Ground winds

Some data formats include a field for the ground wind speed and direction. At first glance, Windsond should be able to report this… but it can’t, since the wind is sensed by the balloon floating with the air movements (i.e. winds) as it rises. Before the balloon starts rising it’s anchored (typically by someone holding it…) and can’t measure winds.

But wait a moment, what exactly is meant by winds on the ground? Due to the physics of fluid mechanics, wind movement very near the ground will be much lower than expected, even approaching zero extremely close to the surface. The ground drag is felt up to 200-400m height, depending on the type of terrain. As it turns out, the standard is to measure ground wind at 10 meters height. Here’s another discussion, by Belfort Instrument. It takes Windsond several seconds to reach 10m height so the GPS has time to sense the change in speed and direction.

Still, the wind close to the ground is more turbulent than at altitude, so a single reading from Windsond cannot capture average conditions like a ground-based weather station can.

By the way, the GPS senses the movement by doppler effect. This is more accurate than calculating heading and distance between successive position reports — GPS position reports may experience sudden jumps and even at the best of times they have a degree of uncertainty that would severely limit the time resolution possible.

At the moment Windsond measures wind once a second and makes an average over the last three seconds when sending data to the ground station. A software extension could transmit all three values to the ground station. A future experiment is to measure five times a second and save this to on-board storage for downloading after the sounding finished.

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