lifting or the slowly rising air around large low pressure systems. Some low-level stratus clouds can be formed when low level turbulence mixes the air and raises it above the condensation level.
Two main types of cloud
Cumulus clouds are piled or tumbled (think of accumulated) and look like giant cotton puffs or cauliflowers floating on high. These clouds are often found in good weather and when they cover 1/4 or less of the sky they are known as fair weather cumulus. Cumulus clouds are created by individual updrafts or convection currents carrying moist air aloft.
🌥️ Two Cloud Types:
STRATUS: Layered widespread clouds with a fairly uniform base. They often appear gray since they block the sun extensively.
CUMULUS: Separate clouds that have piled or rounded tops at various levels. These clouds can be very small or of great extent when they develop into thunderstorms.
Spreading stratus clouds with a few cumuliforms on the rightCumulus clouds. Note the presence of towering clouds and dissipating cloud in the top foreground.
To provide more information we further distinguish clouds by their general altitude.
Cirrus (meaning curl in Latin) are the highest clouds and consist of wisps or streaks of ice crystals at altitudes from 18,000 to 40,000 feet (6 to 13 km) in the temperate climates. Figure below shows classic "mare's tails" cirrus forms and indicates how ice particles falling from high wind layers into a lower velocity wind layer produces the wispy shape. Understanding this allows us to recognize the wind direction at the cloud's altitude.
Cirrus Clouds
The chart below gives the general classification of clouds according to type and altitude in temperate climates:
40,000ft ~ 13 km = High Clouds: Cirrus | Cirrocumulus | Cirrostratus
18,000ft ~6 km = Medium High Clouds: Altocumulus | Altostratus Nimbostratus | Nimbocumulus
We use the prefix cirro- to refer to stratus and cumulus clouds in the upper atmosphere as well. We use the prefix alto- (Latin meaning high like the singing voice) to refer to medium high clouds. No prefix is used when speaking of clouds below 7,000 feet (2 km).
The prefix nimbo- means a cloud from which rain is falling. These clouds may look like the others in their class except they are darker. Note that nimbocumulus clouds are also commonly known as cumulonimbus clouds. We have placed them in the medium high category but in reality they can be much lower and when it is a thunderstorm we are talking about it can have a base as low as 3,000 feet (1000 m) and tops up to 75,000 feet (25 km). Stratocumulus clouds are often formed when cumulus clouds created by thermal currents reach an inversion layer which they cannot penetrate so they spread out into stratus layers. This is the overdevelopment situation and these clouds often have a somewhat lumpy bottom even though they are layer types.
Altostratus layers with altocumulus clouds along the edges. A layer of cirrostratus is seen in the upper right of the photo.
The chart below lists their characteristics as well as their abbreviation and the symbol by which they often appear on weather charts and reports.
Different cloud types and their altitudes
Different cloud types and their form
🌍 SEASONAL AND LOCATION VARIATIONS
Cloud heights and types vary with the season and latitude. Since we now know how clouds are formed we can readily figure this out. When the air is colder as it is in Polar regions or in winter, the relative humidity is higher, and the air is more nearly saturated. Cold winter air may feel dryer than summer air in the house, but outside the air is usually much nearer to its saturation point in winter. As a result any lifting creates clouds sooner and bases are lower. In addition, greater heating of the earth's surface produces much more vertical convection causing condensation and rain which tend to remove water from the atmosphere thereby raising saturation points. As a result, the clouds tend to be much higher at the equator than the poles and clouds tend to be cumulo-type at the equator and stratus-type at the poles. Likewise clouds tend to be higher and more cumulo-type in summer and more stratus-type and lower in winter. Figure below shows cloud height variation over the earth with cloud symbols indicated.
Cloud Heights and Latitude
🌫️ LESSER KNOWN CLOUDS
Cloud watching isn't as exciting as bird watching (of either type) perhaps, but there are enough different forms of clouds to provide interest to pilots, especially since they can give us further information about conditions. We'll list the various clouds and give their description as well as what they portend.
🌁 FOG
This cloud form is well-known and needs no description. It is found when warm, moist air from sea moves over the land (advection fog), or when the land radiates heat at night to cool a moist layer lying above it (radiation fog).
Here are a few old-timey sayings that tend to be true about fog:
A summer fog for fair, a winter fog for rain,
A fact known everywhere, in valley and on plain.
