Tuesday Weather Topic in Greater Depth

From: Subject: Tuesday Weather Topic in Greater Depth Date: Fri, 30 Oct 2009 14:15:26 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w08t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 27 = October 2009=20

WIND OBSERVATIONS

We have learned that the wind can be of use to us =97 helping power = our=20 sailboats or turn our wind generators or cooling us during the summer. = But, we=20 also know that it can make us feel uncomfortable during the winter, = robbing us=20 of body heat when the wind-chill equivalent temperatures drop to low = values. We=20 can also look to see how the wind changes over time, and we can then = make an=20 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=20 sophisticated system. All you need is a free flying flag. A windsock, = similar to=20 those used at small airports, could be used to help you estimate both = wind speed=20 and direction. Many novelty stores also stock windsocks. You could even=20 construct a windsock easily from inexpensive materials, keeping in mind = that the=20 small end should remain open. The decorative wind vanes on rooftops may = be an=20 adequate indicator of wind direction, but often they may be affected by = a=20 distorted wind flow over the roof. If possible, the flag or windsock = should be=20 away from obstacles that could influence the wind. Ideally, the wind = indicator=20 should be mounted approximately 10 meters above the surface.

You can determine the direction of the wind motion by looking at the=20 direction that a free flying flag or windsock is pointing. If you are = making=20 your observation from home or school, you should determine the cardinal = points=20 (such as North, East, South and West) from local landmarks using the = noonday sun=20 or a compass. When you make your observation, remember that = traditionally the=20 wind is named for the direction from which the wind blows. That = is, a=20 north wind blows from the north. You can record your wind directions = using a=20 simple 8-point notation (N, NE, E, etc.).

You can estimate the wind speed by looking at the movement of = recognizable=20 objects produced by the wind, such as flags, trees, smoke plumes or the = water=20 surface. The Beaufort scale was devised by Admiral Francis Beaufort = (1774-1857)=20 of the British Navy in the early 19th century to provide an expedient = means for=20 estimating wind speed for use on sailing ships. With a little practice, = together=20 with the modernized Beaufort wind force table appearing in Table 8.1 of = the=20 Weather Studies text, you can gain the experience needed to = estimate wind=20 speed. Compare your observations with the current weather observations = from your=20 local National Weather Service Office.

RECORDING YOUR OBSERVATIONS

If you keep track of your wind observations for any length of time, = you will=20 soon realize that some graphical means for displaying the results would = help you=20 visualize the time sequence of the winds more so than a tabular listing = of the=20 wind speed and wind direction. By now you should be familiar with the = meteogram=20 (short for meteorogram) format for several selected cities on the AMS = Weather=20 Studies website. You could use this same type of meteogram format to = plot=20 your observations for one or more days. In a meteogram, the hourly wind = speed=20 and direction observations are plotted along the entry marked WINDS, = using the=20 same familiar wind arrow format used on the surface weather map. Using = this=20 format, the direction of the plotted arrow on the meteogram indicates = the=20 observed wind direction (a south wind would be plotted as wind arrow = directly=20 below the circle) and the number of feathers denotes the wind speed, = where a=20 half feather identifies a 5 knot increment and a full feather is a 10 = knot wind=20 increment.

WHAT TO LOOK FOR

By keeping track of the winds for several days and comparing your = record with=20 the daily surface weather maps, you can begin to formulate some = relationships=20 between the wind and weather systems. You can locate the directions that = the low=20 and high pressure systems are from you by facing downwind (or back to = the wind)=20 and noting that your outstretched left hand would point in the direction = of low=20 pressure in the northern hemisphere (assuming for the time being that = frictional=20 effects are negligible). As the winds shift direction over time, you = should be=20 able to tell how the pressure systems move. You may also be able to note = that=20 wind speeds often increase and become gusty as a storm system or front=20 approaches. (A gust is a variation in wind speed of at least 10 knots = between=20 peaks and lulls.) Wind speeds usually slacken as a high-pressure cell = nears.=20 Unless local or regional factors such as nearness to a large body of = water=20 predominate, winds generally from the south usually signal a warming = trend as=20 warm air advection occurs.

The following descriptive terms are often used by the local National = Weather=20 Service Offices in public forecasts to describe the prevailing wind = speed in=20 miles per hour. The range of wind speeds may differ in various parts of = the=20 country due to factors such as terrain and elevation.

WINDS
(from NWS, Media Guide to = National=20 Weather Service Terminology, 1996)

Sustained Wind Speed =	Descriptive Term
0 - 5 mph	Light, or light and variable wind
5 - 15 mph, 10 - 20 mph	None
15 - 25 mph	Breezy (mild weather)
"	Brisk or Blustery (cold weather)
20 - 30 mph	Windy
30 - 40 mph	Very Windy
40 mph or greater	Strong, dangerous, damaging. High wind warning=20 required

REFERENCE:

Thompson, S. and A.F. Kapela, 1996: Media guide to National = Weather=20 Service Terminology. National Weather Service. NWS Forecast Office,=20 Milwaukee/Sullivan, WI. 21 pp.

Return to the Tuesday=20 Daily Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological = Society.