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Reading the Clouds |
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Compliments of
Environment Canada |
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On most storm days,
small cumulus grow gradually larger, group together, decrease in number
and then exhibit signs of a breakout as taller, larger towers rise here
and there. This pacing of cloud growth gives you time to observe cloud
evolution. Occasionally, the atmosphere has a stable layer in place,
preventing strong convection and sometimes even small cumulus until
later in the afternoon. When heating reaches the critical point or when
cooling aloft erodes the stable layer, it breaks down rather suddenly,
triggering explosive cloud development. In such cases, the day's
forecast, patience and a wary eye are all you can depend on to prepare.
Storms which break out in this abrupt fashion are more likely to be
severe because the bottled-up heat and moisture are available for an
immediate and concentrated release. |
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Pictorials 21-26
detail the sequence of a developing severe storm. |
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A large bank of
cumulus in 21 becomes a single, focused updraft in 22 (20 minutes later)
in the first breakout phase. Tops are hard and crunchy, but the new
anvil (left) is still irregular. By 23, only 5 minutes later, a very
high tower signals the burst phase. A minute later in 24, the burst has
initiated a steady stream of close updraft pulses which form a
persistent flaring crown 3 min. later in photo 25. Now, the anvil has
become more uniform and solid. The new growth is shifting the storm to
the right of the upper winds from the SW. In photo 26 4 minutes later),
an overshooting top over 39 Kft high persists above the anvil edge and
this storm is now steady-state severe. To its right is the large flange
formed by the spreading of the burst phase. Once begun (in 22) the
change from small Cumulus to severe storm took less than 15 minutes! |
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Assessing the scene - does
the storm have any of these features? |
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Feature |
what you see |
this implies |
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Longevity |
long, flowing anvil |
able to regenerate and
persist |
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Size |
large, solid cloud mass
large, dark rain curtain |
strong updrafts
large core, very heavy precipitation |
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Growth Rate |
boiling cloud tops
backside of cloud almost vertical |
very rapid growth, sustained
updrafts
extreme growth, may be breaking out |
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Organization |
sharply defined flanking
line
nearby clear sky (west-south)
overall compact appearance
cloud bands/lines leading to storm |
sustained inflow separate
from outflow
system has control over nearby flow
controlled, focused inflow |
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Structure |
evidence of lowering
overshooting tops above anvil |
focused, intense updraft
intense updrafts, very tall storm |
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