ATOC 181 Lecture Notes - Lecture 18: Tropopause, Occluded Front, Squall
7 Jun 2018
School
Department
Course
Professor
LESSON 18
MID-LATITUDE STORMS (or LOW or CYCLONE)
• Not affect the tropics much, but affect us directly.
• Not the same thing as extra-tropical cyclones
• Tropical cyclone vs tropical storm = different
Mid-Latitude Storms
• Have different stages of development
• Affect Northern Hemisphere (mid-latitudes, higher
latitude) and in South Hemisphere from 60o S
• Found there due to general circulation of
atmosphere around the Earth
• Have the polar fronts where the winds are going
towards the East (easterlies): region where these
storms will stop
• Westerlies: moving towards the West
• Around the Equator where the earth receives
maximum of heat, air is going up, have convective
clouds. At the junction of ferret cell and polar cell, the
air is ascending and have lots of convective clouds
• A front should be considered in 3-D.
General circulation: Jet streams
• Polar front rotates around 60oN, can move North and
South and can also have discontinuity
• Have subtropical jet that rotates around Earth (30o N)
• Intertropical convergence zone
• Along polar front, air is ascending again.
• Front: is a discontinuity also in the vertical
• Have polar jet on top of the front, is located ahead of the
front of the surface
• The polar front represents a cold frontal boundary that
separates colder from warmer at the surface and aloft
• Warmer in front of the front and warmer in the back. Polar jet is
located in the warm sector
• Polar jet: strong winds in altitude, 100-150km/h, used by airplanes.
When travels towards the East, use the polar jets to save fuel as
pushed by polar jets. From Europe to here, longer flight time.
• Polar front is the whole surface in the vertical, not just the
intersection at the surface
Life Of A Mid-Latitude Cyclones
• Polar front is fairly linear. No waves. In the back, the winds are easterlies and in the front, the winds are
westerlies.
• Stationary polar front separates cold easterlies and warmer westerlies.
find more resources at oneclass.com
find more resources at oneclass.com
• Developing stage: have wave-like distortion which can be created either by a surface --if have regions
with different temperature, have some disturbances near surface, have mountains--or can be created
by jet streams aloft. Happen at intersection of the two winds.
o Warm air ahead of location moves poleward.
o Also creates cyclonic rotation, so whole system of air starts to move.
o The pressure will decrease.
o If have jet on top, it will create a divergence, and the air tends to climb.
o Can last one day or half a day
• Mature stage: low pressure centre forms fronts with clouds and precipitation. Air is ascending and
deeper low-pressure centre.
o Cold air tends to move towards the South/Equator,
always faster than warm air that also moves polewards
o Warm air will be replaced by cold air and vice versa and
the whole system moves
o As the whole system moves, the air in centre moves up
→ creation of clouds → precipitation
o More stratus clouds in front of the surface front. Ahead
of front, have hail, thunderstorms, Cumulonimbus with
anvil in cold front. The cold front moves faster.
o Have well-developed surface fronts
o Cold front: depicted by blue triangles
o Warm front: depicted by red semi-circles
Fronts
• Transition between 2 air masses of different densities
• Density is what trigger an instability. If one is heavier than the other,
the heavier one tends to go up, while the less dense one tends to
rise
• Associated with:
o Sharp temperature change
o Sharp change in dew point: amount of humidity in the air (more humid in warm sectors)
o Shift in wind direction
o Sharp pressure change
o Clouds and precipitation
Cold Front
• Goes up to tropopause
• Steep slope
• Goes up very fast with lot of E
• Have Cb go up tropopause and spread along
tropopause. Have Cs and Ci clouds
• Cold, dry stable air replaces warm, moist unstable
air.
• Warm air is pushed aloft
• Clouds of vertical development: Cu, Cb. Get precipitation in form of hails, thunderstorms
• Thunderstorms, squall lines
find more resources at oneclass.com
find more resources at oneclass.com
