Kooks - Estimating Wind Speed from Surface Analysis and Prognosis Charts

Kooks - Estimating Wind Speed from Surface Analysis and Prognosis Charts Weather

The following is taken from:

Understanding Weatherfax
Mike Harris
Sheridan House 1997
(ISBN 1-57409-031-3)

Estmating Wind Speed

Winds are caused by by differences in air pressure. The greater the difference, or, in other words, the steeper the the pressure graidient, the stronger the wind. On weather charts the pressure gradient can be found by measuring the spacing between isobars, but in relating this to wind speed other factors need to be taken into account. These are:

Latitude A given wind speed is denoted by a greater isobar spacing in the tropics than at the Poles.
Direction of isobar curvature For a given isobar spacing and latiitude, winds blowing round high pressure pressure systems are stronger than those blowing round a low (cyclostrophic effect).

How to Estimate Wind Speed

The following steps are recomended by MetService.

Estimate the isobar spacing in nautical miles and the mean latitude of the point where you wish to measure wind speed. (Use the latitude scale 1º of latitude = 60 nautical miles.)
Enter the distance in the first column of the table below under the isobar spacing used on your particular chart, then read across to find the wind speed at your particular latitude.
Correct for isobar curvature.
- Increase wind speeds around a high by 20%
- Decrease wind speeds around a low by 20% to 40% (use higher reductions for higher speed winds blowing close to the center)

Isobar spacing (nautical miles)		Speed (kt) 20º mean lat		Speed (kt) 30º mean lat		Speed (kt) 40º mean lat		Speed (kt) 50º mean lat
4 MB Interval Charts	5 MB Interval Charts	Speed (kt) 20º mean lat		Speed (kt) 30º mean lat		Speed (kt) 40º mean lat		Speed (kt) 50º mean lat
4 MB Interval Charts	5 MB Interval Charts	Speed	Gusts	Speed	Gusts	Speed	Gusts	Speed	Gusts
48	60	98	146	67	100	52	78	44	65
96	120	49	73	33	50	26	39	22	33
144	180	33	49	22	33	17	26	15	22
192	240	24	37	17	35	13	19	11	16
240	300	20	29	13	25	10	16	9	13
288	360	16	24	11	17	9	13	7	11

(For the most part you will encounter charts with 4 MB spacing and,
estimating in the area of 40N mean latitude.)

Notes on Wind Direction Surface winds blow towards the center of lows and outwards from highs, making an angle of about 20º to the direction of the the isobar. This angle is due to surface frictional effects, and is a little more over land, but diminishes to zero at 6 km.

Some Practice? For some practice try one of the charts from NWS.

01/26/1998
kcasey@capecod.net

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