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Thermals
How Thermals Work
By
Colin Cutler
If you're flying a glider,
you're always sinking. And the only way to stay aloft is to find
air that's going up faster than you're going down.
So where is the fast-rising air?
Glider Fuel
There
are three types of lift that power gliders: thermals, ridge lift, and
mountain wave.
But the most common by far are
thermals. And because thermals rise relatively quickly, they're the perfect
spot for gliders to hang out.
How Thermals Form
NOAA
research scientist Wayne Angevine sums up thermals by saying that "thermals are like fat trees, with
small, chaotic roots near the surface and large trunks above. The trees tilt
and sway with the wind and change with time, and sometimes they let go of their
roots and drift."
So to start, thermals begin with
two ingredients: the sun, and the ground. As the sun warms the ground, the
ground warms the air directly above it. And that's where the thermal starts.
Spots where there's a lot of surface
heating is usually the most common location for thermals. So places like
asphalt parking lots, junk yards, and rock outcroppings are great places for
thermals to form.
As the air above these spots starts
to heat, small plumes of warm air begin to rise.
Think of it like bubbles rising up from the bottom of a champagne
glass (cheers!). As the small plumes rise, they spin around randomly, and
eventually, they start bumping into each other.
The plumes continue to bump into each other as they rise, turning into
larger and larger blobs. Eventually, as they leave the surface layer (100-200
meters AGL, or roughly 300-600 feet), they're a full-blown thermal.
As the thermals rise, they twist and
flow with the wind. They typically rise at 1-3 meters per second - which
computes to about 200-600 feet per minute.
But they don't keep rising forever.
Some thermals only last for a matter of seconds, while others can last up to
10-20 minutes.
As thermals continue to rise, they
cool, and eventually, they reach the same (or slightly cooler) temperature as
the surrounding air. And this usually happens at a place called the boundary
layer.
The boundary layer is the part of the
atmosphere that's affected by the earth's surface.
It's typically very shallow at night
(100-200 meters), and grows throughout the day (500-2000 meters), as the day progresses
and the ground is warmed by the sun.
When a thermal hits the boundary
layer, it typically flattens and spread out.
And the boundary layer is usually
pretty identifiable, because the dust that the thermals are carrying up with
them stop rising, which is why you see the recognizable dust/haze layer from
the air.
Not Everything Is Rising
As
thermals columns are rising, the air outside the columns is cooler, and more
dense, which means it's sinking.
But since the sinking air usually
covers more area (and has more mass), it sinks at a slower rate than the rising
thermal columns.
The Best Areas To Find Thermals
The
best area to find thermals are dark areas that absorb energy from the sun, and
rapidly heat the air directly above them.
Dark fields and parking lots are
perfect places for thermals to form. Because they heat up quickly, they rapidly
heat the air directly above them, creating strong thermals.
But there's another place that's great for creating
thermals, especially early in the day. And that's mountain ridges.
Ridges create strong thermals early in the day,
because they have a more direct angle to the sun than the ground does. Boulder,
CO, is a perfect example of this. When the sun rises, it shines directly on the
Flatirons ridge line, quickly warming it, and creating incredible amounts of
lift.
The Worst Areas To Find Thermals
The
worst areas to find thermals are typically wet spots.
When there's a lot of moisture on the
ground or in the air, a huge amount of energy is absorbed through evaporation.
And when energy is absorbed by the 'wet' air, there's little to no energy left
to heat the air and make it rise as a thermal.
One of the worst places for thermals
is in something called a 'blue hole.' They aren't exactly like black holes in
space, sucking up everything in their path, but they're pretty close.
Blue holes exist above lakes and
other large bodies of water. Above blue holes, the air is cool, which means
it's sinking.
You can easily tell where a blue hole
is on a cloudy day, because you'll see a large gap in the clouds. And where
there are no clouds, there's no lift.
Putting It All Together
Thermals
are created by the sun heating the ground, and the ground warming the air above
it. As small plumes of warm air rise, they group together and form thermals,
and make the perfect spot for gliders to fly and stay airborne.
So the next time you're flying and
you feel a little bump as you cross a dark field or parking lot, just remember
that while it might not do much for your powered airplane, if you were in a
glider, you could hang out there all day.
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