What Causes Our Daily Weather To Change?

Change in our weather

is a result of a change in air

masses.

What Is An Air Mass? An air mass is a large body of air in

the lower troposphere that has similar characteristics throughout.

An air mass can be several thousand kilometers in diameter and several kilometers high

The temperature and humidity are nearly uniform throughout

The temperature and humidity depend on where the air mass originates.

Where Do Air Masses Originate?

Air masses originate in source regions.

Source regions have flat, uniform composition and light winds.

How Are Air Masses Classified?

The temperature of each air mass depends on whether the air mass originates in an arctic, polar, or tropical source region.

The humidity depends on whether the air mass originates over land ( continental ) or over water ( maritime ).

Air masses are classified by temperature and humidity.

Air Mass ClassificationAir Mass Symbol Temperature &

HumidityWhere Air

Mass Forms

Area of the Country that is Impacted

Southwest and

Southeast U.S.

Northeast and Northwest

U.S.Midwest to

Eastern U.S.

Southwest U.S.

Northern U.S. in winter

Maritime Tropical

Continental Tropical

Maritime Polar

ContinentalPolar

ContinentalArctic

mT

mP

cA

cT

cP

Warm and Wet

Very Cold and Dry

Cold and Wet

Warm and Dry

Cold and Dry

Low latitudesover water

High latitudesover water

High latitudesover land

Low latitudesover land

High latitudesover land

mP – West Coast• Tends to be

unstable • Heavy rains as

cool moist air flows over mountains along west coast • mP is modified by

time it reaches interior of US, though is milder that cP• (how???)

mP – East Coast• Not as common

as west coast mP • Colder than west

coast mP• Usually brought

onshore by high pressure to the north of us and/or low pressure to the south moving up the coast.

Maritime Tropical Air Mass

West Coast East Coast• Brings showers and thunderstorms • Summer - Brings showers and

thunderstorms• Winter – snow or heavy rain

Continental Tropical Air Mass – Summer only

Continental Polar Air Mass

Types of Air Masses

Global Source Regions For Air Masses

Stability of an Airmass Changes to the stability of an air mass can

result from temperature differences between an air mass and the surface

The stability of an air mass may be shown using a third letter:

"k“ = un-stable or "w“ = stable

Air Mass Source Region – Practice Quiz

Continental Arctic

ContinentalPolarMaritime

Polar

MaritimePolar

ContinentalTropical

MaritimePolar

MaritimeTropical

MaritimeTropical

1.

2.3.

4.

6. 8.

7.

5.

Weather Patterns• Weather patterns are caused by the

movement of air masses and what happens when different air masses collide.• Front – boundary between two air masses.• Air masses meet but do not mix.

The boundary line between the offensive and

defensive lines would be a front.

Front

Great Air

Mass

Poor AirMass

Name Formation Clouds Weather Symbol

•Wind, heavy rain•Thunderstorms•Lightning•Colder air after front passes

•Clouds then gentle, steady rain arriving before the front•Steady rain for days along front if stationary too long•Strong winds and heavy rains

Cold•Altocumulus•Cumulonimbus

Warm

Stationary

Occluded

Air masses stop moving

•Cirrus•Cirrostratus•Altostratus•Nimbostratus•Cirrus•Cirrostratus•Altostratus•Nimbostratus

•Cumulus•Cumulonimbus

Fronts Cold Front: boundary between

advancing cold air mass & a warmer air mass it is displacing Rising warm air usually produces

precipitation if wet Air becomes colder after front passes

Fronts Warm Front: boundary between

advancing hot air mass & a colder air mass it is displacing 1st clouds days in advance, then RAIN Air becomes warmer after front passes

Fronts Occluded Front: when cold front ‘catches

up’ to a warm front, producing clouds & precipitation

Fronts Stationary front: when a front stops

moving forward, producing clouds & precipitation – causes floods if stationary too long

The Life Cycle Of A Cyclone (Low Pressure System)• The United States often experiences weather due to

the boundary between the cold air masses of the Polar Easterlies and the warm air masses of the Prevailing Westerlies. This arrangement results in a weather conditions called a Traveling Cyclone.• The formation of a cyclone

is called "cyclogenesis". Cyclones form in areas of low pressure.

The Life Cycle Of A Cyclone (Low Pressure System)•The process begins when two air masses of different densities and temperatures move in opposite directions along a polar front resulting in a large temperature gradient.

The Life Cycle Of A Cyclone (Low Pressure System)•Cyclones are the result of instability forming along the polar front, resulting in a wave developing between the cold and warm air masses. The fronts are separated, and the warm front begins to move north as the cold front begins to move south

The Life Cycle Of A Cyclone(Low Pressure System)

•The pressure at the center of the cyclone continues to get lower as the cyclone continues to form. Clouds ahead of the warm front begin to precipitate over a large area of surface, and clouds along the cold front begin to precipitate over a narrow stretch [along the cold front]. As the pressure is lowered, wind speeds increase.

The Life Cycle Of A Cyclone ( Low Pressure System)

•The cold front continues to move eastward, speeding to overtake and undercut the warm front. This is called occlusion. It is at this point which the cyclone is at its peak. The area of the warm sector is decreasing.

The Life Cycle Of A Cyclone (Low Pressure System)

•By the time the cold front completes occlusion with the warm front, the air in the warm sector is lifted upward as the cold air replaces it at the surface. The system is stabilizing.

Weather Forecasting

Satellites

Radiosondes

Surface observations

Station Model

Surface Weather Map

Locating a Front Wind direction changes Temperature changes sharply Dew Point changes sharply

Forecasting Computer models take current data & plug it

into equations to predict weather

Meteorologists take computer models & tweak them to fit their experience with local conditions

ForecastingTrend Method: using past movement of a front &

precipitation to predict future movement

Practice reading a weather map