The table below, which is based on clear day data, shows the average time, in minutes, required to obtain a UV-B exposure of 1 MED (i.e. for a fair-skinned person who does not tan easily to get a light sunburn) at Southwestern, based on data from 1995 through 1998. The average standard error of the time interval quoted for 13:00 is less than one minute. The time of day is local clock time, which is CST for March 21, but CDT for all other dates.
 
 

Monitoring of solar UV-B Irradiance at Southwestern began in 1994 when Dr. Rob Roeder and student Dennis Moore, with funding from a Mundy Faculty Fellowship and the Lazenby Chair in Physics, installed a Solar Light model 501 UV Biometer on the roof of the science building. The instrument was recalibrated once a year, usually in February, to maintain accuracy as recommended by the WMO. If stratospheric ozone varies, the solar ultraviolet flux at ground level will also vary, and this will be seen in the meter output. The output is a measure of biologically active radiation in the wavelength range from 290 to 320 nanometers. Excessive exposure to this solar UltraViolet-B (UV-B) radiation is suspected to be a cause of skin cancer.The NASA ozone data obtained with the Earth Probe TOMS instrument when over Georgetown, TX, can be seen by using this link.

The monitoring program continued until the end of November 2001, producing seven years of data and making it the most comprehensive study of solar UV-B conducted in central Texas. Some of the important results of the program were published in the Journal of Geophysical Research (Atmospheres), vol. 107, number D22, ACH17-1 through ACH 17-7, for November 2002. They are summarized at the bottom of this page.

The data plotted in the scattergram below came directly from the UV-Biometer on the roof of Fondren-Jones Science Hall and show the time for a horizontal surface to absorb one MED (Minimum Erythemal Dose) of solar ultraviolet-B (UV-B) radiation at local noon at Georgetown. A person who absorbs 1 MED of UV-B has received 21 milliJoules of UV-B per square centimeter on the skin, and if that person is of the fair-skinned type which does not tan easily, a slight reddening of the skin will result - a sunburn.

From the scattergram, one can see that at times near the summer solstice, which typically occurs on June 21, it is possible to absorb 1 MED of solar UV-B in as little as 10 or 12 minutes at local noon. In December at local noon, it takes close to 40 minutes. If days are partly cloudy, it can take either a somewhat longer or shorter time depending on the number and type of clouds.  On extremely cloudy or overcast days, longer times would be needed.
 
 
 

The term "local noon" means the time when the sun is on the observer's meridian, that is, when the sun is highest in the sky at a particular location. Because the longitude of Georgetown is 97.5 degrees West, Georgetown lies a half-hour West of the standard meridian for Central Standard Time (CST). Thus local noon at Georgetown occurs close to 12:30 pm CST or 1:30 pm when we are on Central Daylight Time (CDT). In the summer, local noon at Georgetown always occurs within 7 minutes of 1:30 pm CDT; the slight variation is a result of the fact that the earth does not move uniformly in its orbit around the sun. The effect of longitude and time of day can easily be seen by looking at the irradiance measurements for a whole day.
 
 

The following graphs show measurements at a solar zenith distance of 55 degrees, morning and afternoon, at local noon, and the total dose per day at Southwestern. The irradiance unit is a MED/hr ( 1 MED/hr = 5.83x10^-6 Watts per square centimeter of biologically active radiation) and is related to NOAA's UV Index by the conversion factor: 1 MED/hr = 2.34 on the UV Index scale. A UV-B irradiance of 4 MED/hr thus corresponds to 9.4 on the UV Index scale.  The current US forecast for the UV Index prepared by the Climate Prediction Center can be found here.  Data on the graphs are shown for all weather conditions, not just clear days.

Irradiance at 55 degrees, morning;
Irradiance at 55 degrees, afternoon;
Daily total dose;
Irradiance at local noon.

 To provide a convenient way of looking for changes in the summer UV-B irradiance during the period of the study Dr. Roeder adopted a quantity called the "Average Summer Peak UV" (ASPUV) irradiance which is similar to a measure first introduced by Dr. Richard McKenzie and his colleagues in New Zealand.  For purposes of standardization, "summer " is defined as the months of May, June and July, an interval which is somewhat symmetric around the summer solstice which occurs on or near June 21 each year when the sun reaches its maximum northern declination.

The first step in finding the ASPUV for any summer is to find the average value of the solar UV-B irradiance for each day measured during the year; this is done by averaging 5 readings, each 10 minutes in length, which are centered around local noon.  This procedure avoids the subjectivity involved in determining whether or not a given day is "clear".  Since data were not collected while the instrument was being recalibrated, complete data sets exist only from April through the following January; an example of the data set from 1 April, 1999 through 31 January, 2000 can be seen here.  Once these data have been obtained, the 5 days of each of the months of May, June and July with the highest local noon averages are picked out and these 15 values are then averaged to find the ASPUV for a given summer. 

The procedure for determining the standard deviation associated with each ASPUV is complicated by the fact that each day's measurement is part of a time series - a group of regularly spaced measurements - and the value of a measurement made on any given day depends in part on the value of the preceding day's measurement.  This leads to what statisticians call an inflation of the variance which must be taken into account if the variance and hence the standard deviation is not to be underestimated. this procedure is explained in detail in the technical paper but the results are shown in the figure below.

       
This figure shows that the ASPUV in 2001 was lower than it was in 1995, even though the change from one year to the next is not statistically significant.  In fact there may not have been any change during the years 1995, 1996 or 1997  but the values for 1999, 2000 and 2001, although possibly not different from one another,  do appear lower than those for the early years.  The most likely cause of the decrease is the increase in automoblie and truck traffic in and around Georgetown as the area has grown in population.  Such traffic tends to generate low-level (tropospheric) ozone which is known to play a disproportionate role in UV absorption (see Bruhl and Crutzen, Geophysical Research Letters, vol. 16, pages 703-706, 1989).  This decrease in the ASPUV implies that it takes longer to acquire 1 MED of UV-B radiation at local noon in the summer of 2001 than it did in 1995, but don't put away your sunblock, because in 2001 the amount of time required was only 13.6 minutes on average!

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