Crossing the boundary: analogue universe, digital worlds

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# 6.3.1 The climate model

We know that the weather is created by the interaction of earth's atmosphere with the land, the oceans and the energy of the sun. Therefore, the key factors are air pressure, temperature, humidity, wind speed, and so on. Any model will have to identify and represent these properties only, ignoring irrelevant ones such as the current government or the size of the Meteorological Office building. After this, the familiar process of splitting up things can begin.

One successful type of atmospheric model is known as a General Circulation Model (GCM). In GCMs the earth's surface is split into a rectangular grid. Each rectangle is the base of an atmospheric column, extending from the surface to high in the atmosphere, and divided into layers which split the whole atmosphere into a network of 3-D boxes. Each box contains a number of points at which the temperature, pressure, humidity, wind speed and direction, and other features are recorded. The size of the grid, height of each column and number of layers in each column depend on the model and what it is being used for. The Goddard Institute for Space Studies Model II, for example, divides the atmosphere into 3312 columns with 9 to 31 layers in each column. The height of each column is about 80,000 feet.

Does this look familiar? It really amounts to nothing other than the sampling and quantisation that we carried out in The model divides up the atmosphere into boxes and each box into points (sampling). Then each point is mapped to a number – in this case a series of numbers (quantisation). But what is the point of building such an elaborate model? Let's leave that question for a moment and cast our net even wider.