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Air Masses and Fronts: Formation, Properties, and Classification, Study notes of Earth, Atmospheric, and Planetary Sciences

An in-depth exploration of air masses and fronts, their formation, properties, and classification. Air masses are large regions of air with similar temperature and moisture content, forming over areas with uniform underlying surface properties and high surface pressure. Fronts are boundaries between contrasting air masses, characterized by zones of temperature and moisture differences. various types of fronts, their identification, and the impact of fronts on weather patterns.

Typology: Study notes

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Lecture 20
Outline of Today’s Lecture
Air Masses
Air Mass
Modification
• Fronts
1
What is an Air Mass?
Air Masses are large regions of air with similar
temperature and moisture content.
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Lecture 20

Outline of Today’s Lecture

  • Air Masses
  • Air Mass

Modification

  • Fronts

1

What is an Air Mass?

Air Masses are large regions of air with similar

temperature and moisture content.

Air Mass

Air masses form over areas with

(1) Uniform underlying surface properties and

(2) High surface pressure, where divergence of surface

winds decreases contrasts in temperature and

humidity.

3

Air Mass Properties

  • Air masses take on the properties of the underlying

surface.

  • Air masses are classified according to their location

of “origin”.

  • Geographical Characteristics
    • Tropical, Polar, Arctic.
  • (^) Surface Properties
    • maritime, continental.
  • Source region characteristics are most predominant if

air mass remains over source region for a long time.

Air Mass Source Regions

Mean sea-level pressure map, January: Air

Masses form where areas of high pressure prevail.

7

Air Mass Source Regions

Air Mass Source Regions

Mean sea-level pressure map, July: Air Masses

form where areas of high pressure prevail.

9

Air Mass

Modification

Air Mass Modification I

cP air from Canada is carried across the Great Lakes or

the Gulf Steam. Contact with the ocean warms and

moistens the air near the surface, transforming it to an

unstable mP air mass.

13

Air Mass Modification I

  • cP air from Canada is carried across the Great Lakes.
  • Contact with the lakes warms and moistens the air near the

surface, transforming it to an unstable mP air mass, and

resulting and lake-effect snow showers.

Annual average snowfall totals.

Air Mass Modification II

When mP air enters the West Coast and moves inland

it crosses several mountain ranges, removing

moisture as precipitation.

15

Air Mass Modification II

When mP air enters

the West Coast and

moves inland it

crosses several

mountain ranges,

removing moisture

as precipitation.

Fronts

A Front - is the

boundary between

air masses.

Thus, a front is

characterized by a

zone of contrasting

temperature and

moisture.

19

Isobars Isotherms

Stationary Front

Note the two air masses, cP and mT, that are

involved in the early formation of this front.

mT

cP

mT

cP

Air mass characteristics can differ

tremendously across a front

April 1976

temperature

contrasts

Jet stream axis is shown by shaded arrow

21

Four Types of Fronts

Warm Front

Cold Front

Stationary Front

Occluded Front

Frontal symbols are placed pointing in the direction of

movement of the front (except in the case of the

stationary front).

Wind Shift Across Cold Front

25

Typical Cold Front Structure

Cold air replaces warm; leading edge is steep in fast-

moving front shown below due to friction at the ground

  • (^) Strong vertical motion and unstable air forms cumuliform clouds
  • Upper level winds blow ice crystals downwind creating cirrus and

cirrostratus

Slower moving fronts have less steep boundaries and less

vertically developed clouds may form if warm air is stable

Cold Front Passage

27

Cold Front on Weather Map

Note that the front is located at the leading edge of the

colder air.

Typical Warm Front Structure

  • In an advancing warm front, warm air rides up over colder

air at the surface; slope is not usually very steep.

  • Lifting of the warm air produces clouds and precipitation

well in advance of boundary.

  • At different points along the warm/cold air interface, the

precipitation will experience different temperature histories

as it falls to the ground (snow, sleet, fr.rain,& rain).

31

Warm Front on Weather Map

The warm front is also located on the warm air side of

the colder air.

Stationary Front can bring Flooding

  • Warm, moist mT air moves into California on Jan. 1, 1997
  • Heavy flooding caused 100,000 people to flee their homes
  • Yosemite NP experienced nearly $200 million in damages

and was closed for two months

The Pineapple Express

brings heavy rain to CA.

mT

mP

33

Frontogenesis: Formation of Fronts

  • Fronts form when air of differing origins converges

(e.g., tropical vs polar).

  • Convergence of air happens in areas of low pressure.
  • Frontogenesis is the hallmark of midlatitude cyclones.

Fronts are an fundamental part of winter storms

Frontogenesis: Formation of Fronts

37

The lifecycle

takes several

days to a week,

and can move

1000’s of km

during this time.

Stationary front

Incipient stage Mature Stage

Mature stage

Occluded stage

Dissipated stage

Lifecycle of Midlatitude Cyclone

Incipient stage

Polar front separates cold easterlies and

westerlies.

Lifecycle of Midlatitude Cyclone

39

Incipient Stage

A kink forms on the front and cold air starts to move

southward. Warm air starts to move northward.