Oct 23, 2001, 11:52 AM
Post #1 of 5
How long after a field as been cut, can you get a thermal draft? If you jump over a field can you expect the draft all over it? I know it might seem thecnical but where i jump in Canada we jump over BIG fields so i was wondering how it could be applied. Thanx! " Seul un parachutiste sait pourquoi les oiseaux chantent"
billvon (D 16479)
Oct 23, 2001, 2:15 PM
Post #2 of 5
This is something that sailplane, hang glider and paraglider pilots spend years learning. But basically, for thermals you need two things - a warm surface to heat the air and a 'trigger event' to cause the thermal to lift off. Once it lifts off, it acts like a big bubble of rising hot air. It is usually smaller than the field. Roads, plowed fields, concrete etc can all cause thermals. Even a corn field in the middle of a forest can give rise to a thermal, because the corn may be warmer than the surrounding forest. The trigger event can be something major (like a sudden change in wind) or something really minor (a seagull flying overhead.) A tall thing in the middle of the field, like a big rock, can help trigger the thermal. Sometimes these can be so predictable that hang gliding sites have "house thermals" where you can nearly always find lift (if you time your flight right.)
Since parachutes glide like bricks compared to sailplanes and hanggliders, generally all we notice from a thermal is some turbulence. You need a >1000fpm thermal to keep you aloft, which is a very strong thermal indeed. Often, such thermals are marked by dust, leaves or even trash in the air. Birds also look for them, so if you see a big bunch of birds circling a field, it's a good bet there's lift there.
i found that there is a really nice thermal coming off the hager at perris, watched a jumper about 200ft in front of me and 100ft below me rise above me to scare the hell outa me and then as soon as i hit the same spot i cought the thermal and was once again above him....
<wingnut> "up my noooossseee"- wingnut, at first euro dz.com boogie
gotta watch out for that to, cause coming OFF the thermal can cause you to pick up some serious forward speed. the hanger roof at Cross Keys is black, and farily large, and typically the prevailing wind direction allows a set up over the building, but as soon as you get off the "lift" you get a pretty good dive - alomst like a front riser. gotta watch out for that.