"Nature Guide Journal"
31 May 2001
Laying in sun-warmed grass watching clouds is a childhood pastime
well worth keeping into adulthood. Besides offering subtle clues to
weather forecasting, the lives of clouds are a fascinating atmospheric
show.
When watching the spectacle, it is helpful to remember that the air
above our heads is layered; each layer typically has different
temperatures and humidities, moving at slightly different directions and
speeds.
Think of clouds drifting across the sky as markers for the moving
layers of air. You may be able to watch several conflicting boundaries
at different altitudes, moving at slightly different directions.
Clouds are evidence of change: change that forces invisible water
vapor in an air mass to condense into small, but visible, droplets.
Particles, such as dust or pollen, serve as nuclei for the vapor's
condensation. Droplets that further coalesce into heavy-enough drops
fall as rain.
"Heap" clouds usually form at the upper boundary of a layer
when moist air punches up into a layer of relatively colder air above
it. A good example of heap clouds are "fair-weather" clouds,
with their rather flat bottoms–which mark the top of the warmer bottom
layer–and domed, lumpy tops.
You can watch a heap cloud grow as their turbulent, roiling knobs
develop and build to shape the billowy top. Most such knobs
("convection cells") have a life span of about ten minutes:
bubbling and filling out, eventually fraying into dissolving wisps as
the droplets evaporate back to vapor.
"Layer" clouds are less turbulent than heap clouds, with no
convection cells, and the resultant mass of condensed water vapor is
flatter and broader. Fog is a layer cloud that hugs the ground.
The mixing of warmer, moist air with colder air that condenses the
vapor is usually driven by ascent of the warmer air, as in wind rising
up a mountain side. Heap clouds form with localized ascent (such a
column of air rising off an especially warm spot of sun-baked earth–"fair
weather" clouds). Layer clouds form with broad-based ascent or
cooling (such as sea-cooled air gently meeting land-warmed air–summer
beach fog).
Conflicting air masses at the same altitude ("fronts") also
generate clouds: a cold front muscling under a warmer mass of air
generates heap clouds; a warm front sliding over a colder mass of air
generates layer clouds.
Clouds can be found at altitudes up to 55,000', and clouds can form
at different altitudes at the same time above a single location if
conditions between air layers warrant. "Cirrus" clouds are
those with bases above about 21,000' altitude; "altos" clouds
are those with bases between 6,500' and 21,00' altitude;
"stratus" clouds are those with bases below 6,500' altitude.
Patterns in the disturbances between layers in the atmosphere can
create beautiful patterns of clouds–waves, streaks, ripples–as the
air masses rub and shear against each other.
These evolving shapes and drifting patterns make a delightful pageant
worthy of a least a few minutes of sky-gazing.
~~~
Visit our pages on related topics:
thunderstorms
our regional climate
the windstorm of 7 February
2002
Wavecrest Discoveries can craft your personal
discovery of this delightful part of our world by customizing one of our
distinctive guided excursions. Our walks,
tours, and special
activities are wonderful ways to explore this fascinating region—and
are the perfect entertainment for guests.
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