<P> Many of the features of Highs may be understood in context of middle - or meso - scale and relatively enduring dynamics of a planet's atmospheric circulation . For example, massive atmospheric subsidences occur as part of the descending branches of Ferrel cells and Hadley cells . Hadley cells help form the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer . The subtropical ridge also helps form most of the world's deserts . </P> <P> On English - language weather maps, high - pressure centers are identified by the letter H. Weather maps in other languages may use different letters or symbols . </P> <P> The direction of wind flow around an atmospheric high - pressure area and a low - pressure area, as seen from above, depends on the hemisphere . High - pressure systems rotate clockwise in the northern Hemisphere; low - pressure systems rotate clockwise in the southern hemisphere . </P> <P> The scientific terms in English used to describe the weather systems generated by highs and lows were introduced in the mid-1800s, mostly by the British . The scientific theories which explain the general phenomena originated about two centuries earlier . </P>

Which direction do low and high pressure systems rotate in the northern hemisphere
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