In 1988-89 a suction trap for insects was placed at a height of about 160 m
above ground level in a TV-mast 17 km from the radar. The suction trap was
made by the Agricultural Research Centre of Finland and had an inlet
diameter of 0.4 m and its total length (heigth) was 2 m. The trap can be seen
in the picture at about halfways of the mast on the right hand side.
The suction trap had a blower that sucked about 3000 cubic meters of air
through a quite dense conical metal net. A plastic sampling bottle was
placed at the end of the cone to catch biological matter in the air
(mainly insects and spiders).
In principal the number density of species can be calculated according to
the number of catched individuals and the air volume that has flowed through
the trap. In practice both the wind velocity and the size of individuals
affects the efficiency of the trap. In the following these factors are not
taken into account.
During the catching periods (Aug 2 - Oct 20, 1988 and May 10 - Aug 4, 1989)
the catched individuals were grouped by Marko Nieminen according to families
and sizes. The relative importance of each group in radar echo was then
calculated assuming simple Rayleigh scattering dependance on size.
In the following figure both results are presented as percentage histograms;
yellow ones for the relative amount of individuals and blue ones for the
relative importance in radar echo.
One 9 mm moth, wingless (small) insects and plant seeds were excluded.
Note the logarithmic scale!
The numbers tell the size classes according to body length in millimeters.
The groups are as follows:
Diptera, Aphidoidea, Coleoptera,
Hymenoptera, Thysanoptera, Auchenorrhyncha, Psocoptera,
Heteroptera, Psyllidae, Neuroptera and Aranea; 2≤Dipt<4 means dipterans whose body length has been 2 to 4 mm.
[clear air echos] [Matti Leskinen] [Department of Meteorology]