Wetenschap - 10 mei 2011

Dust devil near Wageningen

Meteorologist Tom van der Spek was in the right place at the right time. The dust devil he filmed may passed close to a Wageningen weather station.

Tornado chasers risk their lives for this experience. But meteorologist Tom van der Spek was handed it on a plate when a dust devil crossed his path. The path in question was his route home from work. It was just before two thirty in the afternoon of Friday 15 April, after the early shift, when Van der Spek was cycling across the Binnenveld in the direction of Bennekom. He was just turning into the Plassteeg when he saw a tractor enveloped in a cloud of dust. This turned out to be a dust devil. A chance in a lifetime for a meteorologist. Dust devils are caused by air rising in hot, dry weather. In contrast to whirlwinds and tornados they are driven from below (by the warm earth) and are fairly harmless. It was not actually terribly warm that afternoon, about 22 degrees Celsius. Van de Spek: 'I just happened to have my camera with me. There are renovations going on at work and I am taking photos of it.' He lost no time in shooting a few photos and a film of this natural phenomenon. He then took a left turn into the field to take some close-ups. Until the dust devil died out.
One hundred KPH
By coincidence, the dust devil passed close to a Wageningen UR weather station.  Plant researchers from PRI are doing research there on the influence of greenery on fine particles. 'We are trying to measure whether you can filter fine particles in livestock sheds using vegetation', explains project leader Annette Pronk of PRI. These measurements had not begun when the dust devil passed by. But the apparatus was already working. On the Monday after the dust devil, Van der Spek spoke to the researchers at the site and told them what he had seen. 'At first they responded rather sceptically. But I have been a meteorologist for almost a quarter of a century so I do know what I am talking about.' The researchers soon became enthusiastic.
Peter Hofschreuder (ASG, Livestock Research) scans the data from that Friday. But he finds no trace of the dust devil. 'There were a lot of strong gusts of wind that day. The measurements of the wind speeds show a tremendous range with peaks of 30 metres per second. That is more than 100 kilometres per hour. It certainly was not an ordinary day.' But whether the dust devil touched the weather station is not clear.
'I already thought the dust devil passed behind the weather station actually', responds Van der Spek. 'I was standing far off, about 700 metres away, so that distorts your view.' He estimates that the dust devil was 100 metres high. 'And you can see very clearly that it is anticyclonic, meaning that it turns clockwise.' You can see Van der Spek's film below.  
Roelof Kleis.

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