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On a roll now, Frank decided to see what it would take to forecast wind. He found that the computer would need to be able to calculate the horizontal pressure gradient, in both directions: along the flow and across the flow. The equation looked something like this:
The NEW WIND is equal to the OLD WIND plus a slight turn to the right from the CORIOLIS FORCE and a slight push by the PRESSURE GRADIENT FORCE, with a little bit of horizontal and vertical MIXING.
But needing to know the pressure gradient everywhere wasn't so bad. The weather computer could already calculate the pressures themselves using the temperatures. So he set up a grid, going 100 miles in all four directions, with a grid point every 20 miles.
Now the computer could compute the pressure gradient, by dividing the change in pressure between adjacent grid points by the distance between those grid points. Now he had all he needed, except...
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