Weather

Weather is an all-encompassing term used to explain all of the many and varied phenomena that can occur in the atmosphere of a planet. The term is normally taken to mean the activity of these phenomena over short periods of time, typically no more than a few days in length. Average atmospheric conditions over significantly longer periods are known as climate. Usage of the two terms often overlaps and the concepts are obviously very closely connected. Weather phenomena result from temperature differences around the globe, which arise primarily because areas closer to the tropics, around the equator, obtain more energy from the Sun than more northern and southern regions, nearer to the Earth's poles.

A secondary cause of temperature differences on the Earth is that different surface areas have differing reflectivity, and therefore absorb and radiate dissimilar amounts of the solar energy they receive. Surface temperature differences cause vertical wind currents. A hot surface heats the air above it, and the air expands and rises, lowering the air pressure and drawing colder air into its place. Rising and expanding air gives up its heat and so cools, which causes it to shrink and sink, increasing air pressure and displacing the air already below it.