Formation of clouds and fog types weather forecast nubes wolken Molnformationer Annuvola Nuages formørker

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Formation of Fog and Clouds
Water vapour is an invisible gas, and a certain amount is always present in the atmosphere; there is a limit to the amount of water vapour which air can hold, and this limit is lower when the air is cold than when it is warm. Thus when air is cooled, the point may be reached when the amount of water vapour it holds is the maximum possible at the lower temperature; any further cooling will result in the condensation of water vapour into liquid water droplets. The steam from a kettle and the misting over of kitchen or bathroom windows are every day examples of condensation taking place in this way. During spring and summer, the sea surface round is usually much colder than the land, thus cooling by contact any warm moist air which passes over it. By gentle stirring in the wind, this cooling spreads upwards and, after a time, may be sufficient to cause condensation throughout a layer of air and produce sea fog.

Clouds are formed when condensation occurs in the atmosphere well above the ground, and in this case the cooling needed to produce it is brought about simply by moving the air upwards. Because pressure decreases with height, rising air must expand; just as energy is required to push the piston in a car cylinder, so too energy is needed to move away the surrounding air when a bubble of rising air expands. The only supply of energy readily available in the atmosphere is the heat of the air itself, which thus always cools when it ascends.

Cloud Types
When the atmosphere is in what is termed an unstable state in which temperature falls rapidly with height, air given an initial upward impetus will continue to move upwards spontaneously. This is a process of convection similar to that which heats water in a kettle; upcurrents, (the so-called thermals sought as lift fur gliders), in which air is cooled are balanced by downcurrents in which air is warmed. Clouds, looking at first like puffs of cotton wooI, may form where the air is ascending. If the instability extends to great heights, the clouds tower upwards and may produce showers or thunderstorms. The typical rounded top of a large Cumulus cloud is becoming fibrous and angular at the edges. A fully developed Cumulonimbus cloud shows an anvil shaped top; a thunderstorm is in progress and rain can be seen beneath the centre of the cloud.

When the atmosphere is stable (i.e. temperature not decreasing rapidly or even increasing with height), air will only rise upwards when forced to do so. The clouds which result in these circumstances form in layers or large patches rather than indiscrete puffs with blue sky in between. A simple example of this process is when stable air is forced upwards to rise over a hill, forming what is described as orographic cloud. If the surface air is very moist, Stratus cloud may envelop the hill top but equally, the hill induced upward motion may cause a patch or band of cloud to form well above the hill. In both cases however, it is characteristic of orographic cloud that it will appear stationary despite a wind blowing through it; this is because cloud is constantly formed in the region of ascent upwind of and over the hill, and is dispersed by warming downwind where the air is descending. Another, very common type of layer cloud arises from the turbulence generated in the lower part of the atmosphere by the friction between the air and the ground. In this zone (which may be several thousand feet thick), large eddies form exactly as they do, visibly, in water flowing alongside the bank of any stream. In the atmosphere the eddies are vertical rather than horizontal and involve a lot of upward movement of air in which cloud can form in layers, perhaps covering a large area. Convection, brographic and turbulence clouds can all be distinguished on the excellent satellite cloud photographs now regularly presented on television. These pictures also not infrequently show broad bands of thick cloud which can be many hundreds of miles lang. The association already mentioned between depressions and rising air, together with the fact that rising air is a pre-requisite for cloud formation, suggest a connection between cloud and low pressure systems; but although the satellite pictures and the accompanying weather charts often show thick cloud bands passing through or into depression centres, it is clear that the connection does not depend on the pressure value alone. To understand the formation of these important clouds it is necessary to consider the origin and characteristics of the air involved.

Clouds with Vertical Development
Fair weather cumulus have the appearance of floating cotton and have a lifetime of 5-40 minutes. Known for their flat bases and distinct outlines, fair weather cumulus exhibit only slight vertical growth, with the cloud tops designating the limit of the rising air. Given suitable conditions, however, harmless fair weather cumulus can later develop into towering cumulonimbus clouds associated with powerful thunderstorms.

Puerto Rico, 19/06/2004
Developing cumulus clouds