Often a fog burning off in the morning indicates a good thermal day.
When fog goes up, the rain is o'er,
When fog comes down, 'twill rain some more.
Evening fog will not burn soon,
Morning fog will burn 'fore noon.
and:
Fog that starts before the night will last beyond the morning light.
Fog at Sapa (Vietnam)
⛰️ CAP OR CREST CLOUD
A cap cloud is formed over the top of a mountain when air is lifted as a general wind strikes the mountain or when upslope breezes due to heating slip up the mountain's sides and over the top to reach condensation level.
Cap cloud grows on the unwind side and erodes on the downwind side
A cap cloud often begins as a thin wisp over the mountain in the late morning and grows until late afternoon. These clouds may readily reach thunderstorm proportions in very unstable air. The cloud bases of cap clouds often lower as the day continues (unlike cumulus clouds created by thermal currents whose bases often rises as moist air near the ground dries out) due to a continued supply of humid air, especially in seaside mountains. This lowered base can obscure the mountain and put a damper on soaring. Tropical islands with all their allure of warm breezes, warm seas and warm natives have one major drawback for pilots: they usually cloud up and rain by early afternoon.
A cap cloud is continually formed on its upwind side and continually erodes downwind. Thus it stays put over the mountain top and doesn't drift with the wind. For this reason it is not a good indicator of wind velocity although it may lean in the direction of the wind aloft. Also by carefully watching it build and erode you may be able to tell the wind direction at cloud height.
🚩 BANNER CLOUD
Another type of cloud that forms on a mountain crest is a banner cloud as shown in figure below. Here rotor air blowing upslope on the downwind side of the mountain as well as drifting snow combine to create a cloud that streams out downwind from the crest. This type of cloud usually foretells high winds at the cloud level and should serve as a warning to sport aviation enthusiasts.
Banner cloud
🌪️ ROTOR CLOUD
When winds blow across a mountain chain or ridge it may form a roll of air downwind from the mountain like a tornado on its side. This is known as a rotor and is often found beneath waves. This cloud tends to stay in the same place and indicates very serious turbulence. The cloud is formed on the upmoving side of the rotor and erodes on the downward side.
Figure show lenticular cloud, wave cloud, rotor cloud and cap cloud
〰️ WAVE OR LENTICULAR CLOUDS
These clouds are known affectionately as lennies for their cross-section in very lens. They form when the air undergoes up and down undulations (waves) caused by the air blowing over hills or mountains. Wave clouds are also more or less stationary since they grow in the upward portion of the wave and erode in the downward portion. These clouds are oriented perpendicular to the wind but do not give much information about the wind velocity (since they are stationary) other than indicate at least a 15 mph (24 km/h) wind.
Lenticular and wave cloud
🌬️ LEE SIDE CLOUDS
In very humid conditions - often when it is raining or clouds are near a mountain top in layers - small broken wisps will appear on the downwind side of the mountain. This is caused by the upslope drift due to the rotor against the mountain. Lee side clouds are indicators of the wind's direction.
🌊 BILLOW CLOUDS
Sometimes clouds that appear like long ripples in water will show up high in the sky. These are billow clouds and they are formed when one air layer (warm) moves over another with enough velocity to create close waves just like in water. What distinguishes billow clouds from wave clouds is their spacing. Wave clouds are much further apart, are often stacked one atop another and are frequently lower than billow clouds. Billow clouds move with the wind, but do not drift as fast as the wind in the upper air mass. Billow clouds often foretell a change in weather as they are frequently formed by an approaching warm front. When the billowing process produces long, close rolls of clouds they are called Roll Clouds.
How billow cloud generated
Billow cloud
💧 MAMMATA CLOUDS
These clouds often appear under the shelfs of thunderstorms and look like udders hanging below the cloud. They are so named because they are someone's idea of mammaries. Mammata clouds indicate slight downdrafts below the cloud as they entrain cloudy air downward before it can evaporate.
Mammatus cloud
🍄 PILEUS CLOUD
The top of a towering thunderstorm often pushes the air above it upward as it climbs rapidly to high altitudes. This pushed up air will often form a cloud that looks like a veil over the thunderstorm top. This is the pileus cloud and its real significance to us is that it indicates lift above the thermal cloud. A pileus cloud is one of the highest clouds and as such has served as a name for various sport aircraft.
