AIR MASSESAIR MASSES

Effects of EarthEffects of Earth Earth is a sphere uneven heating

convection currents world-wide wind patterns

Rotation of Earth Coriolis Effect wind direction changes to the right (northern hemisphere) and to the left (southern hemisphere)

Tilt of Earth seasons change as move through orbit

Air is usually in motion(Convection and the Coriolis effect)

BUT

Large portions of air often remain in nearly the same place

long enough to take on the temperature and moisture

characteristics of the land or ocean below

These characteristics vary greatly

Differences between the land and ocean can have a significant influence on the properties of the air above

When a large portion of the air is nearly uniform in temperature and humidity, it is considered an air mass

ClassificationAir masses can be classified due to

where on the planet they form -1. The amount of moisture in the air

continental or maritime

(depends on location – over land or ocean)

2. The temperature of the air polar (cool) tropical (warm)

Air masses are not stationaryAir masses are not stationary

Often move according to prevailing Often move according to prevailing wind patternswind patterns

Air masses can interact as they move Air masses can interact as they move and even collide= weather.and even collide= weather.

Highs, Lows, and FrontsHighs, Lows, and Fronts

Frontal SystemsFrontal Systems ““An air mass is a parcel of air that spends An air mass is a parcel of air that spends

enough time over a region to take on the enough time over a region to take on the temperature and humidity of the region.”temperature and humidity of the region.”

Can remain in the same place for some Can remain in the same place for some timetime

Eventually moves due to worldwide wind Eventually moves due to worldwide wind currentscurrents

The zone b/w interacting air masses is The zone b/w interacting air masses is called a frontcalled a front

There are 4 general types: There are 4 general types: Warm Warm ColdCold Stationary Stationary Occluded Occluded

Cold FrontsCold Fronts1. Cold air mass overtakes a

warm air mass2. Cold air is more dense3. Moves underneath, pushing

warm air upwards4. Warm air rises rapidly

forming a cumulus cloud5. Vapour condenses

If this occurs long enough, a cumulonimbus cloud is created

Rain often comes with a cold front

If the cold front is fast moving intense thunderstorms may develop sometimes bringing hail and tornadoes

Warm FrontsWarm Fronts Warm air mass overtakes a cooler oneWarm air mass overtakes a cooler one Warm air gently rides up over the trailing end of the cold air massWarm air gently rides up over the trailing end of the cold air mass Stratus clouds usually form (altostratus and cirrus clouds form as Stratus clouds usually form (altostratus and cirrus clouds form as

warm air climbs higher)warm air climbs higher) Nimbostratus clouds form close to the front bringing light rain or snowNimbostratus clouds form close to the front bringing light rain or snow Precipitation may last longer than with a cold frontPrecipitation may last longer than with a cold front Once it passes, the air is left warmer than before the front arrivedOnce it passes, the air is left warmer than before the front arrived

STATIONARY FRONTSSTATIONARY FRONTS Two air masses Two air masses

(warm/cold) in contact (warm/cold) in contact but neither is movingbut neither is moving

Some warm air rises Some warm air rises and condenses and condenses creating stratus clouds creating stratus clouds and often rainand often rain

The front may remain The front may remain this way for several this way for several daysdays

= dreary, gloomy sky= dreary, gloomy sky

OCCLUDED FRONTSOCCLUDED FRONTS A faster moving cold A faster moving cold

front catches up to a front catches up to a slower moving warm slower moving warm frontfront

All the warm air is All the warm air is pushed up, away from pushed up, away from the ground, contacting the ground, contacting the leading cold air the leading cold air massmass

Many types of clouds Many types of clouds are present, one after are present, one after anotheranother

Often cause a period of Often cause a period of steady precipitationsteady precipitation

Investigating Weather Fronts

High Pressure SystemsHigh Pressure Systems Any region where air is descending (putting more Any region where air is descending (putting more

pressure on the region below) is a HIGH PRESSURE pressure on the region below) is a HIGH PRESSURE SYSTEM.SYSTEM.

An air mass cooled by conduction and convection An air mass cooled by conduction and convection when it forms over cold ground becomes more when it forms over cold ground becomes more dense and settles lower to the grounddense and settles lower to the ground

Creates more pressure, continues to settle, draws Creates more pressure, continues to settle, draws more air from abovemore air from above

Air is also pushed out toward low pressure areas at Air is also pushed out toward low pressure areas at the perimeter in all directions the perimeter in all directions

This air curves to the right due to the Coriolis EffectThis air curves to the right due to the Coriolis Effect = ANTICYCLONE = clockwise rotation of air due to = ANTICYCLONE = clockwise rotation of air due to

the high pressure cellthe high pressure cell

High pressure systems create a High pressure systems create a clear clear skysky (since air is descending not rising) (since air is descending not rising)

Air contains little moisture due to cold Air contains little moisture due to cold in upper atmosphere = in upper atmosphere = Clouds do not Clouds do not formform

High pressure systems are usually High pressure systems are usually located within a single air masslocated within a single air mass

May be May be 100’s of km100’s of km across across Due to size, weather conditions are Due to size, weather conditions are

stablestable Prevents most turbulencePrevents most turbulence

Low Pressure SystemsLow Pressure Systems1. Very intense heating of the ground 1. Very intense heating of the ground

heats air above it (conduction) heats air above it (conduction) Air rises, pulling more, less dense air in beneath itAir rises, pulling more, less dense air in beneath it Air curves to the right (Coriolis Effect) so rising air Air curves to the right (Coriolis Effect) so rising air

rotates = CYCLONE (in the N.hemisphere rotates rotates = CYCLONE (in the N.hemisphere rotates counterclockwise)counterclockwise)

2. Warm air rises in a frontal system 2. Warm air rises in a frontal system creating low pressure systems alsocreating low pressure systems also

Generally smaller than high pressure systemsGenerally smaller than high pressure systems Likely to occur b/w air massesLikely to occur b/w air masses Bring unstable weather conditions (cause Bring unstable weather conditions (cause

precipitation and often storms)precipitation and often storms)

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1902/es1902page01.cfm?chapter_no=visualization