[ TERC VHS-1 Sun Photometer ] [ Manual TOC ]
Figure 11 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 12 is a line graph which 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 13 is a bar graph which 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
Fig. 13, 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.
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. 12 and 13 is generally higher
than in Fig. 11 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 Fig. 11 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?