The best answer is Abrasion.
<span>"Because the Earth is spherical in shape, the areas between the Poles and the Equator benefit from solar energy to different extents over the year. Those areas on the Equator receive the highest levels of solar energy, and that energy declines towards the Poles. The temperatures of atmospheric masses thus differ as one moves from the Equator to the Poles." (http://assandra-sciences.blogspot.com/2010/07/how-does-shape-of-earth-affect-climate.html)
</span>
Usually when the clouds are heavy because of too much evaporation
