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Posted by Ray
on 10/19/2008, 7:53 am, in reply to "Soaring season not over"
72.230.154.40
Doug's exhortation should be taken seriously. Not only is the soaring season not over, actually in the fall it's much better than in summer almost all of the time, albeit with a few exceptions. Why?
There are two main reasons:
First, in the middle of the summer the jet stream is well to our north. As is the flow generally at all levels, since momentum is transported downward to the surface, which frictionally retards it. So we get those big monster highs, with hot, hazy, stable conditions. To get a "super day", one needs instability through a good deep layer, and that can only be maintained by the importation of relatively cold air aloft, leading to an adiabatic lapse rate. So for a day like yesterday one needs a "polar express". Those do sometimes happen in the middle of the summer, which in conjunction with the long day gives folks a chance to complete their diamond badge. But now, in the fall, the seasonal pattern is changing. The jet is moving northward and is now almost directly, on average, over us. So we get lots more cold advection events. To look for this if you're doing your own soaring forecasting, look at maps of isobars and isotherms superimposed. Sparing you the vector calculus, there is zero cold advection when the isobars and isotherms are parallel, and maximum when they are perpendicular.
Second: In the fall, it's much more scenic. But that's a bonus, incidental fact. A beautiful leaf is a dead leaf is a dry leaf. The average tree loses by evapotranspiration about 200 gallons per day. Multiply that by the number of trees per unit area, and the latent heat of evaporation (energy required to vaporize that water), and then compare that with solar insolation (energy per unit area coming in from the sun), and you'll find that a substantial fraction of the sun's energy is needed to evaporate the water, as opposed to having all of it go into heating the surface. Consequently for example, at Dansville nowadays one can still be getting usable thermals off the ridge well after the rest of the convective field has collapsed. If you're very lucky, you can use that to stay up as the sun sets, then take advantage of the cool air draining into the valley to stay up indefinitely.* That mechanism (catabatic flow or "valley effect) is rarely strong enough to support a sailplane. Not so for waves, which are now coming into the picture. There's no excuse for any persistent Dansville pilot to not get Gold altitude near the airport.
* I suggest landing before the runway lights come on. I have done that. You will get an enlightening conversation from the airport manager and/or the FAA.
--Previous Message--
: Come out and soar, the season is not over.
: Today the soaring was great with thermal
: strengths up to 700fpm up to cloud base
: which ranged from 4600 to 5600ft msl. There
: were extensive cloud streets and the
: conditions for cross country flight were
: outstanding. Jake and Doug B. flew the
: Bergfalke to an off-field landing at Seven
: Gullies, Jari and I flew the Grob and
: Pegasus respectively, for over 2 hours each,
: while John Caldwell flew the K21 after Tom
: finished several instruction flights. Come
: out and play, but dress warmly.
:
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