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Rain and Cloud

In general, air temperature decreases with height – typically at about 0.5 °C per 100 meters. This known as the lapse rate.

When air near sea level becomes warmer than the air above it, it tends  to rise. As a ‘bubble’ of air rises, it moves into progressively reducing pressure, so ir expands. As it expands, it gets cooler, at a rate of about 1 °C per 100 metres, untill it reaches a level at which it is the same temperature as the air around it.

If the temperature of the surrounding air is reducing more quickly than the bubble temperature, the bubble of air will continue to rise: this produces conditions described as ‘unstable‘.

If the temperature of the surrounding ir is reducing more slowly than usual, the bubble of air will not be able to rise as far or as quickly: this produces ‘stable‘ conditions.

In some situations, the air a few hundred metres above the surface may be warmer than that at ground level: this produces an extreme form of stability known as inversion.

Precipitation

Precipitation is a generic word for drizzle, rain, hail, snow, or slet, and is likely whenever moist air is cooled, e.g.:

  • by passing over a cool surface
  • by being forced over high ground
  • by rising over cooler air (e.g. at a front)

When air is warm, ot can hold a considerabl amount of water vapour. As it cools, the water vapour condenses to form water droplets or ice crystals. These produce cloud or fog (fog is effectively just cloud which has formed at surface level).

If the air is cooled still further, the water droplets eventually merge together by the rising air and which therefore fall as rain or hail.

If the air is fairly dry, the water vapour will not condense until it reaches temperatures below freezing point. If this happens at ground level, the condensing water immediately forms ice crystals called frost. If it happens in low clouds, the ice crystals clump together to form snow.

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