We know that atmosphere is heated from below and warm air rises, but air can be prevented from rising by a condition known as inversion. An inversion refers to a situation characterised by warm air floating over cool air. One of the causes of inversion is the flow of winds across vast expanse of cold oceans. Here the moving air is cooled from the bottom instead of being heated. These winds are comparatively dry even though they cross oceans. Because of the low temperature of the surface winds the land under such an inversion can become desert. The Galapago's Islands which Darwin visited are desert because they lie under an inversion zone, even though they are at the equator. as they happen to be in the path of stable inverted air moving towards the intertropical convergence. Local inversions assume importance since pollution levels increase during such conditions. Because of low wind velocities, the air above the inversion is not able to mix with the low level air, makingit stagnant. This is common during low temperature conditions; dew, frost or fog often forms. The ground thus remains cool and the trapped pollutants are unable to escape.