Where is the next thermal?

I am writing this to ask experienced soaring pilots to correct me
where you have acquired different wisdom, or your local conditions are
different, or where you have something more to add.

I hope this turns into a thread on "How I keep going" - there must be
scores of pilots more excellent than I who can say more than me.

More than 20 years ago, before I had become able to pursue soaring (as
distinguished from gliding), a letter to the editor appeared in
Soaring Magazine. A SE-WI hang-gliding pilot who'd done a lot of self-
taught XC soaring, innocently thought that SSA members might have a
lot more wisdom that he, so he asked for clues on how to find
thermals. He never got a response that was printed in the magazine...

He did tell the one thing he knew about finding thermals: "The best
lift is at the most downwind corner of the field."

This, in my own experience, is the single most important clue on where
to go when scratching low, here over the prairies. Identifying what
is "the field", where the wind's coming from, and what's "a corner"
have sometimes been challenging, and figuring out just how small hills
and valleys will affect the thermals is always interesting (hills tend
to help, valleys tend to channel them - flying downwind up a valley
above ridgetop is usually a route to a save).

I've also noticed that any isolated sorta-bare hill or rock pile on
flat ground is a pretty reliable source...

On the negative side, being anywhere downwind of any lake wider than
about a mile, or wet river flood plain, is definitely associated with
a lack of organized thermals, sometimes for miles. Forests are bad,
but the clearings in them are good. Swamps in the prairie are often
bad (wet) but in the woods often create boomers due to the
differential, and moist air is buoyant.

I am not very good at identifying the best field when they all look
good (spring planting or fall harvest). On the first cold day after a
warm spell in the fall, some fields generate thermals on overcast
days: the best seem to be cut bean fields, covered with a thin layer
of stems and leaves.

Early in the day, thermal sources tend to be small (a single field or
parking lot) and late in the day, they tend to be very large (e.g., a
3-10 mile-diameter prairie).

And at the beginning of the day, all clouds have lift (of quite
variable strength), mid-day, as few as 20% do, and late in the day,
the appearance of the cloud is a reliable indicator of whether it's
"working."

When high, the corollary to the field is of course the flat cu: the
first place to look for the thermal is at the most upwind part of the
cloud. Although again, what's a "cloud", where the wind's coming from,
and the direction of the wind above cloudbase all affect what will be
found.

Clouds with flat bases are better that clouds with fuzzy bases, except
that a new wisp always has something, and an "old" flat cloud may be
dying.

Clouds with enthusiastic tops are sucking air, and under these, the
lift is usually under the tower (useful when aiming from a distance)
and there's often a dimple in the bottom where the suction is
greatest. I was once told that streamers usually indicate condensing
lift, but I've found this usually wrong -- except when the thermal is
unusually humid.

Clouds sometimes are lined up. My experience is that the streets are
fake (but not necessarily useless) if the surface wind is less than 10
kt, and usually real above that; but that all streets end.

The hardest time for me is when I'm sort of midway between cloud and
ground. Do I fly under the best part of the cloud and look up? Or do I
fly toward the thermal source? Or, how can I reliably identify a good
intermediate point?

In this regard, I've found that thermals are curved, sometimes
strongly (if imagined in "section"). This can be seen with swamp
fires, where the smoke skims along the ground for awhile, then begins
to rise in a great slow curve to the small cu created by the humidity
released from the ground by the fire's heat. This makes physical
sense, when one stops to think about it, for the wind first whisks the
thermal bubble along horizontally (at, say, 10 mph = 900 ft/min). The
warmed air must accelerate; this rate of acceleration depends on the
buoyancy of the thermal relative to the lapse rate. If it's a boomer,
say 900 ft/min, then the angle will go from nil to 45 degrees as it
accelerates, and this feels almost vertical. But down low, below about
1500 ft agl, the clouds are sometimes more a distraction than a help
because of this. The curvature of thermals, I think, is a special
mystery when trying to fly xwind on a blue day. I, at least, often
feel mystified.

More topics, about which I can't say much:

Staying alive late in the day (e.g., "there's zero-sink over
freeways")

Identifying mountain thermals

"Good terrain" in deserts

sea-breeze and dry-front lift

other...