We can also identify a few other types of clouds based on their peculiar shape. These are Altocumulus Lenticularis which are simply high wave clouds that show some cumulus activity due to instability created by the wave lifting. Altocumulus Castellanus are cumulus type clouds connected in rows or spread out groups with high tunel-like structures. These clouds often portend thundery weather. Fracto-Stratus or Fracto-Cumulus are simply ragged clouds of a given general type (stratus or cumulus). They may be broken apart by high winds or irregular vertical motions.
Pileus cloud
🔍 WHAT CLOU TELL US
Clouds are up there where we want to be. Because they are in the air environment they can tell us what the air is doing. From this we can often discern what the current conditions are as well as what is going to happen.
💨 WIND VELOCITY
Clouds can generally tell us the wind velocity (speed and direction) at their height. However, as we just learned, certain types do not drift with the wind (cap, banner, rotor and wave clouds). Furthermore, stratus clouds will not demonstrate any drift if they are so undifferentiated or widespread that we cannot see them move. Another problem arises when cumulus type clouds are growing so rapidly that even their upwind edges appear to move outward. Finally, cumulus clouds formed on thermals arriving at altitude with a slow horizontal velocity acquired below may actually drift slower than the surrounding air due to the inertia of the thermal air (their mass can measure into the thousands of tons).
Given all the above exceptions, we still can get a very good idea of the wind velocity by watching the cloud drift. The best way to do this is to stand next to a building or tree and compare the cloud's position as it moves in time. Figure below shows a cumulus cloud in various wind velocities. If you are flying, there is no way of separating the cloud's drift from your own, but you can still observe the cloud drift and hence the wind velocity by watching the movement of the cloud shadows along the ground.
When two or more layers of clouds exist there is often a parallax problem whereby their relative motion makes the high clouds look like they are moving backwards or drifting slower than they really are. Here the upper level clouds appear to be moving slower because they are much further from us. The way to overcome this problem is to use a building or tree to hold your eye in one place as mentioned above.
🌦️ WEATHER INDICATIONS
Clouds can give us an idea of what weather to expect. Lower clouds give us hints of what's in store several minutes to hours in the future, while higher clouds can predict what's happening in hours or days ahead. Every cloud has a message of some sort and it's not always about the wind.
Cumulus clouds that flatten out into a layer indicate that the air has stopped rising upon reaching a temperature inversion (warmer air). Such an inversion is often associated with an approaching high pressure system. This indicates clear weather ahead for the air sinks slowly in a high pressure system which clears out all clouds except isolated cumulus created by thermals able to rise through the sinking air.
Bands of clouds are frequently seen in the sky. They can be anything from different thicknesses of stratus layers to a line of towering cumulus to cloud streets. The latter is dealt with in Chapter X. A line of towering cumulus often preceded a quickly advancing cold front with all the attendant changes brought by the advancing cold mass. Great turbulence and a wind direction change should be expected.
High bands of clouds are often good indicators of future weather. Long bands of high cirrus associated with the jet stream parallel this high river of wind (see Chapter V) and often foretell the future direction of the surface wind. When upper level clouds move in a direction markedly different from lower winds (say 90º or more) it generally means the wind on the surface is going to change-usually to that of the upper level. If the band of clouds in the jet stream is stationary, the weather is unlikely to change for the next twelve hours or so.
If high cloud is moving away and the sky is clearing, the system that created the cloud has probably passed and better weather is on the way. On the other hand, if a layer of stratocumulus cloud is approaching with little or no surface wind the line of the advance indicates the upper wind direction with the surface wind blowing beneath it at about 20º to the left or counterclockwise as we go down (clockwise in the southern hemisphere).
When bands of cirrus in its various forms are followed by thickening clouds and lowering clouds, there's a good chance that a warm front is on the way and will be in the area within 24 hours. The speed, strength and attendant severity of the front can be predicted by noting how fast the clouds increase and are moving.
There are other causes of cloud bands in the air such as thermal rolls along a ridge or mountain chain as well as a long area of convergence. In general, moving bands of clouds are the important signs of changing weather.
📈 SIGNS OF LIFT
Soaring pilots are always looking for lift and besides other gliders and birds climbing as well as dust devils, cumulus clouds are a glider pilot's friend. Of course, the cumulus cloud must be of the lower variety or they won't be based on ground thermals and are thus not readily usable. Even when cumulus clouds are thousands of feet above a pilot it often pays to move under them for thermals tend to feed in multiples and the whole extent of the lift can be rather spread out and reach to the ground.
