When currents of air with differing temperature and moisture content meet at an angle, the warm and moist air will be forced to rise over the heavier air (which always remains in the lower position).
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In addition, when air masses from different directions converge toward a centre, as is always the case in cyclonic circulation, some of the air is forced up. In both these cases of convergence, cloudiness and precipitation result.
In tropical regions, where there is no marked contrast in the temperature and humidity of the converging air masses, the lifting is almost vertical and is generally accompanied by convection.
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In such a condition, convergence provides the initial impetus to the upward movement of convectively unstable air masses and causes cumulo-nimbus clouds and heavy showers.