Formation and Structure

As previously mentioned, clouds are formed when water vapour condenses into liquid water or ice crystals. The amount of water vapour in the Earth’s atmosphere varies and tends to be greatest near source regions such as oceans, lakes and vegetation. Condensation nuclei are always present in sufficient quantity for condensation to occur, however, if they are particularly abundant, condensation can even occur at less than one hundred percent relative humidity. The stability or instability of the air will also determine the type of cloud formed. Clouds in stable air will develop into horizontal sheets and clouds in unstable air will develop vertically into cumuliform type clouds.

The extent and type of cloud and precipitation produced will depend on the amount of water vapour available, the abundance of nuclei, stability of the air and the amount of cooling that the air undergoes

Clouds typically form due to adiabatic expansion, as air expands it cools, and water vapour condenses into liquid. The major processes that result in the upward movement of air and its subsequent expansion are described below:

  • Convection: Occurs when an air mass near Earth’s surface is heated and rises upwards through the atmosphere. As it rises, it expands and cools. If sufficient moisture is present, clouds will form.
  • Frontal Lift: Air masses do not readily mix and when two air masses with different properties come into contact with one another the denser (colder) air mass will force the less dense (warmer) air mass upwards.
  • Orographic Lift: Occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it cools down adiabatically which can raise the relative humidity to 100% and create clouds.
  • Convergence: In areas of low pressure, air converges towards the centre of the low and is forced upwards. If the air is humid and unstable, clouds and even thunderstorms may develop.
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