Clouds based on wave lift and convergence lift are also good indicators. I have witnessed several competition pilots cross a five mile valley to get under a flat, long convergence cloud. They were rewarded with a flight in solid lift that continued for tens of miles.
When extending oneself by gliding long distances to find lift under a cloud it is obviously important to know what type of cloud it is and how active it is. In general, less active clouds tend to be flatter although they also tend to have less sink around them. In Chapter IX we explore the nature of thermal clouds in more detail.
🌀 SIGNS OF TURBULENCE
Turbulence is a mixing of the air. Because a cloud is borne on the air it can often indicate what amount of turbulence we should expect. Figure below shows various clouds in turbulent conditions.
Cloud shape and how far it is stretched reveal the wind speed aloft.
Cumulus clouds indicate turbulence caused by thermals. The amount of cloud boiling in a cumulus head and how ragged it is help determine how strong the lift and how strong the wind is. Strong conditions generally relate to strong turbulence. Any time clouds are torn apart we should expect turbulence.
Other clouds such as rotor clouds and cumulonimbus thunderheads are signs of extreme turbulence. Stratus clouds in general are signs of gentle conditions, but when they are formed where two different layers of air are mixing or shearing, they can indicate turbulence.
Stratocumulus clouds caused by mixing or overturning layers should be expected to be turbulent. Watch the details and fine edges of clouds for signs of turbulence.
🌧️ RAIN SIGNS
Clouds can take on many hues, depending on how they pass the sunlight. When it is illuminated low a cloud can produce awe-inspiring sunsets. This lends us the old adage which holds up to modern science: "Red sky at night, sailor's delight; red sky in the morning, sailors take warning."
The darkness of a cloud is often dependent on how it is illuminated-when the sun is in back of it, it absorbs the light and appears darker. However, in general we can tell a cloud's load of moisture and thus how apt it is to produce rain by noting how dark it is, especially at the base. The darker a cloud becomes, the more likely we are to encounter drops from heaven.
💡 CLOUDS TELL US ...
💨 Wind velocity: Note isolated cloud drift, lean and raggedness.
🌦️ Weather: Note changing wind directions at different levels and changing types of clouds. Cloud bands often indicate changes.
📈 Lift: Note the presence of cumulus, wave or convergence clouds.
🌀 Turbulence: Note cloud types and raggedness.
🌧️ Rain: Note cloud shading and changes in shade as well as build-up in size.
🪂 FLYING IN CLOUDS
This book is not a book about how to fly. However, certain characteristics of clouds that we have learned here should be pointed out to all pilots contemplating entering their depths. When clouds form they release latent heat. This makes them more unstable which often results in greater turbulence. Taking a flight on a small airplane will demonstrate the difference in turbulence above, in or below the clouds. In the clouds is usually the roughest with below next. We should mention the fact that turbulence associated with rotor and thunderstorm clouds can tear an airplane apart.
A greater danger than turbulence in clouds is disorientation or vertigo. Because the normal visual references are not available in clouds, the eyes and sense of balance are not in agreement and total spatial disorientation can occur. The only way clouds can be safely flown on a continuous basis is by an instrument rated pilot with a minimum of a turn and bank indicator and a compass (preferably a gyro type since magnetic compasses are not accurate in turns).
Cloud signalling rain — a sign to land early.
✨ SUMMARY
Flying above a cloud with the sun at your side produces a strange shadow of you and your craft on the cloud surrounded by a bright halo tinted in rainbow colors. Pilots know this spectacular sight as a "glory." It is caused by the normal shadow process and the reflection and refraction of light by the cloud.
The glory is but one of the visual beauties that clouds offer uniquely to pilots. There are also billows and canyons to play in, light pillars and silver linings to fill our eyes. All of this and more is brought to us by clouds in the sky.
Clouds also tell us much about the nature of the sky for they are offspring of moisture originating at ground level and carried aloft by many processes. They are altered in character by the surrounding air in which they pass their life cycle. It seems that every pilot should have a more than average interest in clouds for by their nature they predict how joyful a flight will be and they can greatly alter the tenor of that flight for better or for worse.
We cannot really touch the clouds but we can learn their lessons and join them for a brief spell in the great expanse of sky.
