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