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.
2
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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
2021/2022
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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