THERMALS.Thermals are vertical convective
currents that result from local heating. They stop short
of the condensation level. Thermal convection is the
usual result of strong heating of the lower atmosphere
by the ground surface. A superadiabatic lapse rate
immediately above the ground is necessary to the
development of strong thermals. They form most
readily over areas of bare rock or sand and in particular
over sand dunes or bare rocky hills. In the presence of a
moderate or fresh breeze, especially in a hilly terrain, it
is impossible to distinguish between turbulent and
thermal convection currents. Pure thermal convection
normally occurs on clear summer days with very light
prevailing wind. In the eastern United States, dry
thermals are usually of only moderate intensity, seldom
reaching an elevation in excess of 5,000 feet above the
surface. The high moisture content of the air masses in
this section in summer reduces the intensity of surface
heating to some extent. This moisture content usually
keeps the condensation level of the surface air near or
even below a height of 5,000 feet above the ground. In
the dry southwestern part of the country, where ground
heating during clear summer days is extreme, dry
thermal convection may extend to a height of 10,000
feet or more. Under these conditions, extremely
turbulent air conditions can occur locally up to
whatever heights the thermals extend, frequently
without a cloud in the sky.
One variation of the dry thermal is seen in the dust
or sand whirls, sometimes called dust devils. They are
formed over heated surfaces when the winds are very
light. Dust whirls are seldom more than two or three
hundred feet high and they last only a few minutes at
most. Over the desert on clear hot days as many as a
dozen columns of whirling sand may be visible at once.
The large desert sand whirls can become several
hundred feet in diameter, extend to heights of 4,000 feet
or higher, and in some cases last for an hour or more.
They have been observed to rotate both anticyclonically
and cyclonically, the same as tornadoes.
An almost identical phenomenon is observed over
water in the form of the waterspout. Waterspouts occur
frequently in groups and form in relatively cool humid
air over a warm water surface when the wind is light.
The waterspout is visible due to the condensed water
vapor, or cloud formation, within the vortex. The
condensation is the result of dynamic cooling by
expansion within the vortex. In this respect it differs
from the sand whirl, which is always dry. Both the sand
whirl and the waterspout represent simple thermal
convection of an extreme type. They are not to be
confused with the more violent tornado.
When dry thermal convection extends to an
condensation level, then cumulus convection takes the
place of the dry convection. A cumulus cloud, whose
base is at the condensation level of the rising air, tops
each individual thermal current. Beneath every building
cumulus cloud a vigorous rising current or updraft is
observed. Thus the local thermal convection pattern
becomes visible in the cumulus cloud pattern. The
cumulus clouds form first over the hills where the
strongest thermals develop. Under stable atmospheric
conditions, little convective cloud development occurs.
However, under unstable conditions these thermals
may develop cumulonimbus clouds.
INDUCED OR DYNAMIC TERTIARY
There are four types of induced or dynamic tertiary
circulations. They are eddies, turbulence, large-scale
vertical waves, and Foehn winds.
An eddy is a circulation that develops when the
wind flows over or adjacent to rough terrain, buildings,
mountains or other obstructions. They generally form
on the lee (downwind or sheltered) side of these
proportional to the size of the obstruction and speed of
the wind. Eddies may have horizontal or vertical
circulations that can be either cyclonic or anticyclonic.
downwind of rough coastlines or mountain chains. An
example of a horizontal eddy is the weak cyclonic
circulation that develops in the channel off the coast of
Santa Barbara, California. The winds frequently blow
parallel to the northern California coastline during the
winter fog and stratus season. The Santa Barbara
channel often remains fog-free because the waters are
protected from winds that transport the fog inland.
However, when the winds are sufficiently strong,
friction along the tough coastal range produces a weak
cyclonic eddy over the channel. This cyclonic flow,
though weak, is sufficient to advect fog into the region.