• A warm front occurs where warm air meets cold air. As the warm air is lighter, it rises over the cold air in front of it. As the warm air rises slowly, it cools forming clouds, which then lead to drizzle and rain.
• A cold front occurs where a cold air mass pushes under a warm air mass in front of it, forcing the warm air to rise rapidly. This creates thick clouds and short heavy bursts of rain.
• An occluded front occurs where a cold front overtakes a warm air mass and pushes it upwards, forcing it off the ground. As the warm air mass is pushed upwards, it cools and condenses quickly leading to short bursts of heavy rainfall. An occluded front first takes on the characteristics of a warm front before producing conditions similar to a cold front.
Depressions/Cyclones A depression or cyclone is an area of low atmospheric pressure that forms from warm air rising from the ground. It is marked ‘L’ on a weather chart, which stands for low pressure or depression. Depressions bring unsettled weather conditions. Winds blow toward the low-pressure centre in an anticlockwise direction (see Fig. 19.5(b)) and bring unsettled, changeable weather such as cloud cover, precipitation and strong winds.
(a) Warm : Fig. 19.2 A warm front Cold
Warm Cold : Fig. 19.3 A cold front
Cold air
Direction of front
: Fig. 19.4 An occluded front
Warm air
Cold air
( ; Fig. 19.5 (a) A low-pressure system moving across Ireland and (b) a low pressure/cyclone
