On Nov 4, 3:38*pm, Westbender wrote:
On Nov 4, 1:52*pm, Dave Nadler wrote:





On Nov 4, 2:26*pm, danlj wrote:

I sorta hate to interrupt the conversations by asking a question about
actual weather, but here we are...
Last fall, I posted a question on "where's the next thermal" that got
helpful responses. Somewhere in this group, a pilot made the
observation that on some days, the thermals vanish mid-afternoon --
and if this pause can be weathered, it's possible to stay up until
sunset.

This led me to think about my own northern-plains experiences. Often
I've gotten low, or thought the day had died, in midafternoon (3-4
pm). One flight in particular, this summer --http://www.onlinecontest..org/olc-2.0/gliding/flightinfo.html?flightId...
-- I had flown ENE, and after making the first turnpoint at about 2:45
pm, I couldn't find any lift at all. At about 600' (175m) agl, I gave
up scratching and restarted my engine while aproaching a field. All
the clouds as far as I could see had turned raggedy and shrinking. I
assumed the day was for some reason done (upper-level dry front?).
After I'd been despondently climbing with engine for a few minutes, I
realized that all the bases look good again, shut down, and had an
easy thermalling flight back home.

I can recall other, similar times, usually with a 20-30 minute episode
of scratching around down low, when I blamed myself, or the small
upwind lake, or the unknowns. But this day and that prior post makes
me wonder whether the boundary layer might not "shift gears"
midafternoon more often than we realize.

In this respect, I have (as others, I'm sure) that early in the day
the thermal sources are single fields and the lift broken and narrow;
late in the day, the thermal sources are savannas and the lift broad
and smooth, whether or not strong.

1: Do you have any meterological knowledge or links to suggest
regarding this?

2: Have you seen this phenomenon?

Thanks,
Dan Johnson

This can certainly happen when there's a wind change, which
messes up the thermal formation. A while after the change,
things get going again...

I'm sure there are other explanations but this is a situation
I've seen a few times.

Hope that helps,
Best Regards, Dave- Hide quoted text -

- Show quoted text -

Here in Wisconsin, this "cycling" is fairly common. We see it a
handful of times each season. Some days it can be pretty exagerated.
Although when it happens, it doesn't typically happen just once a day.
We can sometimes see cycles as frequent as a half hour to an hour.

Here's my take on why this happens. The clouds build to the point
where they create enough shadow to reduce the heating on the ground.
Once the thermals weaken and the clouds dissipate, the ground gets
more sun and begins heating again starting a new cycle. Not sure if
this is the case the original poster is describing, but it appears to
be what's going on around here.

When we fly on these kind of days, we have to take advantage of the
strong part of the cycle to make distance, then loiter around in a
weak thermal or fly very conservatively until the weak part of the
cycle ends.- Hide quoted text -

- Show quoted text -

You'll often hear racing guys talking about being "in-synch with the
day" or "out-of-synch with the day" (the latter condition usually
being described with expletives preceeding). When you're in-synch
with the day, you seem to hit a thermal just when you'd be starting to
dial back from go-fast mode to search-mode. I'm thinking of a 60
mile leg at Mifflin this year where there were 6kt climbs spaced at 15
mile intervals with 6,000-7,000 foot bases. Climb at 6 kts, blast
off 'till you're getting down around 4,000, hit 6kts, repeat... UNTIL
it stopped working. Suddenly, the two good climbs were right next
to each other, followed by 20 miles of nothin, nada, nichts.
Different airmass? Different terrain? Something else?

Certainly in mountainous or hilly areas, there are mountain/valley
effects which can wash out large areas. Over flat lands, we
defininitely see the overdevelop/cool-off/regenerate cycling. In
addition, warmer air aloft can advect or an excess of moisture at the
LCL can lead to larger scale spreadout. Still other times wave can
set up with the right wind conditions and enhance some areas which
suppressing others. All of these can turn a great day into a
difficult day in short order.

P3