The atmosphere is a layer of gases surrounding our planet, kept in place by its own weight under gravity. The outer fringes extend to about 10,000 km, but the bulk of the atmosphere is compressed into the first 16 km (more like 9 km at the poles). Most of our weather takes place in this lower layer of the atmosphere, known as the troposphere. The layer above the troposphere is known as the stratosphere.
The atmosphere is a fluid and its motions are governed by the effects of pressure, rotation, gravity, friction and microphysical processes such as condensation, evaporation and precipitation. To forecast the weather, we need to first understand the processes which affect the atmosphere and then quantify and simulate them in a numerical forecast model.
Atmospheric processes occur on a very wide range of spatial scales — from the smallest gust of wind to weather patterns as large as continents. In numerical models, the processes are partitioned into two categories; those that are large enough to be represented by the model directly, and those that are too small and whose effects must be represented indirectly by further equations and approximations known as parametrizations.
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