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Sylvia's solution

End of week tutorial

How did you get on?

In this video, Sylvia demonstrates the solution to the air mass exercise in the previous Step.

You may have noticed the ‘geostrophic wind scale’ in the top left of these and other weather maps. You may remember that the closer the pressure contours are to each other, the faster the wind blows. This scale allows you to calculate the wind speed from the pressure contour separation.

Close up of wind scale

To find the wind speed at a given place, first of all, see what latitude you are at – the curved lines of latitude are labelled, at 10° intervals. Then, measure the shortest distance between the contours at that place. In the example shown below, at 60° North, the distance is shown in red.
Now, measure the same distance from the left hand side of the scale, at the appropriate latitude. Again, we’ve shown it in red in the example below. You can then use the curved lines to read the wind speed – in our example, the same length red line takes you to somewhere between the 15 and 25 knot lines, so the wind speed at the marked place is about 20 knots (1kt =0.5 m/s or metres per second, or 1.2mph or miles per hour).
Black and white weather map

The actual wind speed at the ground will always be less than the geostrophic wind speed because of friction. Friction will slow the wind more over land than over water. Rules of thumb suggest that the wind will be 2/3 of the geostrophic speed over water and 1/3 over land, but it will vary considerably.

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Come Rain or Shine: Understanding the Weather

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FutureLearn - Learning For Life

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