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 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 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 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 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 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