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Recognizing Incoming Weather


Weather forecasts are simply educated estimations or deductions based on general scientific weather principles and meteorological evidence. Forecasts based on past results may or may not be accurate. However, even limited experience in a particular mountainous region and season may provide local indications of impending weather patterns and increased accuracy.




Bad Weather

Signs of approaching bad weather (within 24 to 48 hours) may include...

  • A gradual lowering of the clouds. This may be the arrival or formation of new lower strata of clouds. It can also indicate the formation of a thunderhead.
  • An increasing halo around the sun or the moon.
  • An increase in humidity and temperature.
  • Cirrus clouds.
  • A decrease in barometric pressure (registered as a gain in elevation on an altimeter).

Storm Systems

The approach of a storm system is indicated when...

  • A thin veil of cirrus clouds spreads over the sky, thickening and lowering until altostratus clouds are formed. The same trend is shown at night when a halo forms around the moon and then darkens until only the glow of the moon is visible. When there is no moon, cirrus clouds only dim the stars, but altostratus clouds completely hide them.
  • Low clouds, which have been persistent on lower slopes, begin to rise at the time upper clouds appear.
  • Various layers of clouds move in at different heights and become abundant.
  • Lenticular clouds accompanying strong winds lose their streamlined shape, and other cloud types appear in increasing amounts.
  • A change in the direction of the wind is accompanied by a rapid rise in temperature not caused by solar radiation. This may also indicate a warm, damp period.
  • A light green haze is observed shortly after sunrise in mountain regions above the timberline.

Thunderstorms

Indications of local thunderstorms or squally weather are...

  • An increase in size and rapid thickening of scattered cumulus clouds during the afternoon.
  • The approach of a line of large cumulus or cumulonimbus clouds with an "advance guard" of altocumulus clouds. At night, increasing lightning windward of the prevailing wind gives the same warning.
  • Massive cumulus clouds hanging over a ridge or summit (day or night).

Strong Winds

Indications of approaching strong winds may be...

  • Plumes of blowing snow from the crests of ridges and peaks or ragged shreds of cloud moving rapidly.
  • Persistent lenticular clouds, a band of clouds over high peaks and ridges, or downwind from them.
  • A turbulent and ragged banner cloud that hangs to the lee of a peak.

Fair Weather

Signs of approaching fair weather include...

  • A gradual rising and diminishing of clouds.
  • A decreasing halo around the sun or moon.
  • Dew on the ground in the morning.
  • Small snowflakes, ice crystals, or drizzle, which indicate that the clouds are thin and fair weather may exist at higher elevations.
  • An increase in barometric pressure (registered as a loss in elevation on an altimeter).

Continued fair weather may be associated with...

  • A cloudless sky and shallow fog, or layers of haze at valley bottoms in early morning.
  • A cloudless sky that is blue down to the horizon or down to where a haze layer forms a secondary horizon.
  • Conditions under which small cumulus clouds appearing before noon do not increase, but instead decrease or vanish during the day.
  • Clear skies except for a low cloud deck that does not rise or thicken during the day.
Source: http://www.globalsecurity.org/military/library/policy/army/fm/3-97-6/index.html

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