 An example of bands of lenticular clouds that have formed in standing waves to the lee of a mountain range. More lenticular clouds. | ||||||||
| I've chosen lenticular clouds as an example of a process because they look less like a process than do other clouds. The reason for their apparent stability is that they stay close to the tops of the hills or mountain ranges above which they form and maintain an apparently firm shape. They may also form 'downstream' of the hills in the tops of the standing waves that form in the lee of the hills. In fact, bands of lenticular clouds in the lee of a mountain range are clear evidence that standing waves have formed in the air stream. Lenticular clouds are an everyday example of a process that looks more like a solid than a process. The process of cloud formation As with everything else, all clouds are a process. Every parcel of air holds a certain amount of water vapour and the higher the temperature the more water vapour it can hold. As air rises the pressure on it decreases so it is able to expand and cool. This means that it cannot hold as much water vapour as before. Eventually a temperature may be reached at which the parcel of air cannot hold all its water vapour, this temperature is known as the dew point and as the temperature decreases below the dew point some of the water vapour will condense into visible water droplets. That is it becomes visible as clouds, most of which drift with the wind. As time passes the clouds either vaporize again or grow so that the droplets form into larger drops and fall as rain, hail or snow. Lenticular clouds are special because, although they are formed by the same process, they stay in the same place, often for several hours at a time, and can look quite solid. Lenticular cloud – lenticularis A lenticular cloud is a cloud that is shaped like a lentil seed, lenticularis is their correct name. They form along mountain ranges and in the standing waves on the lee side of mountain ranges. They usually have very well defined lens shaped outlines and may be quite long, like a long seat cushion.
Unlike other clouds, lenticular clouds don't move. No matter how strong the wind is, lenticular clouds stay in a fixed position in relation to the mountain range or the observer. The reason for this is that the air being forced up by the mountains, or by the standing waves, cools off as it rises and expands. As with other clouds, this causes the water vapour that it contains to condense and form the cloud but, in the case of the lenticular cloud, the air, or wind if you like, keeps going and soon starts to descend again after it passes the top of the mountain or wave. This causes the air to heat up and revapourise the condensed water so the air becomes clear again.
This forms the cloud shape, but the material that is making the shape is constantly forming and disappearing as the wind flows through it.
Perhaps you've also realised by now that the hill or mountain range below the cloud is also a process. |
