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Subtropical Highs

The sun is the ultimate source of energy that drives the earth's weather. Most of the energy reaches the equatorial regions and the least energy reaches the poles, causing the tropics to warm and the poles to cool. The earth's atmosphere redistributes this heat imbalance through a complex set of atmospheric circulation patterns. The warm air at the low latitudes rises and moves toward the poles. The rising air, and the subsequent clouds and precipitation, cause the tropics to be very wet. As the air moves towards the subtropics, it descends over the oceans and creates semi-permanent circulation features called subtropical highs. In the Northern Hemisphere, these high pressure systems are located over the North Pacific and North Atlantic oceans. The North Atlantic High is generally centered over Bermuda, so it is also known as the Bermuda High. The descending air under subtropical highs warms and dries as it descends, resulting in generally sunny skies and dry weather. Cold air from the poles flows toward lower latitudes in order to complete the redistribution of the heat imbalance in the atmosphere. This cold polar air collides with warmer subtropical air in the mid-latitudes, resulting in frontal precipitation and low pressure cyclonic storm systems.

This entire system of fronts, subtropical highs, and tropical rain migrates with the seasons, moving northward during the Northern Hemisphere summer and southward during the Northern Hemisphere winter. Sometimes during the summer, the Bermuda High will extend further to the west than usual, encompassing a significant part of the southern and eastern United States. Its descending air inhibits precipitation and its anticyclonic circulation pattern deflects tropical storms and hurricanes to the south and weakens cold fronts to its north, resulting in heat waves and droughts.