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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 midlatitudes • Air takes on the characteristics of the source region • Air masses migrate from their source regions, bringing changes in weather to other places Air Masses Fronts • 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 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 Warmer air displacing cooler air Move slower than cold fronts Gently sloping surface marked by stratus-type clouds Can produce extended periods of precip Cold Front and Warm Fronts Stationary Front • Nonmoving boundary between warmer and cooler air masses • Often gray skies, possibly prolonged precip 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 Drylines • • • • 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) • Large systems that travel thousands of miles over several days, bringing precipitation, sometimes severe weather • Consists of well-developed fronts circulating about a lowpressure center • Not to be confused with tropical cyclones (hurricanes) Distinctive comma-shaped cloud pattern of a mid-latitude cyclone Life Cycle of MLC’s • Begin along linear stationary polar front 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 • Distinct fronts develop and low pressure intensifies • Weather associated with fronts develops • Storm migrates in a generally eastward direction Occlusion Develops • Faster cold front begins to catch up with slower warm front Cyclone Dissipates • Occluded front grows, eventually the storm system dies out MLC Life Cycle MLC in Motion Upper-level Winds and MLC’s • Rossby waves – long waves in the flow of the midlatitude 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 • 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 • MLC’s are steered by upper-level winds, following the path of the jet stream Common Winter Storm Tracks GY205 Weather and Climate End of Lecture 5
