Tuesday Supplemental Information

ONLINE WEATHER SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Tuesday, 30 March 1999

WIND OBSERVATIONS

We have learned that the wind can be of use to us -- helping power our sail boats or turn our wind generators or cooling us during the summer. But we also know that it can make us feel uncomfortable during the winter, robbing us of body heat when the wind chill equivalent temperatures drop to low values. We can also look to see how the wind changes over time, and we can then make an intelligent forecast of the weather for the next several hours.

WHAT IS NEEDED

In order to make reasonably accurate wind observations, you do not need a sophisticated system. All you need is a free flying flag. A wind sock, similar to those used at small airports, could be used to help you estimate both wind speed and direction. Many novelty stores also stock wind socks. You could even construct a wind sock easily from inexpensive materials, keeping in mind that the small end should remain open. The decorative wind vanes on roof tops may be an adequate indicator of wind direction, but often they may be affected by a distorted wind flow over the roof. If possible, the flag or wind sock should be away from obstacles that could influence the wind. Ideally, the wind indicator should be mounted approximately 10 meters above the surface.

You can determine the direction of the wind motion by looking at the direction that a free flying flag or wind sock is pointing. If you are making your observation from home or school, you should determine the cardinal points (such as North, East, South and West) from local landmarks using the noonday sun or a compass. When you make your observation, remember that traditionally the wind is named for the direction from which the wind blows. That is, a north wind blows from the north. You can record your wind directions using a simple 8 point notation (N, NE, E, etc.).

You can estimate the wind speed by looking at the movement of recognizable objects produced by the wind, such as flags, trees, smoke plumes or the water surface The Beaufort scale was devised by Admiral Francis Beaufort (1774-1857) of the British Navy in the early 19th century to provide an expedient means for estimating wind speed for use on sailing ships. With a little practice, together with the modernized Beaufort wind force table appearing in Table 8.1 of Online Weather Studies text, you can gain the experience needed to estimate wind speed. Compare your observations with the current weather observations from your local National Weather Service Office.

RECORDING YOUR OBSERVATIONS

If you keep track of your wind observations for any length of time, you will soon realize that some graphical means for displaying the results would help you visualize the time sequence of the winds more so than a tabular listing of the wind speed and wind direction. By now you should be familiar with the meteogram (short for meteorogram) format for several selected cities on the Online Weather Studies Homepage. You could use this same type of meteogram format to plot your observations for one or more days. In a meteogram, the hourly wind speed and direction observations are plotted along the entry marked WINDS, using the same familiar wind arrow format used on the surface weather map. Using this format, the direction of the plotted arrow on the meteogram indicates the observed wind direction (a south wind would be plotted as wind arrow directly below the circle) and the number of feathers denotes the wind speed, where a half feather identifies a 5 knot increment and a full feather is a 10 knot wind increment.

WHAT TO LOOK FOR

By keeping track of the winds for several days and comparing your record with the daily surface weather maps, you can begin to formulate some relationships between the wind and weather systems. You can locate the directions that the low and high pressure systems are from you by facing downwind (or back to the wind) and noting that your outstretched left hand would point in the direction of low pressure in the northern hemisphere (assuming for the time being that frictional effects are negligible). As the winds shift direction over time, you should be able to tell how the pressure systems move. You may also be able to note that wind speeds often increase and become gusty as a storm system or front approaches. (A gust is a variation in wind speed of at least 10 knots between peaks and lulls.) Wind speeds usually slacken as a high pressure cell nears. Unless local or regional factors such as nearness to a large body of water predominate, winds generally from the south usually signal a warming trend as warm air advection occurs.

The following descriptive terms are often used by the local National Weather Service Offices in public forecasts to describe the prevailing wind speed in miles per hour. The range of wind speeds may differ in various parts of the country due to factors such as terrain and elevation.

**WINDS** (from NWS, ***Media Guide to National Weather Service Terminology***, 1996)
Sustained Wind Speed	Descriptive Term
0 - 5 mph 5 - 15 mph, 10 - 20 mph 15 - 25 mph " 20 - 30 mph 30 - 40 mph 40 mph or greater	Light, or light and variable wind None Breezy (mild weather) Brisk or Blustery (cold weather) Windy Very Windy Strong, dangerous, damaging. High wind warning required

REFERENCE:

Thompson, S. and A.F. Kapela, 1996: Media guide to National Weather Service Terminology. National Weather Service. NWS Forecast Office, Milwaukee/Sullivan, WI. 21 pp. (also appearing as http://www.crh.noaa.gov/mkx/media. htm)

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