Weather Patterns
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Weather Patterns and Severe Storms Ch. 20 PowerPoint PPT Presentation

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Weather Patterns and Severe Storms Ch. 20. air mass. a large body of air that has the same temperature and humidity throughout classified according to where they originate

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Weather Patterns and Severe Storms Ch. 20

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Weather Patterns


Severe Storms

Ch. 20

air mass

  • a large body of air that has the same temperature and humidity throughout

  • classified according to where they originate

    •as it moves, the characteristics of an air mass change and so does the weather in the area over which the air mass moves.

Air Masses Are Classified by Region

Originates over the ocean

humid air mass (m) (maritime)

Originates over the land

dry air mass (c) (continental)

Originates in cold air

cool air mass(P) (Polar)

Originates in warm air

warm air mass (T) (Tropical)

Which one affects us?

  • Maritime Tropical Atlantic (mT)

  • moves northward across eastern US

  • brings mild, cloudy winters and hot humid summers with hurricanes and thunderstorms

View satellite movies of air masses moving acrossNorth America.


small balloon carried observatory that carries a radio transmitter that sends out signals about air pressure, temperature, and humidity

fronts: boundary between two air masses having different temperatures and humidity

Types of Fronts

warm front

  • forms when warm air moves into an area formerly covered by cooler air

  • brings hot, humid days and precipitation over a large area

cold front

  • forms when cold, dense air moves into a region occupied by warmer air

  • brings strong storms (squall) with clear days following

Compare and contrast warm and cold fronts.

occluded front

  • two cold masses sandwich a warm mass

  • brings strong winds and heavy rains

stationary front

the flow of air on either side of the front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front- the surface position of the front does not move


  • small-area storms formed by the strong upward movement of warm, moist, unstable air

  • always accompanied by lightning, thunder, rain and sometimes hail

  • formed from cumulonimbus clouds

Observe an animation of a thunderstorm.

Two groups of thunderstorms:

  • Air-mass thunderstorm:

    • form in warm, moist air mass

    • occur in spring and summer- last less than 1 hour

    • single, widely scattered

  • Frontal thunderstorm:

    • occur in lines along a frontal surface

    • stronger and may last several hours

    • can produce heavy rain and flooding

All thunderstorms produce lightning!

  • Lightning:

  • a discharge of electricity from a thundercloud to the ground, or cloud to cloud, or ground to cloud

  • temperature inside lightning flash can reach 28,000°C

  • at this temperature, air expands explosively-sudden expansion makes thunder!


a narrow, funnel-shaped column of spiral winds that extends downward from the cloud base and touches the ground

  • strongest winds between 360 and 500 km/hr

  • funnel less than 500 m across

  • always travel with a parent thunderstorm at speeds ranging from 40 to 65 km/hr

funnel is a mixture of clouds and dust

pressure gets lower in center

air flows toward the funnel and cools to dew point- drops form

lowering of condensation level due to low pressure causes cloud to funnel

Extremely low pressure-when it touches ground, acts like a giant vacuum

Waterspout: tornados over the water- weaker than tornados

Tornados usually occur during spring and summer and most likely occur in late afternoon

Fujita Scale: scale used for categorizing tornados

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