Weather Patterns and Severe Storms Air Masses and

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 § c. P - cold & dry in winter and cool and dry in summer; “lake-effect” snow § m. T - warm, moist and unstable § m. P - North Pacific and Atlantic; rain and snow § c. T - least influence for NA; very hot and dry; droughtlike 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-tomoderate 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 (m. T meets c. P) § “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 doughnutshaped wall (eye wall) where greatest winds and rain are found § Weakens over land colder water (no “fuel”)

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