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AIR MASSES CONDUCIVE
TO THERMAL SOARING
Generally, the best air masses for thermals are those
with cool air aloft, with conditions dry enough to allow
the sun’s heating at the surface, but not too dry so
cumulus form. Along the West Coast of the continental
United States, these conditions are usually found after
passage of a pacific cold front. Similar conditions are
found in the eastern and mid-west United States,
except the source air for the cold front is from polar
continental regions, such as the interior of Canada. In
both cases, high pressure building into the region is
favorable, since it is usually associated with an inversion
aloft, which keeps cumulus from growing into
rain showers or thundershowers. However, as the high
pressure builds after the second or third day, the inversion
has often lowered to the point that thermal soaring
is poor or no longer possible. This can lead to warm
and sunny, but very stable conditions, as the soaring
pilot awaits the next cold front to destabilize the atmosphere.
Fronts that arrive too close together can also
cause poor post-frontal soaring, as high clouds from
the next front keep the surface from warming enough.
Very shallow cold fronts from the northeast (with cold
air only one or two thousand feet deep) often have a
stabilizing effect along the Plains directly east of the
Rocky Mountains. This is due to cool low-level air
undercutting warmer air aloft advecting from the west.
In the desert southwest, the Great Basin, and intermountain
west, good summertime thermal soaring
conditions are often produced by intense heating from
below, even in the absence of cooling aloft. This dry
air mass with continental origins produces cumulus
bases 10,000 feet AGL or higher. At times, this air will
spread into eastern New Mexico and western Texas as
well. Later in the summer, however, some of these
regions come under the influence of the North
American Monsoon, which can lead to widespread and
daily late-morning or early afternoon thundershowers.
CLOUD STREETS
Cumulus clouds are often randomly distributed across
the sky, especially over relatively flat terrain. Under
the right conditions, however, cumulus can become
aligned in long bands, called cloud streets. These are
more or less regularly spaced bands of cumulus clouds.
Individual streets can extend 50 miles or more while
an entire field of cumulus streets can extend hundreds
of miles. The spacing between streets is typically three
times the height of the clouds. Cloud streets are aligned
parallel to the wind direction, thus they are ideal for a
downwind cross-country flight. Glider pilots can often
fly many miles with little or no circling, sometimes
achieving glide ratios far exceeding the still-air value.
Cloud streets usually occur over land with cold-air outbreaks,
for instance, following a cold front. Brisk surface
winds and a wind direction remaining nearly
constant up to cloud base are favorable cloud street
conditions. Wind speed should increase by 10 to 20
knots between the surface and cloud base, with a maximum
somewhere in the middle of or near the top of
the convective layer. Thermals should be capped by a
notable inversion or stable layer.
A vertical slice through an idealized cloud street illustrates
a distinct circulation, with updrafts under the
9-12
clouds and downdrafts in between. Due to the circulation,
sink between streets may be stronger than typically
found away from cumulus. [Figure 9-13]
Thermal streets, with a circulation like Figure 9-13,
may exist without cumulus clouds. Without clouds as
markers, use of such streets is more difficult. A glider
pilot flying upwind or downwind in consistent sink
should alter course crosswind to avoid inadvertently
flying along a line of sink between thermal streets.
THERMAL WAVES
Figure 9-14 shows a wave-like form for the inversion
capping the cumulus clouds. If the winds above the
inversion are perpendicular to the cloud streets and
increasing at 10 kts per 5,000 feet or more, waves
called cloud-street waves can form in the stable air
above. Though usually relatively weak, thermal waves
can produce lift of a 100 to 500 fpm and allow smooth
flight along streets above the cloud base. [Figure 9-14]
So-called cumulus waves also exist. These are similar
to cloud-street waves, except the cumulus clouds are
not organized in streets. Cumulus waves require a capping
inversion or stable layer and increasing wind
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Glider Flying Handbook(113)
