Lets start with the most basic, though, important ideas first. to understand what happens with the air around us we need to understand clearly the difference between a low pressure and a high pressure system.
i) Low pressure systems
Low pressure refers to the process where warm air rises. This rising air could be caused by thermal heating, orographic uplift or cold air masses forcing warmer air to rise. When air rises the temperature of the rising air will start to decrease adiabatically (average of 0.65˚C for every 100m). If enough moisture is in the rising air it will condensate once the rising air temperature reaches dew point temperature at the condensation level. The air associated with a low pressure does not only rise, but it spirals upwards in a clockwise direction (for southern hemisphere). A low pressure also “sucks in” air from a high pressure. Thus, air always flow from a high pressure to a low pressure.
ii) High pressure systems
A high pressure system is associated with descending air. Cold upper air is more dense and heavy than warm air and will start to sink towards the surface. As it reaches the surface it causes higher pressure onto the surface. While the air is descending it spirals in an anti-clockwise motion (in the southern hemisphere). The air that “accumulates” at the surface must flow to somewhere and that is to a n area of low pressure. Due to the subsidence of air in a high pressure no clouds can form, hence the absence of rain. As the cold air descends, the air starts to heat up adiabatically and thus increases the temperature on the surface. Therefore, high pressure systems are associated with fair weather conditions.
