Chapter 20: Weather Patterns and Severe Storms

Air Masses and Weather Air masses: large bodies of air that move similar temperatures and amounts of moisture at any given altitude area experiences constant weather as it moves, characteristics of air mass change and weather in the area changes

Classifying Air Masses Named according to source region Polar (P) masses (cold) form at high latitudes toward Earth’s poles Tropical (T) masses (warm) form at low latitudes Continental (c) masses (dry) form over land Maritime (m) masses (humid) form over water

4 Types of Masses continental polar (cP): dry and cool continental tropical (cT): dry and warm/hot maritime polar (mP): colder maritime tropical (mT): hotter

Weather in North America Influenced by continental polar (N. Canada, Alaska, and Arctic) and maritime tropical (Gulf of Mexico, Caribbean, or Atlantic) air masses

Continental polar air masses (cP) cold and dry in winter cool and dry in summer not associated with heavy precipitation lake-effect snow localized storms large difference in temperatures between land and water

Maritime polar air masses (mP) come from North Pacific accompanied by low clouds and showers uplift produces heavy rain or snow Maritime tropical air masses (mT) warm with moisture usually unstable source of most precipitation

Continental tropical air masses (cT) least influence on weather hot and dry begins in SW and Mexico during summer can cause drought-like conditions

Formation of Fronts Front: a boundary that separates 2 air masses occurs when 2 air masses meet narrow (15-200 km wide) Warm, less dense air overlies cooler, denser air

Types of Fronts Classified according to temperature of advancing front

Warm Front warm air moves into area formerly covered by cooler air Diagram: warm air moves into area formerly covered by cooler air represented by red arrows and semicircles point toward cooler air low, gradual slope as warm air rises, it cools to produce clouds and precipitation slow rate of movement gradual increase in temperature

Cold Front cold, dense air moves into region of warmer air shown by blue line with blue triangles point toward warmer air becomes steeper as it moves moves rapidly more violent weather temperatures drop winds shift Diagram:

Stationary Front position of front does not move shown by blue triangles on 1 side and red semicircles on the other Diagram: can produce gentle to moderate precipitation Diagram:

Occluded Front active cold front overtakes warm front cold air wedges warm front upward Diagram:

Middle-Latitude Cyclones main weather producers represented by “L” large centers of low pressure gradually travel from west  east in counterclockwise direction warm air moves toward poles cool air moves toward equator cause stormy weather cold front pushes warm air up, strengthening storm weakens pressure stops cyclone

Role of Airflow Aloft maintains cyclonic and anticyclonic circulation air high up in atmosphere fuels middle-latitude cyclone surface air that feeds cyclone generally originates as air flowing out of an anticyclone cyclones and anticyclones found next to each other

Thunderstorms

Thunderstorms generates lightning and thunder produce gusty winds, heavy rain, and hail can be produced by a single cumulonimbus cloud or clusters of them can influence small area or km wide along a cold front

greater numbers in the tropics: warm, moist, and unstable 45,000 a day 16 million a year in the world 100,000 a year in the US form when warm, humid air rises in an unstable environment

Cumulus Stage strong updrafts or upward movement of air cloud grows vertically

Mature Stage amount and size of precipitation too great for updrafts to support heavy precipitation is released from cloud most active stage gusty wind, lightning, heavy precipitation, and hail

Dissipating Stage downdrafts/downward movement of air cooling effect of falling precipitation flowing in of cooler air storm dies down

lifespan 1-2 hours if storm moves, it can regenerate

Tornadoes

violent windstorms as a rotating column of air called a vortex extends downward from a cumulonimbus cloud 770 a year in the US can occur at any time during the year greatest from April to June rare in December and January

most form with severe thunderstorms development of mesocyclone vertical cylinder of rotating air develops in updraft of a thunderstorm

strong winds high up cause winds lower in atmosphere to roll strong updrafts cause rolling to tilt low pressure within tornado air near ground rushes in and spirals upward around core

Tornado Safety tornado watches: alert to possibility in a specific area for a time period tornado warnings: actually been sighted in an area of indicated by radar

Hurricanes

whirling tropical cyclones produce winds of at least 119 km/hr called typhoons, cyclones, and tropical cycles outside the US

most powerful storms on Earth most form between 5° and 20° N and S latitudes formed/fueled by energy given off when huge amounts of the water vapor condense develop mostly in late summer water temperatures warm enough to provide heat and moisture to the air most powerful storms on Earth most form between 5° and 20° N and S latitudes formed/fueled by energy given off when huge amounts of the water vapor condense develop mostly in late summer water temperatures warm enough to provide heat and moisture to the air

begins as tropical disturbance disorganized clouds and thunderstorms low pressures little or no rotation

inward rush of warm, moist air moves toward core turns upward and rises to cumulonimbus clouds surrounds center of storm (eye wall) greatest wind speeds heaviest rainfall

Eye center of storm no rain or wind warmest part of the storm

Storm Surge dome of water 65-80 km wide sweeps across coast where eye moves onto land

weakens when it moves over cool ocean water or over land Saffir-Simpson Scale: describes intensity (range from 1-5)