Science - June 5, 2008

Meteorologist measures ‘occult precipitation’

There’s so little of it that hydrologists hardly even notice it, but that small amount of dew that forms at night in a desert is crucial for the stability of an ecosystem. PhD graduate Bert Heusinkveld developed an instrument that can measure dew droplets exactly, providing evidence that life in the desert would not be possible without the ‘mysterious precipitation’.

The optical wetness sensor.
‘Mankind has always been fascinated by dew,’ tells Heusinkveld. Dew is a form of precipitation that occurs when the night sky is clear. ‘You can’t feel it, you can’t see it, but everything around you gets wet. That’s why meteorologists call it occult precipitation.’

For a long time, little attention was paid to the phenomenon of dew formation, as there was no way of measuring exactly the volume. ‘The process of dew formation is extremely subtle,’ says Heusinkveld. In the absence of the sun at night, the earth’s surface cools down as it radiates heat into the atmosphere. When the cooling process reaches condensation point, drops of moisture develop on the earth’s surface. ‘Every disturbance immediately above the soil can interfere with this process.’ And that includes an instrument for measuring moisture.

Heusinkveld managed however to create a ‘unique optical instrument’ that can measure moisture content night and day without being in contact with the soil. Using infrared light the optical wetness sensor (OWS) measures the light that the surface reflects. Part of the light is absorbed by the moisture. By measuring the part that is left over, you can determine how much moisture there is on the surface.

Heusinkveld tested the OWS in the Negev desert in Israel. His measurements show that about 0.2 litres of dew forms per square metre in a night. It is a miniscule quantity as far as many hydrologists are concerned, says Heusinkveld. ‘But if you add it up for a whole year, it amounts to almost as much as the annual rainfall. But rainfall only falls during a few months. The rest of the year it’s dry. Dew provides moisture throughout the whole year, and therefore it’s very important.’

Small animals, such as spiders and ants, depend on dew for their survival. ‘You see them in the morning licking up the dew that has formed on the ground. It’s a fleeting but crucial moment.’ And not only for small insects, but also for micro-organisms, such as blue-green algae which form a biological crust on sand dunes. The crust keeps sand in place and thus prevents erosion. ‘Without dew there would be more desert formation,’ concludes Heusinkveld.

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