DATASTREME SUPPLEMENTAL SUMMARY
To complement the Daily Summary for Tuesday, 31 October 2000
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 DataStreme Study Guide Part A, 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 DataStreme 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. As discussed in Tuesday's Concept
of the Day, you can locate the direction from you of the low and high pressure
systems by facing downwind. 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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URL: datastreme/learn/t_sup.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.