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Figure 1. An illustration of IFS model levels, showing
how they follow the terrain near the surface of the
Earth. Level=1 is near the top of the atmosphere
and Level=137 is near the surface of the Earth. The
left hand axes are altitude (km) and pressure (hPa),
while the right hand axis is level number.

Figure 2. An illustration of model half-levels and model layers. The pressure
on model levels is in the middle of the layers defined by the model half
levels immediately above and below. The uppermost layer is adjacent to the top of the atmosphere
(where p=0), while the lowest layer is adjacent to the surface of the Earth
(where p=sp).