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In the module on wind, it was stated that the large-scale winds are in geostrophic balance. In other words, the wind speed and direction tends to be such that the coriolis force exactly balances the horizontal pressure gradient force. With the wind blowing in the right direction parallel to the pressure contours (or height contours on a constant-pressure map), the coriolis force tries to turn the air to the right while the pressure force tries to turn the air to the left, and a state of balance (non-acceleration) results.
Here's a sample upper-air map, with winds and heights plotted.
Notice how the winds are nearly parallel to the height contours everywhere. Furthermore, notice that where the height contours are closer together, the winds are stronger. These are the characteristics of geostrophic balance.
Suppose I took the wind symbols away. Would you still be able to infer the wind speed and direction?
The wind direction should be easy. The wind should everywhere be parallel to the height contours, with low pressure to the left. (That way, the pressure gradient tries to move the air to the left, while coriolis (as always) tries to move it to the right.)
The wind speed could be computed, if I gave you the formula and a calculator. But it's rarely necessary to get actual numbers for the wind speed. It's usually adequate to know where the wind speed is large and where it is small. And it will be large where the contours are close together, and small where the contours are far apart. That's because the closer the contours, the stronger the pressure gradient. It's like contours on a topographic map: the closer together they are, the steeper the slope.
Another way of looking at it is to imagine that two adjacent height contours represent walls between which the air flows. Where the walls are far apart, the flow is weak. Where the walls are close together, the air is squeezing through a smaller space, and therefore must move much faster to prevent air from piling up behind the constriction. This is not what's really happening to the air, but it's a useful way of visualizing the pattern of winds from the pattern of height contours.
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