Cyclogenesis, or the convergence of winds in troughs with lower atmospheric pressure, causes rising air in depression weather systems.
- Depression weather systems
Depression weather systems are also known as 'lows' or areas of low pressure. They occur when the atmospheric level of pressure at sea level is lower than the pressure level in the surrounding area. The term given to the formation of these systems is 'cyclogenesis', and they form in areas which experience wind divergence. Two areas of wind divergence are formed, and create upwards motion by climbing ahead of troughs in the environment.
More relevant to the area of Pakistan, the monsoon trough experiences long areas of low atmospheric pressure as a feature of the Hadley cell circulation. The low pressure areas seen in the monsoon trough extend as far as Australia and East Asia depending on which season it is, and the resulting pressure gradients are responsible for driving the monsoon circulations.
- The effects of convectional rainfall in depression weather systems
In Pakistan, low pressure and rising air are often the cause of heavy rainfall. Convectional rainfall occurs when the ground is heated for a number of days, leading to a large amount of moisture rising to form heavy storm clouds which give way to thunderous rainstorms. Obviously, depression systems that cause rising air will aid the process of convectional rainfall. This weather is typically seen around more coastal areas, where the sea has another part to play in the complex weather system over the country.
