Weather Patterns and Severe Storms

Air Masses and Weather An air mass is an immense body of air that is characterized by similar temperatures and amounts of moisture at any given time. Carries temp and moisture conditions with it when it moves, but can change (modify) over time

Classifying Air Masses Named according to their source region Continental (land) Maritime (water) Polar (high latitudes) Tropical (low latitudes) Describe both moisture and temperature

Weather in North America cP - cold & dry in winter and cool and dry in summer; “lake-effect” snow mT - warm, moist and unstable mP - North Pacific and Atlantic; rain and snow cT - least influence for NA; very hot and dry; drought-like conditions

Front Formation When two air masses meet, they form a front, which is a boundary that separates two air masses. Often associated with some form of precipitation. When collide, pressure differences influence how the air masses will behave (one moves faster). Classified according to the temperature of the advancing front.

Warm Front Forms when warm air moves into an area formerly covered by cooler air Warm air rises (less dense) on top of colder air (more dense) Gradual slope, so light-to-moderate precipitation Gradual increase in temperature when passes

Cold Front Forms when cold, dense air moves into a region occupied by warmer air. Steep slope as cold air wedges and forces warm air aloft; moves faster than warm front Can lead to heavy downpours and severe weather Temperatures drop when a cold front passes Stationary front forms when air masses run parallel to each other

Occluded Front When an active cold front overtakes a warm front Complex weather patterns Much warm front precipitation, yet advancing cold air produces its own precipitation pattern

Middle-Latitude Cyclone Main weather producer Large centers of low pressure that generally travel from west to east and cause stormy weather Counter-clockwise air movement Most have a cold front and often a warm front Forceful lifting so much precipitation

Thunderstorms A storm that generates lightning and thunder. Frequently produce gusty, winds, heavy rain, and hail. May be produced by a single cumulonimbus cloud or clusters of along a cold front About 45,000/day Most frequent in tropics

Thunderstorm Development Form when warm, humid air rises in an unstable environment Cumulus stage: strong updrafts provide moisture Mature stage: heavy precipitation (winds, lightening, hail) Dissipating stage: falling precipitation and descending cold air from above causes storm to die down

Tornadoes Violent windstorms that take the form of a rotating column of air called a vortex which extends from a cumulonimbus cloud. 770/year in U.S. Greatest frequency April through June (mT meets cP) “Tornado Alley”

Tornado Development Typically form with severe t-storms Often begin as a mesocyclone (“rolling” air at surface pushed up from rising air) Air pressure 10% lower than surrounding air, so air near ground rushes in and spirals upward Fujita scale measures intensity

Hurricanes Whirling tropical cyclones that produce winds of at least 199 kilometers per hour Generate high waves at sea and strong winds and flooding inland Most form near equator (warmer ocean waters) Most powerful storms on Earth

Hurricane Development Fueled by energy (latent heat) when large quantities of water condense Develop most often in late summer when water temperatures very warm Begin as tropical disturbance (t-storms) As warm air rushes in to core, air turns upward and rises creating doughnut-shaped wall (eye wall) where greatest winds and rain are found Weakens over land and colder water (no “fuel”)

The bottom line… Severe weather occurs when unstable air is forced up Severe weather is seasonal Frontal wedging between cP and mT (Spring) Warm ocean waters (Summer and Fall) Isolated T-Storms (localized convective lifting in summer) Lake-effect snow (cP moves over water)