If you look at a sky filled with cumulus, you may notice they have flat bases, which all lie at the same level. At this height, air from ground level has cooled to the dew point.
This is common in the summer, with morning Cumulus developing into deep Cumulonimbus thunderstorm clouds in the afternoon. Near the ground, Cumulonimbus are well defined, but higher up they start to look wispy at the edges. This transition indicates that the cloud is no longer made of water droplets, but ice crystals. When gusts of wind blow water droplets outside the cloud, they rapidly evaporate in the drier environment, giving water clouds a very sharp edge.
Cloud Types
On the other hand, ice crystals carried outside the cloud do not quickly evaporate, giving a wispy appearance. Cumulonimbus are often flat-topped. Within the Cumulonimbus, warm air rises by convection. In doing so, it gradually cools until it is the same temperature as the surrounding atmosphere. At this level, the air is no longer buoyant so cannot rise further.
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Instead it spreads out, forming a characteristic anvil shape. Cirrus form very high in the atmosphere.
They are wispy, being composed entirely of ice crystals falling through the atmosphere. If Cirrus are carried horizontally by winds moving at different speeds, they take a characteristic hooked shape. Only at very high altitudes or latitudes do Cirrus produce rain at ground level. But if you notice that Cirrus begins to cover more of the sky, and gets lower and thicker, this is a good indication that a warm front is approaching.
Where the Clouds Can Go by Conrad Kain
In a warm front, a warm and a cold air mass meet. The lighter warm air is forced to rise over the cold air mass, leading to cloud formation. The lowering clouds indicate that the front is drawing near, giving a period of rain in the next 12 hours. Stratus is a low continuous cloud sheet covering the sky. Stratus forms by gently rising air, or by a mild wind bringing moist air over a cold land or sea surface.
Stratus cloud is thin, so while conditions may feel gloomy, rain is unlikely, and at most will be a light drizzle. Our final two cloud types will not help you predict the coming weather, but they do give a glimpse of the extraordinarily complicated motions of the atmosphere. Smooth, lens-shaped Lenticular clouds form as air is blown up and over a mountain range.
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Once past the mountain, the air sinks back to its previous level. As it sinks, it warms and the cloud evaporates. But it can overshoot, in which case the air mass bobs back up allowing another Lenticular cloud to form. This can lead to a string of clouds, extending some way beyond the mountain range.
The interaction of wind with mountains and other surface features is one of the many details that have to be represented in computer simulators to get accurate predictions of the weather. And lastly, my personal favourite.
A cloud is like the steam in your bathroom, or the fog you might sometimes walk through — it is a parcel of air with a whole lot of very tiny droplets of water in it, just suspended in the air. It is mainly air, because the droplets are really tiny and far apart, usually ten of them side by side would only be as wide as one human hair. The droplets are so small they do not fall, unless by chance two or more get thrown together in the cloud and combine to form a heavier drop. If they do this, and start to fall, they are quite likely to sweep up other droplets underneath them and grow bigger and bigger and fall faster and faster, until they can fall out of the cloud as a raindrop.
A cloud is formed when warm, moist air rises and cools. The air always contains some moisture think about where the water goes to when the washing on the line dries. When the sun comes out and it is warm, puddles, rivers, the sea all evaporate, and moisture from them goes into the air. Cool air can hold less moisture than warm air.
So if warm air with lots of moisture in it cools down, the moisture forms little cloud droplets in the air. Air cools as it rises. This is because higher in the atmosphere the pressure is less so the air expands and air cools as it expands. So if moist air rises in the atmosphere, some of the water vapour in rising air condenses forming the tiny cloud droplets. The shape and appearance of the cloud depends on the shape of the rising parcel of air.
If the air rises because it is being pushed up over a range of hills the cloud may look long and smooth, called a lenticular cloud. These often form over the Tararua ranges, between Wellington and the Wairarapa, and also over Canterbury where it is called the nor-west arch cloud.