A ventimeter, a kite and a bicycle

For some time I have been studying wind speed distributions from a variety of sources including airports, offshore buoys, weather balloons and backyards and whilst this has been instructive, it has told me little about the wind where I live. Weather reports from an airfield on open ground 10 km to the west, often don't describe what I see from my house on the west side of an urban valley.  About a year back, a personal weather station appeared on pole a few streets away and since then I have changed my dog walking route to so I can observe the anemometer and wind vane, hopefully without invading anyone's privacy.  Unlike many PWS, this one is clear of roof and tree tops.  It clearly responds long period gusts and the frequent changes of direction which seem to be a feature of urban wind, often the anemometer is gently turning with the vane fluttering in the direction of the prevailing all of which suggests a wind speed of less than 5 m/s.

 


I have attempted to gain some understanding of wind in the area I live in by cycling around with a hand held ventimeter type wind speed measuring device, these efforts have been described in a couple of previous posts (fortunately, my family only rarely read this blog).  This too was quite useful, but my arms only extend to 2 metres which is close to the ground for serious wind speed measurement.  Recently, I have started messing with a small kite, not only is this instructive and three dimensional, it is also fun.  The ventimeter attracted little attention, except from an alsatian dog called Trooper, but most people smile at the kite, especially old ladies.  Maybe there is some point to be made about making environmental science accessible, but life is too short.

The kite is difficult to fly in even light turbulence, sometimes on a gusty day, a random swirl of wind will take it into relatively smooth air where it fill fly contentedly as long at the air keeps moving, once the gust subsides, it returns to Earth..  Having spent a lot of my life crunching numbers, it has been a surprise, how much can be learnt without a single keystroke.  Whilst its short on numbers, the kite provides an indication of the change of turbulence with height.  In the sketch below, the layers and circles are very roughly at 10, 20 and 30m, the arrows are relatively smooth air and the circles represent turbulence.

Wind blowing off the sea is usually smooth, however, wind coming from the land can become turbulent if it passes over trees and buildings.  On a coastal plane, hedges can create local turbulence below relatively smooth air.  In urban areas, the wind is almost always turbulent, at least up to treetop height.  I would not recommend flying a kite in an urban area to investigate.  I have always been curious about the performance of three medium sized wind turbines in the vicinity of Dagenham which I have seen turning smoothly.  On ridges above the surrounding land, gusting is less than on the approaching slopes.  If the wind is flowing up the slope it can be relatively smooth, but if the slope is in the wind shadow of a ridge, it can be almost calm with only long period gusts.

These comments have the caveat that they are drawn from a small sample collected across a relatively small area using a kite.