Hands On Physics


The graph below shows the AOT measured by a Sun photometer since the spring of 1990. This instrument is the first known Sun photometer to use an LED as a detector. Notice how warm weather haze causes the AOT in South Texas to rise in the summer. Cold weather in this region often means very clear air, which causes the AOT to reach very low values. The big bulge in AOT from the fall of 1991 to the end of 1994 was caused by a thick cloud of aerosols that formed high in the stratosphere after the eruption of Mount Pinatubo on 15 June 1991.

Figure P7
Aerosol Optical Thickness

The line graph below shows the AOT measurements made by the prototype TERC VHS-1 from noon until evening at my observing site in South Texas on 9 March 1996, a day when the sky had a few cirrus clouds but was otherwise very clear. Notice that the clarity of the atmosphere remained reasonably stable during this time. On some days there can be significant changes in AOT. The dip at 18:00 (6:00 p.m.) is caused by the error that occurs when the air mass exceeds 10 or so.

Figure P8
AOT Measured with TERC VHS-1

The bar graph below shows the AOT measured by the prototype TERC VHS-1 near solar noon during each of its first 8 days of testing. Even though only 8 days of data are shown in Figure P9, this graph clearly shows how the arrival of cool, dry air over South Texas caused relatively low AOTs on 9-11 March. When warmer, more humid air returned to South Texas on 12 March, the AOT increased sharply. On 14 and 15 March, wispy cloud haze boosted the AOT even higher. A cool front brought in dry air on 17 March. Although the AOT fell sharply, it was higher than the earlier lows. The question was what caused this? A gigantic grass fire brought on by very dry conditions produced considerable smoke over Central Texas. Apparently the cool front arriving from the north blew some of this smoke over my site, thus increasing the AOT above what it should have been.

Figure P9

A powerful cold front with very strong winds is blowing through South Texas as this manual is completed the morning of 18 March. Will the AOT at solar noon be lower than on 17 March when thin smoke drifted high overhead? Not necessarily. The absence of rain that has provoked grass fires has also allowed the soil to dry, and the weather forecast predicts blowing dust to arrive from North Texas. Already there is a thin film of dust on exposed glass windows outdoors. Exactly how this dust will affect the AOT remains to be seen, but it will certainly have an effect!

Incidentally, as some sharp-eyed readers have probably noticed by now, the AOT in Figs. 9 and 10 is generally higher than in Figure P8 for the same time of the year. This is because AOT increases as wavelength decreases. The TERC VHS-1 detects green light (about 525 nm), and the instrument used to collect the data in Figure P8 detects near-infrared (about 850 nm). This means that an annual graph of AOT data made with the TERC VHS-1 will be less distinct (fuzzier or, as a scientist might say, noisier) than a graph of data collected in the near-infrared. But the TERC VHS-1 graph will closely reflect what your eye actually sees. And it will more closely indicate the effect of haze on direct sunlight near the peak of the sunlight spectrum.

As you can see, considerable information can be gleaned from AOT data--especially when the weather is changing, if a major volcano eruption occurs or if you are near significant sources of air pollution. What will your measurements show?

Previous Page || Up a Level || Index || Next Page