This weeks background information is available in Chapters 10.

Geography is so crucial in the way that smallscale winds develop. The diversity of America’s landscapes and meteorology creates a wide assortment of winds, spanning most of the types observed worldwide.

The friction in a fluid, such as air, is called viscosity. Viscosity comes in two scale-dependent varieties. There is friction at the smallest scales when molecules bump into each other. This happens in particular near boundaries,
such as the ground (which is a rough surface). This is called molecular viscosity. If molecular
viscosity were the only kind of friction, however, then the atmosphere from just
above the ground on up would never feel the effects of friction.

The real “friction” in the atmosphere arises from the jostling of the wind with
human-sized swirls of air, not tiny molecules. These swirls are called eddies, the same
name given to swirls of water in a stream or in the ocean. They arise in the atmosphere
when the wind blows over or around obstacles such as trees or buildings. Daytime heating
by the Sun also leads to eddies; in addition, the atmosphere naturally develops eddy
motions, especially near the Earth’s surface. At the smallest scales, the eddies themselves
lose their energy to molecular viscosity.

These invisible eddies impede the smooth flow of wind by causing slower-moving air
to mix with higher-speed air. It is similar to traffic merging onto a crowded highway: The
right-lane traffic jams up as slower cars from the on-ramp mix into the main flow of traffic.
In the same way, eddies mix air from the surface, where winds are slow, with fastermoving
air higher up. As a result, the overall wind slows down.

The atmosphere contains wind patterns at all different scales. At the smaller scales, winds are
slowed down and made irregular—turbulent—by the effect of eddies. This friction-like
process is a “brake” on the natural tendency of the pressure gradient force (PGF) to push
air from high to low pressure at all scales. At the tiniest scales, true friction—the rubbingtogether
of molecules—does take place and robs the eddies of the energy they steal from
the larger-scale wind.

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Nature doesn’t always provide a visual indicator
for turbulence. Sometimes, in nearly clear skies high up at
cruising altitude, planes will suddenly encounter the same sort
of jarring bumpiness. This is called clear-air turbulence, abbreviated
“ CAT.” It is one of a pilot’s worst nightmares.
What is clear-air turbulence? One of the main theories is
that wind shear develops its own gravity waves. These waves then rapidly break, like ocean waves on a
beach. Waves breaking on a beach generate a lot of foam; the
“ foam” of a breaking atmospheric gravity wave is turbulence,
and planes flying through it will encounter bumps and jolts.

On December 28, 1997, United Airlines Flight 826 carrying
393 people to Honolulu from near Tokyo hit heavy turbulence
over the Pacific Ocean. Passengers who happened to be wearing
their seat belts at the time described floating “like we were
in an elevator falling down,” according to the Associated
Press. Those not wearing seat belts crahsed their
heads into the cabin ceiling. One woman was killed
as a result of severe head trauma and at least 102 people were
injured, some of them seriously.

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Winds occur on every imaginable scale. Small-scale winds are
driven primarily by the pressure gradient force and are slowed
down by the effects of “friction.” The main type of friction in a
fluid such as the atmosphere is actually turbulence caused by
swirling eddies of different sizes. These eddies are stirred up by the
Sun, by wind blowing around obstacles, and by the atmosphere
itself. This causes the characteristic gustiness of winds that is frequently
observed.

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