Seeing conditions and atmospheric transparency play a major role in successful study and observation of celestial objects for earth based astronomers. Unfortunately, most of us do not have access to the Hubble Space Telescope where these two factors do not play a role. We have to live with whatever Mother Nature has in store for us and do the best we can under the conditions she provides.
A new study at ASO deals with the possibility of forecasting "windows of opportunity" when the seeing and transparency might be at their best by using Skew-T/Log-P (skewed temperature/logarithmic pressure) diagrams available from many sources on the web. Many in near real time.
The Skew-T diagram is used to determine upper atmospheric meteorological conditions over a given position on earth. A device is carried into the upper atmosphere with a weather balloon, and during it's ascent, data is transmitted or collected through all the layers of the atmosphere up to nearly 10 miles. For the most part, this data consists of air temperature, dew point, wind speed, and of course the altitude at which the measurements are taken. The Skew-T chart combines this information together in a graph that gives us a picture of the vertical column of air over many key points on earth.
(Chart 1)
The horizontal axis represents temperature in Celsius (increasing from left to right), and the vertical axis represents pressure in millibars which always decreases in altitude above sea level. 700mb is equal to 10,000 feet above sea level. Many charts also indicate the height in feet or meters as the one above.
Seeing (Steadiness of the air)
The heavy red line in this chart indicates temperature, and the blue line is dew point. The temperature is read from the slanted (skewed right) thin red lines emerging from the X-axis (temperature). The important thing to note here is the purpose of slanting the lines; If the temperature were to cool at what is considered a "normal rate" with increasing altitude, the red line would be perfectly straight up and down. With all else aside, a perfectly straight red line would seem to indicate astronomical seeing (steadiness) would be perfect! This is due to the fact that sudden changes in temperature versus altitude above your head would likely cause boiling effects in the air due to the interaction of these unlike temperatures. It's unlikely you will ever see a perfectly straight temperature line on any Skew-T diagram, but you might find a time when it is fairly smooth, which should tell you the seeing might be great. A sudden and wide "dog-leg" in the red line might tell you to expect some pretty unsteady planetary images.
The next step in this project is to determine if these sudden temperature vs. altitude changes have more effect on seeing at certain levels in the atmosphere. Due to the fact there is less air at the higher altitudes, a change of temperature vs. altitude would probably be less noticeable than an equal change at lower altitudes.
Transparency (Clearness of the air)
It is possible that transparency could also be accurately determined with the Skew-T chart, here's why; As mentioned, the heavy blue line which runs nearly parallel with the red temperature line represents "dew point". The farther to the left this line is from the temperature line, the dryer the air. If it runs close enough, it could indicate a high density of moisture, or even clouds. If these lines cross at any point, there would be complete saturation, or 100% humidity. At low levels, the crossing of the lines would almost always be in the form of rain or other precipitation. In regard to our transparency index, we would want to see a Skew-T chart with these lines separated a good distance all the way from the ground to the top of the chart!
Chart 1 above was taken on 29 Nov 2001 over Russellville, AR where there had been heavy rain all day, and a thick overcast. Note the convergence of the two lines up to about 800 mb, or around 7000 feet, and then another convergence at about 500 mb, or around 18,000 feet. This indicates two cloud layers. One being thick, nearly 7000 feet, and the other being high and thin, probably less than a few hundred feet. Had the lower layer not been there, I might have been able to see stars just fine that night, but the transparency would not have been perfect due to the layer at 18,000 feet.
(Chart 2)
Chart 2 was taken of conditions over Wichita, Kansas the same night as Chart 1. Reported weather conditions there were Clear and 27 degrees F. Humidity was around 80% (note closeness of lines at ground level).
Even though the weather was stated as clear, note the near convergence of the two lines at around 15,000 feet. This would surely have some effect on astronomical seeing, even though at first glance outside, the sky looked clear as a bell. In this chart the temperature line appears a bit more vertical than in the previous chart, but I suspect seeing was still not too good due to a few sudden bends in the curve.
The major drawback we encounter with the Skew-T, is that at best there are only forecasts for 12 hours in advance, which may or may not prove accurate, as they use a method of interpolation to predict the variables.
Another important consideration is wind speed, which in the early stages of my data correlation, does appear to have a distinct role in seeing conditions, particularly those at high altitudes. More on this later.
This is a new and ongoing project. As information is collected and studied on the usefulness of this method, more will be published about it's possible benefits in forecasting astronomical observing conditions. Stay tuned!
Brian Sherrod
Arkansas Sky Observatory - Cascade Mountain.
webmaster@arksky.org
Copyright Arkansas Sky Observatory © 2001 [A.S.O.] All rights reserved. Revised: May 30, 2002
