FRONTS

Chapter 12

This week onwards

• Air masses

• What are fronts

• Different types of front

• Weather associated with fronts

• Mid-latitude Cyclones– Weather and cyclones

Synoptic Scale - Air Masses

• Air Mass

– A huge volume of air that is relatively uniform horizontally in temperature and water vapor concentration.

– Properties of an air mass are usually defined by the type of surface it develops over ---- the source region.

Air Masses

• Source Regions

– Relatively flat

– Uniform Surface Compositions

• Oceans

• Great expanse of snow covered ground

• Air needs to be in contact with the source region for a long period of time to develop uniform characteristics.

• Temperature– Cold Polar (P)– Warm Tropical (T)

• Moisture– Dry Continental (c)– Moist Maritime (m)

• Results in four basic air mass types.

Classification of Air Masses

Classification of Air Masses

• Continental Tropical (cT)– Hot, Dry– Develops over the deserts of Mexico and

the Southwestern United States

• Maritime Tropical (mT)– Warm, Humid– Develops over the tropical and subtropical

oceans and the Gulf of Mexico

Classification of Air Masses

• Continental Polar (cP)– Cold, Dry– Develops over the northern interior of

North America -- Central Canada

• Maritime Polar (mP)– Cold, Moist– Develops over the cold ocean waters of the

North Pacific and North Atlantic

Classification of Air Masses

• Arctic Air– Very Cold and Usually Dry– Develops over the snow or ice covered

regions of continents– Similar to polar air masses but usually

develops in regions north of 60N where there is little or no insolation during the winter.

Classification of Air Masses

mP

cT

mTmT mT

mPcP

Arctic

Air Mass Modification

• Air masses do not remain over their source region.

• If the air moves over a region that is different from where it originated, the air mass will be modified, or changed, by the land that the air is travelling over.

• Changes include: warming, cooling, adding or reducing moisture

Air Mass Modification

cP

WarmerLand

The cP air mass willbe warmed by thewarmer land that itpasses over.

Fronts• Front

– A narrow transition zone between air masses of differing densities.

– The density differences usually arise from temperature differences.

– Density differences may be a result of humidity differences (summer - drylines).

• A front is the boundary or transition zone between different air masses.

Fronts

• The transition zone is usually narrow (15 to 200 km in width).

• Fronts may be over 1000 km long.

• Fronts are 3-Dimensional and slope

• On maps, we draw the frontal boundary on the warm side of the transition zone.– This is the position of the front at the surface

Different fronts

• There are 4 different types of front we are going to look at:– Cold front– Warm front– Occluded front– Stationary front

Fronts on maps

Cold Front

Warm Front

Occluded Front

Stationary Front

Fronts

• Fronts are associated with ascending air (warm over cold)

• They usually result in cloud formation and frequently produce precipitation

• This precipitation can be very widespread

• There is often a change in wind direction (and temperature) at a front

Idealized Cold Front

• Cold Front

– A boundary that moves in such a way that the colder (more dense) air advances and displaces the warmer (less dense) air.

– The largest temperature differences are normally associated with cold fronts.

Idealized Cold Front

Isotherms

0C 5 10 15 20 25 30C

100 km

Frontal Zone

44 75

COLDFRONT

Idealized Cold Front

Cross-sectional Characteristics

Cold Air

Warm Air

Strong Lifting near thefrontal boundary can produce convective clouds.

Idealized Cold Front

Cold Air Warm Air

Idealized Cold Front

• Precipitation:

– Located on either side of the front.

– Convective, showery in nature

– If warm air is stable

• Nimbostratus, Altostratus

– If warm air is unstable

• Cumulus, Cumulonimbus, Thunderstorms

Idealized Cold Front

© 1999 Prentice-Hall, Inc. -- From: Aguado and Burt, Understanding Weather and Climate

Cold Air Warm Air

Surface Front