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Lecture 5

EESA09H3 Lecture 5: Midlatitude Cyclones - Storms & Ice Storms

Environmental Science
Course Code
Tanzina Mohsin

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EESA09 Lecture 5: Midlatitude Cyclones
Con’t from last lecture…
- Primary explanation for the decline in hurricane frequence during El Nino yrs = due to increased
wind shear in environment
- El Nino yrs = wind patterns are aligned in a way that vertical wind shear increased over
Caribbean + atlantic
o Increase wind shear = prevent tropical disturbances from developing into hurrricanes
o In eastern Pacific, wind patterns are altered to reduce wind shear in atmosphere
contributing to more storms
Midlatitude Cyclones referred to as ‘low’/’low pressure’/’frontal system’ major weather makers in
midlatitudes (35-60C); most storms in S.Ontario in fall, winter, spring occur approx. every 4-7 days
- 100s-1000s km in extent (larger than hurricanes)
- Less intense winds; can have thunderstorms + tornadoes associated w/them (cold front)
o Climatology of midlatitudes = polar front region b/w polar + Ferrel cells
o Three Cell Theory
o Air Masses
Midlatitudes battle ground b/w cP (continental polar) + mT (maritime tropical)
o Division b/w air masses
Stationary Front

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Stable(lack of latent heat to fuel storm), low pressure trough
w/horizontal wind shear
Alternating blue triangles + red semi-circles
Cold Front
Cold air pushing into warm air mass; designated by blue line w/triangles
facing warm air.
Strong temp.gradient w/ change in moisture (dew point)
Shift in wind direction + Pressure change
Frontal slope 1:50 15-25 knots (7-13 m/s)
Heavy precipitation along front where mT air = forced up (cloud +
precipitation patterns)
Warm Front
Warm air pushing into cold air mass; designated by red line w/semi-
circles pointing towards cold air
Slope 1:150 1:300
Gentle precipitation (drizzle)
Occluded Front
Cold front catches up w/warm front; alternating blue triangles + red
Warm air forced above surface; warm front-style precipitation
- POLAR FRONT THEORY low pressure/cyclone = principal weather maker @ midlatitudes;
development of low pressure begins w/ small perturbation/disturbance along the polar front
(division b/w polar + ferrel cells)
o Polar front = semi-continuous boundaries sep. cold polar air from more moderate mid-
latitude air
o Mid-latitude cyclone (wave cyclone) form + move along polar front in wavelike manner
o Frontal wave, warm sector, mature cyclone, triple point, secondary low, family of
o STEPS OF THE THEORY succession of highs + lows
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