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GY205 Weather and Climate. Lecture 5 (Please turn in homework on the table). Air Masses. Very large bodies of air with fairly uniform temp and humidity characteristics Form in high and low latitudes, not mid-latitudes Air takes on the characteristics of the source region

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gy205 weather and climate

GY205 Weather and Climate

Lecture 5

(Please turn in homework on the table)

air masses
Air Masses
  • Very large bodies of air with fairly uniform temp and humidity characteristics
  • Form in high and low latitudes, not mid-latitudes
  • Air takes on the characteristics of the source region
  • Air masses migrate from their source regions, bringing changes in weather to other places
  • Fronts are boundaries between air masses
  • Cold air is more dense than warm air, they don’t easily mix
  • Colder air will stay on the surface, warmer air will be forced above cooler air
  • Adiabatic cooling of the warm air can lead to clouds and precip

Map symbols

cold front
Cold Front
  • Cooler air displacing warmer air
  • Fastest moving of fronts
  • Steep frontal surface marked by cumuliform clouds
  • Can produce short-lived heavy precip, t-storms
warm front
Warm Front
  • Warmer air displacing cooler air
  • Move slower than cold fronts
  • Gently sloping surface marked by stratus-type clouds
  • Can produce extended periods of precip
stationary front
Stationary Front
  • Nonmoving boundary between warmer and cooler air masses
  • Often gray skies, possibly prolonged precip
occluded front
Occluded Front
  • An occluded front forms when a cold front catches up with a warm front
  • Warm air is cut off from the ground
  • Often produces heavy precip
  • Boundary between humid and drier air
  • Humid air is less dense, will be lifted by advancing drier air
  • Form during spring and summer in the southern Great Plains
  • Produce t-storms, often tornadoes
mid latitude cyclones mlc s
Mid-latitude cyclones (MLC’s)
  • Large systems that travel thousands of miles over several days, bringing precipitation, sometimes severe weather
  • Consists of well-developed fronts circulating about a low-pressure center
  • Not to be confused with tropical cyclones (hurricanes)

Distinctive comma-shaped cloud pattern of a mid-latitude cyclone

life cycle of mlc s
Life Cycle of MLC’s
  • Begin along linear stationary polar front
cyclogenesis a storm is born
Cyclogenesis (A Storm is Born)
  • A disturbance (topographic or from upper-level winds) along the polar front causes a bend, or wave, to form
  • Warm air pushes north, cold air pushes south, starting counterclockwise rotation
mature cyclone
Mature Cyclone
  • Distinct fronts develop and low pressure intensifies
  • Weather associated with fronts develops
  • Storm migrates in a generally eastward direction
occlusion develops
Occlusion Develops
  • Faster cold front begins to catch up with slower warm front
cyclone dissipates
Cyclone Dissipates
  • Occluded front grows, eventually the storm system dies out
upper level winds and mlc s
Upper-level Winds and MLC’s
  • Rossby waves – long waves in the flow of the mid-latitude Westerlies, formed by air flow along troughs and ridges
  • Air flow along Rossby waves produces high pressure (anticyclones) and low pressure (cyclones) at the surface
Air entering a trough produces upper-level convergence, pushing air downward to create high pressure at the surface
  • Air leaving a trough produces upper-level divergence, drawing air upward to create low pressure at the surface
weather associated with mlc s
Weather Associated with MLC’s
  • As a mid-latitude cyclone migrates eastward, it brings changes in the weather along its fronts
  • Clouds and precip move through, winds shift, pressure changes, temps rise then fall
paths of mlc s
Paths of MLC’s
  • MLC’s are steered by upper-level winds, following the path of the jet stream