A Clear Sky Record

Forrest M. Mims III

Weather records are made to be broken, and that occurred during a recent string of clear skies over South Texas. This event provides a good example of how amateur and student scientists can use a digital camera to collect significant scientific information.

During the 11 days from 23 October to 02 November 2007, no clouds could be seen over the field we call Geronimo Creek Observatory. At night the clear skies were ideal for watching the little fuzz ball in the northeast sky known as Comet Holmes.

The clear skies also simplified my daily measurements of the ozone layer, haze, solar ultraviolet, and total water vapor. All these measurements require direct sunlight without intervening clouds.

I began measuring the sky in September 1989. The previous record number of clear days occurred in October 1995, when we had nine clear days in a row. After an interruption of two cloudy days, that event was followed by five additional clear days.

In May 1995, we had eight clear days in a row, and we had seven such days in October 1992. We had six days of clear skies in February 1995 and October 2003.

Figure 1. These fisheye photos of the sky over the Mims place on Geronimo Creek in South-Central Texas show a string of 11 clear days beginning 23 October 2007, the longest series since sky measurements were begun in 1989. Photographs by Forrest M. Mims III.


A clear sky is usually a dry sky, and this is one reason clear skies are usually blue and free of haze. But sulfur emitted by coal burning power plants and gases from certain trees can make a clear sky hazy, especially when the humidity is high.

A sky with puffy cumulus clouds can also be deep blue, but this usually occurs only when clean tropical air arrives from the Gulf of Mexico and the Atlantic.

Satellites that measure the ozone layer and other stuff in the air do a better job when clouds don't get in the way. Clear skies also provide good opportunities for outdoor photographers.

Clear skies provide an ideal way to compare measurements of sunlight made over the years without interference from clouds. So let's compare clear days in October of 1990 and 2007.

One way to express the clarity of the sky is to measure how much sunlight it blocks. On an average clear day in October 1990, 96.6 percent of a near-infrared (860 nm) beam of sunlight reached the ground. The number fell to 95 percent during the recent string of clear days in October and November. This 1.6 percent decline is very significant, so let's find out why it occurred.

The first half of the string of clear skies back in October was actually cleaner than in 1990 (97.3 percent). But the second half was dirtier due to sulfate smog from Eastern States and possibly some sugar cane smoke from Louisiana. In short, the reduced clarity of the sky was mainly caused by dirty air from outside Texas.

You don't need fancy instruments to track the clarity of the sky. A camera will work fine, especially if you can manually set it for the same setting for each picture.

Amateur scientists, science fair students and others who would like to do some real science please take note: You can make an important record of the clarity of the sky by making a daily photo of the sky over the north horizon. See my web sites here and here for details.

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