High and low pressure systems are synoptic scale weather features that produce very different conditions. High pressure systems, such as the Azores high and other features of the subtropical ridge, are associated with diverging and subsiding air, clear skies, and settled conditions. Generally, the winds associated with high pressure systems decrease the closer you get to the centre of the high and the winds around high pressure tend to be lower than those around low pressure systems. Air travels outwards from the centre of the high. They rotate clockwise in the northern hemisphere and counter clockwise in the southern hemisphere.
Low pressure systems rotate counter clockwise in the northern hemisphere and clockwise in the southern hemisphere. They are associated with convergence and rising air and as such tend to produce adverse weather conditions including squalls, clouds, rain, and frontal systems. Because air flows towards the centre of a low pressure system, the conservation of angular momentum means that wind speeds intensify as the winds approach the centre of the system. Low pressure systems that form in the tropics frequently intensify into hurricanes.
Outside of the tropics, low pressure systems can rapidly intensify in a process known as bomb cyclogenesis. Generally, bomb cyclogenesis occurs when the pressure inside a low drops by over 24 hPa in a 24 hour period. Bomb cyclogenesis produces the strongest and most intense storms in the mid latitudes and are particularly dangerous for sailors because they can be difficult to forecast.
While most intense low pressure systems tend to form over the sea, large thermal low pressure systems develop over land masses during summer. These systems are the drivers of monsoon circulations such as the West Africa and Asia-Australia monsoons.
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