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The Beaufort Scale
Submitted: Monday, 21st August 2006 by Alan Sheehan

Air movements affect astronomical seeing. Some air movements that can affect seeing are convection currents, the jet stream, and of course wind.

Convection currents are also known as “thermals” and are caused by warmer air rising through the atmosphere. Thermals occur during winter over heated houses, they occur in OTAs while scopes are cooling, etc.

The jet streams are high altitude; high velocity winds that circle the globe. Their path varies from time to time and can influence the weather. They also dramatically influence seeing. There are numerous charts available on meteorological websites to show the current jet stream maps (also see the weather section here on IceInSpace).

The wind, of course, is also well documented in weather reports and meteorological websites. Keen observers may consult the aviation weather forecasts for details of wind speed and direction at various heights throughout the atmosphere, though this data does take some interpreting.

Observers routinely estimate seeing and transparency during observation and imaging sessions. While many consult weather and jet stream information as well, as we know, sometimes the weather information isn’t accurate. So a convenient way of observing your local wind conditions may also help to get a handle on what affects your seeing.

If you have a weather station, great! You have hard data to work with. If not, all is not lost. The Beaufort Scale allows wind estimations to be made from simple observations.

The Beaufort Scale was developed for mariners for estimating wind speed especially in the days of sail power. It has since been expanded to include terrestrial observations so that the same scale can be used to estimate wind speeds on land. This can be very handy for operating elevated work platforms, model aircraft, kites, handling tarps and tents, working on rooves, etc.

The following table includes both the traditional maritime observations and terrestrial observations. With a little practice, you can learn to estimate the wind speed around you. This in turn can help you to better judge seeing conditions, or explain why the seeing is as good, or poor, as you experience.

The Beaufort Scale

Force Description On Land On Sea Speed (knots)
0 Calm Smoke rises vertically Sea like a mirror 0
1 Light air Smoke drifts. Wind vane still Small ripples 1-3
2 Light Breeze Wind felt on face. Leaves rustle; vane moves Small wavelets. Crests break. Glassy foam. 4-6
3 Gentle Breeze Leaves and small twigs in constant motion. Light flag extended. Large wavelets. Crests break. Glassy foam. 7-10
4 Moderate Breeze Raises dust; moves small brances Small waves but fairly frequent 'white horses' 11-16
5 Fresh Breeze Small trees in leaf begin to sway Moderate waves, more pronounced long form. Many white horses. 17-21
6 Strong Breeze Large branches in motion Large waves. Extensive foam crests. Probably some spray. 22-27
7 Near gale Whole trees in motion Sea heaps up. White foam streaks along wind. 28-33
8 Gale Breaks twigs off trees. Impedes progress. Moderately high waves of greater strength; edges of waves begin to break into spindrift. Foam blown in well-marked streaks along wind. 34-40
9 Severe Gale Slight structural damage possible. Tree branches may break. High waves. Dense foam streaks along wind. Wave crests begin to topple and roll over. Spray may affect visibility. 41-47
10 Storm Seldom experienced inland. Considerable structural damage. Trees uprooted. Very high waves with long overhanging crests. Dense large patches of foam blown along wind. Sea surface white. Visibility reduced. 48-55
11 Violent Storm Rarely experienced. Widespread damage. Exceptionally high waves. Sea covered with long white patches of foam along wind. Everywhere edges of wave crests blown into froth. Visibility much reduced. 56-63
12 Huricane or Cyclone Widespread severe damage. Air filled with foam and spray. Sea completely white with driving spray. Visibility seriously affected. 64+
Article by Alan Sheeny B.E (sheeny). Discuss this article on the IceInSpace Forum.
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