The validity of UV-measurements during May &
June in Athens and Vienna has been discussed. After intercomparison of
sensors in August in Athens, we now are convinced about the correctness
of our readings, which has been doubted by some co-workers. In fact, at
a first glance, the results are somewhat puzzling:
1. During the measurement period, the readings in Schwechat where almost
always higher than those in Athens.
2. The readings in Schwechat also showed a very irregular behaviour with
distinct maxim and minima compared to Athens with a relatively stable
increase during the measurement period. This gave rise to the assumption,
the Schwechat sensor would be measuring garbage instead of UV.
In fact, our feeling "the hotter, the more UV" is simply wrong, - anyone
who ever got sunburnt during skiing would agree upon this.
The simple explanation for points 1&2 lies in the different atmospheric
conditions between Vienna and Athens: The keyword is "Turgidity" of the
atmosphere, meaning the degree of clearance of the air, as governed
by various factors like: Clouds, Humidity, Aerosols (Dust, atmospheric
pollution with various gases like NOx, SO2... which all absorb UV-B to
a great extent), water vapour and temperature layering of the lower atmosphere.
During spring time in Vienna, a typical day might exhibit a total cloud
overcast with rain, resulting in low UV-B-intensities. These days often
are followed by sunny days without clouds and very clear atmosphere, which
has been washed out of aerosols by the previous rain. On these days the
air turgidity is very low, giving us views of well above 50 km horizontally.
It would not be unusual for such a day to show higher intensities of UV-B
than during a summer noon. (As higher ground temperature during July and
August causes upwelling of dust from the ground to the lower atmosphere,
resulting in higher turgidity, less air clearance and thus stronger absorption
of UV-B).
On the other hand, the weather conditions in Athens seemed to be more
stable, with less periods of full overcast and rain. Therefore the stability
in UV - readings (point 2). Plus high turgidity of the atmosphere in and
around Athens, caused by dust upwelling and air-pollution - accounting
for the general lower readings (point 1). The often cited "northern hemispherical
ozone hole" cannot be made responsible for these effects.
From the teacher's point of view, these measurements are ideally suited
to demonstrate the differences between subjective, personal impression
and the objective, scientific data.
| << Previous Page |
|
