20.1 Air Masses

An immense body of air that is characterized by similar temperatures & amts. of moisture at any altitude

As air masses move the characteristics of an air mass change and so does the weather in the area over which the air mass is moving.

Air Masses & Weather

Source region – the area over which an air mass gets its characteristics of temperature & moisture

Named according to their source regionIn addition to their overall temperature, air

masses are classified according to the surface over which they form

Classifying Air Masses

Continental Polar (cP) – dry coolContinental Tropical (cT) – dry warm or hotMaritime Polar (mP) – form over water, coldMaritime Tropical (mT) – form over water,

warm or hot

4 Basic Types of Air Masses

Much of the weather in N. America, esp. weather E of the Rockies, is influenced by continental polar (cP) & maritime tropical (mT) air masses

Weather in North America

AIR MASS FOLDABLE

20.2 Fronts

When 2 air masses meet, a front is formed. Front is a boundary that separates 2 air

masses

Fronts can form btw any 2 different air masses

Fronts are often associated with

precipitation

Formation of Fronts

Classified according to the temperature of the advancing front

4 types: Warm, Cold, Stationary, Occluded

Warm FrontForms when warm air moves into an area

formerly covered by cooler airRed line with red semicircles that point toward

the cooler air

Types of Fronts

Cold FrontForms when cold, dense air moves into a

region occupied by warmer airShown by blue line edged with blue triangles

that point toward the warmer airAdvance quicker than warm fronts do

Continued…

Stationary FrontThe flow of air on either side of a front is

neither toward the cold air mass nor toward the warm air mass. It is about parallel to the line of the front

In that case the front does not move

Continued…

Occluded FrontWhen an active cold front overtakes a warm

frontDevelops as the advancing cold air wedges the

warm front upwardWeather associated is typically complex

Continued…

Low pressures are shown by the letter LMiddle-latitude cyclones are big centers of

LP that generally travel W to E and cause stormy weather

Air moves in a counter-clockwise direction & in towards the center

Pg. 567-68 How does a Cyclone form?Bring on stormy weather

Middle Latitude Cyclones

Plays an important role in maintaining cyclonic & anticyclonic circulation.

More often than not, air high up in the atmosphere fuels a middle latitude cyclone

The Role of Airflow Aloft

20.3 Severe Storms

A storm that generates lighting & thunderProduce gusty winds, heavy rain, & hailForms from a single cumulonimbus cloud and

only affects a small area OR by a cluster of cumulonimbus clouds and impact a larger area

US experiences about 100,000 a yearForm when warm, humid air rises in an

UNSTABLE environment

Thunderstorms

Violent windstorms that take the form of a rotating column of air called a vortex. The vortex extends downward from a cumulonimbus cloud

Form in association w/ severe thunderstorms (developing a mesocyclone)

Mesocyclone – a vertical cylinder of rotating air that develops in the updraft of a thunderstorm

SPC (Storm Predication Center) monitors different kinds of severe weather

Tornadoes

Whirling tropical cyclones that produce winds of at least 119 km per hr.

Most form btw 5 – 20 degrees N & S latitude Develop most often in the late summer when

water temp. are warm enough to provide the necessary heat & moisture to the air

The center is known as the eyeCan cause storm surgesWeakens when it moves over cool oceans AND

even greater when it moves over landCategory 1-5

Hurricanes