Royal Meteorological Society 2
THE MOST COMMON TYPE
ˇ Warm, humid air rises
to form cloud.
ˇ Rain falls and thunder heard.
ˇ Very little or no windshear.
ˇ Falling rain and downdraughts
cause the storm to decay.
ˇ Short-lived: 30 to 90 minutes.
When unsaturated air rises, it cools at 9.8ēC per km of ascent. If it cools
enough, then water vapour will condense onto particles suspended in the air
(called cloud condensation nuclei) to form cloud droplets that we can see. Air
must rise to form cloud.
The mechanism forcing air to rise may be surface heating, causing bubbles of
warm air to rise, or the meeting of two dissimilar air masses. Ascent can also
result from air being forced up and over hills or mountains. Upper-level