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The Earth with its atmosphere, making their daily revolution together, could be likened to an enormous grapefruit having a skin that is thinner than rice paper. The difference in this analogy is that the 'skin' around the Earth is an invisible gas termed the 'atmosphere' and held to the Earth by gravitational force. Its upper boundary has not yet been positively defined. In meteorology, we are concerned almost entirely with the lower region of the atmosphere called the troposphere, which extends from the surface to a maximum height of about 10 miles compared with the Earth's diameter of about 6,900 miles. Because of its gaseous state, internal motions and physical effects, it is mainly responsible for all our weather (state of sky, clouds, precipitation, fog, mist and other meteorological phenomena). Dry air is composed of a mixture of gases; within about 10 miles of the Earth's surface, which is the zone in which we are interested, the principal ones are nitrogen (about 78 percent) and oxygen (about 21 percent). There are also small quantities of other gasses such as argon, carbon dioxide, helium and ozone. Finally, there is a variable amount of water vapour in the atmosphere. These gases are all, except carbon dioxide, more or less constant in proportional composition and are essential to life, but meteorological interest is centred chiefly on the amount of moisture (water vapour) in the atmosphere. The amount of water vapour present at any time is very varied because of changes in temperature and the amount of evaporation from water surfaces and in condensation and precipitation. The changing quantities of dust and salt particles in the atmosphere are also of great meteorological importance.