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In the subtropics, easterly winds called,Trades winds, blow. They blow from the High pressure belts to the Low Pressure Belt (subtropics to equatorial region ). So they blow from higher latitude to lower latitude and face more evaporation than condensation. So, they become moist in the eastern margins where they provide plenty of rain . But while they cross the continent to reach the western parts,all their moisture get evaporated and they become dry and hence do not provide rainfall.
So, deserts are formed in the western margins of continents in the subtropics.