Apart from Venus, ours is the only planet which has clouds. Clouds are visible water vapour floating in the sky. They can be white or grey. Clouds are formed when air is cooled and some of its water vapour condense into tiny droplets at the temperature which is known as the dew point temperature. The shape, height and movements of clouds indicate the type of weather which is about to occur.
The cloudy part of the atmosphere lies below an invisible ‘lid’ called the tropopause. Clouds cannot penetrate this top that is put on the weather and about the only time when we actually can see its effect is when thunderclouds spread their anvils under it. The layer below the tropopause is called the troposphere and all weather processes of interest to us occur in the troposphere.
The clouds of the troposphere split themselves rather neatly into three height layers or decks. The highest layer is 6km to 12km above sea level. Feathery ice clouds exists in this layer. All clouds in this layer have names starting with ‘cirr-’. Three main types of clouds exist in this layer, they are the cirrus, cirrocumulus and cirrostratus. The middle layer stretches from 2km to 6km above sea level. Clouds in this layer have the prefix ‘alto-’. Clouds in this layer include altocumulus and altostratus. There are also important forms of these two basic types connected mainly with thunder. The lowest layer of clouds are below 2km above sea level. The clouds have no prefix. They include stratocumulus, nimbostratus, cumulus and stratus. Lines drawn through places having the same amount of cloud are called isonephs.
There are two major shapes of clouds. They are either cumulus or stratus, thus we can make up most of the names of the clouds. If these were the only clouds we would get very little rain because only deep clouds rain at all heavily. The two cloud types which produce most of the precipitation span all the three height decks. The dark cloud type which produce most of our continuous rain is called nimbostratus while most of our showery rain comes from eruption of cumulonimbus. The word nimbus means ‘rain-bearing’.
The following types of clouds are discussed below:
Cumulus Clouds
Cumulusis the cloud of fair weather. To be ‘fair weather Cumulus’ it must not be very deep. Visually, the base seems to be very close to the ground. This is used to distinguish a sky which will remain fair and one which might conceivably shower on you later.
Each cloud element is in the shape of a cauliflower because of the way thermals rise into the base and curl over down the sides rather than like a fountain might do. Cumulus over or from the sea is less well formed than cumulus over or from the land. Often in the latter case the bases are all at the same level and the tops are usually all about the same height as well. The wind should only be moderate or less.
When Cumulus grows deeper than the rule above suggests , it implies possible shower.
Stratus Clouds
Stratus is fog above the ground and often not very far above either. Thus it is a great hazard to hill walkers and mountain climbers and also to road users who have to venture into high places. It is a cloud particularly associated with the SW wind after the warm fronts have passed. When it forms over coastal regions, it does so overnight on the back of moisture-laden winds. Even over low ground the base may not be more than 30-60m up. Over-night fog may lift into stratus as the wind gets up in the morning.
Stratocumulus Clouds
Stratocumulus is cumulus in a layer and is a cloud of situations which are not changing appreciably. Much of the cloudiness stems from stratocumulus layers. Many old fronts are nothing more than thick banks of stratocumulus.
Lands which lie in the path of winds from the sea will often not see the sun even when the charts show that an anticyclone covers the region. In the depths of winter, extensive layers of dark, dense stratocumulus settle over the centres of static anticyclones leading to the days of ‘anticyclone gloom’ with poor air quality. At other times stratocumulus is a benign type of cloud often seen in globules or waves with chinks between the elements.
Altostratus Clouds
Altostratus is the layer cloud of the medium levels which is associated with coming rain. While altostratus itself does not rain, it immediately precedes the nimbostratus that does. It is the cloud which gives a ‘watery sun’ and is often grey and featureless, looking like inverted mud-flats. That is when it is going to rain soon. At other times it just provides extensive islands of dark cloud about the sky. Such sheets altostratus are often the ‘bones’ of old depressions and fronts but the weather associated with them has generally died out.
Sometimes it becomes unstable and produces showers which can be thundery in nature when the rest of the weather regime is ripe for it.
Altocumulus Clouds
Altocumulus consists of small globules spread in rafts and islands about the sky. It can take many forms. Altocumulus is often found where frontal clouds are dispersing or maybe where they are developing. It does not become important until is accompanied by altostratus.
Cirrus Clouds
Cirrus is feathery and diffuse around the edges because it is an ice-crystal cloud. Obviously ice clouds are going to be the highest in the sky and so the fibrous appearance of cirrus gives away its height. However the shape of cirrus elements is also unlike other any other cloud. In the top centre we see a dense ‘head’ of cirrus with a ‘tail’ sinking down towards the right. The tails are called ‘fallstreaks’ and a whole line of cirrus heads with fallstreaks combines to form a layer. To recognise cirrus clouds we need to look for the individual cirrus clouds as the layers are the most obvious which can be found in other cloud type. There is another important recognition point : because they are ice crystals, cirrus clouds are white and in the sun show no dark shadows.
Cirrostratus Clouds
Cirrostratus is a milky veil of ice-crystal cloud that spreads across the sky usually following the appearance of cirrus clouds. It is easily recognised because it is the only cloud type to produce a ring halo. A ring whose radius is about the same as that of the outstretched hand at arm’s length and is only very vaguely coloured, forms about the sun or moon.
Cirrocumulus Clouds
Cirrocumulus is a very rare cloud. They are small, white globular patches arranged in rafts. It may often be impossible to tell whether the cloudlets are actually cirrocumulus or small altocumulus. Neither cloud has an important prognostic value. They are pretty but inconsequential. Figure 8 shows the clouds at sunset.
Nimbostratus Clouds
Nimbostratus is a dark, deep rainbearing cloud of bad weather. Only if one take off from an airport in rain or climb up through the murk into the sunshine above can one realise how deep and solid it can be. Under the overcast, it is difficult to decide if the cloud is nimbostratus or other lesser layer cloud like stratus or stratocumulus. However if the cloud rain continually, it is very likely to be nimbostratus. Practically , when altostratus clouds begin to rain, it is known as nimbostratus also. Nimbostratus is the cloud of front and depression centres and it contributes most of the winter rains as well as quite a proportion of the summer ones.
Cumulonimbus Clouds
Cumulonimbus is the cloud which produces showers and thunderstorms. Each cumulonimbus cell is an overgrown cumulus but often the masses of individual cells combine together to produce what might be some nasty nimbostratus. However, the give-away is that with cumulonimbus the rain starts heavily and tends to tail away before the next deluge. With nimbostratus the rain is not truly showery in nature. It may stop and start somewhat, or some heavier patches come along mixed in with lighter rain but it is not like showers.
The thunderstorm cell is over the sea with its head pressed up against the tropopause and false fcirrus around its anvil top. Surrounding it at low level is an entourage of cumulus clouds which may screens one view of the ‘monster’ behind. Halfway down the cloud mass we see downward projecting lumps resembling cows’ udders. This is characteristic of big cumulonimbus clouds and they are called mamma. It is taken as a sign of possible tornado formation in US. In Atlantic Europe, while tornadoes occur, mammas means that a mass of cumulonimbus clouds are moving away.
Not all cumulonimbus clouds are massive. In winter and spring snow showers will occur from much smaller cumulonimbus clouds and it may then be difficult to tell that the snow which seems continuous, is not a front. At other times of the year showers can fall form cumulonimbus clouds which are much less massive.