Down drafts and microburst’s. (Virga)
Just after I flew in the 1985 Nationals at Chelan Butte Washington there was a jet plane crash at Dallas Ft. Worth International Airport on August 2, 1985 most likely due to a microburst.
http://en.wikipedia.org/wiki/Delta_Air_Lines_Flight_191Shortly after that there was much talk about installing weather radar at airports to detect downbursts or microburst.
While the FAA was hemming and hawing about why these jets were falling out of the sky I had already figured it out.
One XC day while landing at Creston WA from the Farmer tow site. (Maybe 45-50 miles) A microburst caused by Virga under a single cloud switched my WSW wind and blew strong out of an easterly direction for maybe 10 to 20 minutes. It happened so fast I just about flipped the glider while I was trying to nose it into the wind. (Wow, I thought, what if that happened while I was landing?)
Flying mostly in Minnesota where the relative humidity is always high being rained on while flying most times was not a problem.
Flying in the much drier air mid Washington State near a rain cloud can cause extremely strong downbursts of cooler air because of the fast evaporation under the cloud. (Virga)
50 to 60 mile per hour gust fronts will explode outward in a big dust ring on the ground directly below the cloud and many times all the rain will evaporate and never reach the ground. (Virga) That doesn’t happen too often in high humidity areas.
The desert SW can pose a deadly situation for pilots that are used to only flying in states that have a high relative humidity.
If you drive through and area, city, or town and see big evaporative coolers on the roofs of houses remember that these coolers only work in dry areas. If you are going to fly in these areas keep a close eye out for overdeveloping clouds.
When these clouds let loose they have the power to hold off a prevailing wind for up to 10 or 15 minutes and actually reverse the wind direction on the ground locally.
The best course of action if you see a dust front almost directly below you is to fly away from the cloud that caused it, try to stay ahead of the dust front until it weakens. There will be an abundance of lift as this cooler air travels under the warmer air that you are flying in. It is not a good idea to seek out this type of lift.
Your best course of action is to try to fly out ahead of this gust front and land beyond it but not too soon.
http://en.wikipedia.org/wiki/Doppler_radar