A condition in the atmosphere where air parcels are heated and therefore are warmer than the surrounding atmosphere resulting in unstable weather conditions such as heavy rains and hailstorms. See Atmospheric Stability & Instability
Cyclones and Anti-cyclones
Pressure on ground is the force exerted by air by the weight of air above.
Cyclones are areas of low pressure usually associated with cumulonimbus coluds and heavy thunderstorms with frequent lightnings. Lows are also associated with tropical storms. Two air masses converge and rise creating an area of LP.
These are areas of high pressure where upper air masses meet and descend. Calm and clear weather is associated with a high. High pressure systems is also responsible for the formation of some major arid regions eg Sahara desert and Kalahari. Here air from the equator rises and produce heavy down pours while the air continues and descends at 30o north and south creating areas of high pressure characterised by aridity.