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How do Cloud Inversions Form? |
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Written by Peter Dickinson BSc
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Sunday, 01 February 2009 |
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Well first things first, what is a cloud? Water has three states, ice, liquid, and vapour. Ice and liquid water we can see, water vapour is colourless. (I know I am stating the obvious at this point, but please remember that I am used to dealing with children (and Peter B) and I do not mean to patronise). There is always water vapour in air, how much water depends on how warm it is. Warm air holds lots of water vapour and as that air cools the water turns from vapour to a liquid. That liquid forms the little drops of water called cloud droplets, and they make up visible clouds.
We all know that the air gets cooler as we get higher, about 1oC for every 160m (for our climate—see Fig. 1).
 Fig. 1.
So if for any reason a mass of air is forced to rise, it cools down, the water condenses and we get clouds. Now cloud inversions are not high in the sky. They must be caused by the air cooling down at ground level. What could be influential enough to cool down all the air at ground level? Why, the ground of course!
 Cloud Inversion
Land is also rubbish at staying warm; so at night it loses heat really quickly. As a direct consequence of this the air near the ground now gets cooled down, and we all know what happens when air is cooled. This cold land will then cool the air above it. On high pressure days descending air helps to keep a lid on it all.
By night in mountain areas katabatic or mountain winds also occur.. This creates more localised inversions in valley areas. At night dense, cold air from the mountain tops sinks down hill. Such flows of cold air get dammed up even behind even minor obstacles such as walls and lines of trees. The air may collect very locally in what we call frost hollows or in larger valleys where dense fog is often found and where an inversion of temperature is created (see Fig. 2).
 Fig. 2.
Here the cold air underlies the warmer just like when you open the door if you own a fridge freezer and in this case you can even see condensation in the form of small clouds as the air cascades across the kitchen floor.
The conditions for this to happen must be right: it must be humid enough, there can not be too much wind, and the air must cool down to a certain point. But when all this happens it is a truly magnificent sight! (Fig 3.).
 Fig. 3.
Inversion layers can also form over very large regions during winter months when warmer moist air from the Atlantic (kept warm by the North Atlantic Drift), moves over the cold land of continental Europe. In these cases the warm, moist air cools and condenses as it ‘brushes’ over the cold land. The layer closest to the ground condenses forming a layer of cloud at or near ground level. The descending air keeps a ‘lid’ on any upward movements and in winter the weaker sun stops any convection (upward movement of air) which might otherwise disrupt the inversion. P.D.All diagrams and photographic annotation by Peter Burgess OFC Members contributions:-  Jayne Hill  Helmut Hudler  Helmut Hudler  Helmut Hudler |
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