## Monday, January 16, 2017

### Buys Ballot Law to find wind direction from isobars

Reading wind speed and direction from the lay of the isobars on a weather map is a basic skill in marine weather. We need it because surface analysis maps only contain spot winds from observations and forecast maps only include winds greater than 34 kts.  On other parts of both maps we are left to deduce the corresponding wind from the isobars alone.

The ubiquitous use of GRIB formatted weather forecasts has dampened the motivation to learn this skill because looking at one of these forecasts you can turn on and off wind and isobars at will to see the correlation, and if that is all we used we would not need to know more. But that is poor policy to rely on these GRIBs alone; for most effective analysis and forecasting we need to look at the actual maps made by the NWS, and to read these we need this skill.  Even with GRIBs at hand, it is valuable to see if the correlation makes sense or not.

The procedures are discussed in Modern Marine Weather and we have several videos on the subject as well.

The most challenging part is usually figuring the speed of the wind, which takes either tables or a formula (Section 2.4 in our text), on top of reading latitudes and distances carefully from the map.  The wind direction should in principle be easier to determine, but we have found there are still some cases where the in-principle easy solution can be evasive.

Thus we take here an all new approach to resolving this that relies only on the Buys Ballot Law. This should work in all cases the same way.  Normally we started with the rule that wind flows (in the NH) clockwise around Highs and counterclockwise around Lows, and assumed that is all we need to figure the wind direction at any point on a map.  But when we do not know where the local Highs or Lows are located it could be distracting.

Here is the short depiction, followed by a (probably longer than needed) video showing it in action.

In the Southern Hemisphere, the wind circles the other direction so the hand are reversed... but don't even think about that now.

Here is then how you can follow up on choosing the wind direction more precisely.

(1) Plot the point you care about on the map.

(2) Through that point sketch in a new isobar that is parallel to the isobars on either side.

(3) Draw a line through the same point that crosses your new isobar at an angle of about 20° pointing toward the lower pressure.

(4) That line is marking with the wind direction. (Put an arrowhead on the end of the line on the low pressure side.)