Using the interactive tool on this page, you should be able to apply the recipe given in order to estimate the geostrophic and surface wind directions at any point on a map of isobars.
Being able to determine wind direction from a map of isobars is a key skill from this lesson. Make sure that you do not move on from this page without this skill. You can use the interactive tool below to practice applying the recipe for determining wind direction that I described in the video in the previous section:
- Start by figuring out the geostrophic wind direction. Remember that the geostrophic wind always blows parallel to the isobars, with lower pressure on the left (in the Northern Hemisphere). Remembering that winds flow counterclockwise around lows (and clockwise around highs) in the Northern Hemisphere helps, too. Note that the direction of circulation around highs and lows is opposite in the Southern Hemisphere because of the opposite orientation of the Coriolis force (so, flow around lows is clockwise and flow around highs is counterclockwise in the Southern Hemisphere). If you're finding the wind direction for an upper-level wind (not at the surface), this is all you need to do. Remember that wind direction is always given as the direction that it is blowing from.
- Next, if you are finding a surface wind direction, you need to take the geostrophic wind (which you just determined) and turn it inward toward lower pressure so that it crosses the isobars at roughly a 30-degree angle. Finally, don't forget to express the wind direction as the direction that it is blowing from.
Are you ready to practice? Using the map below, pick a point on the map and estimate the wind direction. To check your answer, hold down the left-mouse button over the location that you chose, and the local wind vector will appear along with the wind direction expressed in degrees. The orientation of the arrow represents the local wind direction, and the length of the arrow serves as a qualitative measure of wind speed. If you'd rather see the wind depicted as it would be on the station model, simply click on the Simulated Station Model in the menu below the surface analysis. This option will also give you a more specific sense of wind speeds, but the wind speeds are merely a reference. This tool does not actually calculate the real wind speed (it's a very complex calculation).
If you can consistently "predict" the correct wind direction at any point you choose on the map, you've got the hang of it, and you're ready to move on to the next section. Up next, we'll start exploring some of the weather "consequences" of patterns of surface winds.