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20,000
30,000
40,000
50,000
Altitude (Feet)
STANDARD ATMOSPHERE
Figure 9-3. Standard Atmosphere.
9-4
oxygen requirements. Layers above the stratosphere,
the mesosphere and thermosphere, have some interesting
features that are normally not of importance to
glider pilots. However, interested pilots might refer to
any general text on weather or meteorology.
SCALE OF WEATHER EVENTS
When preparing forecasts, meteorologists consider
atmospheric circulation on many scales. To aid the
forecasting of short- and long-term weather, various
weather events have been organized into three broad
categories called the scales of circulations. The size
and life span of the phenomena in each scale is roughly
proportional, so that larger size scales coincide with
longer lifetimes. The term microscale refers to features
with spatial dimensions of 1/10th to 1 nautical mile and
lasting for seconds to minutes. An example is an individual
thermal. Mesoscale refers to horizontal
dimensions of 1 to 1,000 nautical miles and lasting
for many minutes to weeks. Examples include mountain
waves, sea-breeze fronts, thunderstorms, and
fronts. Research scientists break down the mesoscale
into further subdivisions to better classify various
phenomena. Macroscale refers to horizontal dimensions
greater than 1,000 nautical miles and lasting for
weeks to months. These include the long waves in the
general global circulation and the jetstreams imbedded
within those waves. [Figure 9-4]
Smaller-scale features are imbedded in larger scale
features. For instance, a microscale thermal may be
just one of many in a mesoscale convergence line,
like a sea-breeze front. The sea-breeze front may
occur only under certain synoptic conditions, which
is controlled by the macroscale circulations. The
scales interact, with feedback from smaller to larger
scales and vice versa, in ways that are not yet fully
understood by atmospheric scientists. Generally,
the behavior and evolution of macroscale features is
more predictable, with forecast skill decreasing as
scale diminishes. For instance, forecasts up to a few
days for major events, such as a trough with an associated
cold front have become increasingly accurate.
However, nobody would attempt to forecast
the exact time and location of an individual thermal
an hour ahead of time. Since most of the features of
interest to soaring pilots lies in the smaller
mesoscale and microscale range, prediction of soaring
weather is a challenge.
Soaring forecasts should begin with the macroscale,
that is, identifying large-scale patterns that produce
good soaring conditions. This varies from site to site,
and depends, for instance, on whether the goal is
thermal, ridge, or wave soaring. Then, mesoscale
features should be considered. This may include
items, such as the cloudiness and temperature structure
of the airmass behind a cold front, as well as
the amount of rain produced by the front.
Understanding lift types, and environments in
which they form, is the first step to understanding
how to forecast soaring weather.
THERMAL SOARING WEATHER
Athermal is a rising mass of buoyant air. Thermals are
the most common updraft used to sustain soaring flight.
In the next sections, several topics related to thermal
soaring weather are explored, including thermal structure,
atmospheric stability, the use of atmospheric
soundings, and air masses conducive to thermal soaring.
General Circulation, Monsoon Circulation
Jet Stream
Occluded Cyclone, Hurricane, Front
Land/Sea Breeze, Lee Wave
Thunderstorm, Downburst
Tornado, Dust Devil, Thermal, Turbulence
Space Scale (n.m.)
10,000
1000
100
10
1
0.1
second minute hour day week month year
mesoscale
macroscale
microscale
Time Scale (Lifetime)
Figure 9-4. Scale of circulation—horizontal dimensions and life spans of associated weather events.
9-5
Convection refers to an energy transfer involving mass
motions. Thermals are convective currents and are one
means by which the atmosphere transfers heat energy
vertically. Advection is the term meteorologists use to
describe horizontal transfer, for instance, cold-air
advection after the passage of a cold front. As a note of
caution, meteorologists sometimes are careless with
the use of the word “convection” and use it to mean
“deep convection,” that is, thunderstorms.
Unfortunately, there is often a fine meteorological line
between a warm, sunny day with plenty of thermals,
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Glider Flying Handbook(107)
