Airmasses and fronts
Review of last lecture
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Tropical cyclone structure: 3 major components,
rotation direction of inflow and outflow, location of
maximum wind and rainfall, 3 feedbacks
Tropical cyclone destruction: 4 reasons? Which
side has the most intense destruction?
Tropical cyclone forecast: track and intensity
Currently which skill is better?
Air masses
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An airmass is a large (usually thousands of km across) volume of air that
has horizontally uniform properties of temperature and moisture.
Airmasses acquire their properties from spending days to weeks over the
same part of the Earth.
“Polar” airmasses are colder than “tropical” airmasses
“Maritime” airmasses are wetter than "continental" airmass
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Other specific airmass types include "arctic", "equatorial", and “monsoon”
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Bergeron classification of air masses
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3 letters: e.g. mTk, cPw
1st letter for moisture properties: c - continental, m - maritime
2nd letter for thermal characteristics: T - tropical, P -polar, A Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air(dry air
formed by significant downward motion in the atmosphere)
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3rd letter for stability: k/w - air colder/warmer than ground
Source regions
The areas where air
masses form are called
source regions.
Surface weather analysis
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is a special type of weather map summarizing the information
from all weather stations.
Wind speed code
5
10
50
Present weather
Type
Intensity
Video: Weather fronts
http://www.youtube.com/watch?v=tkK4
_F0VKhM
Fronts
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A weather front is a boundary separating two air masses
Types: cold front, warm front, occluded front, stationary
front, dry line, squall line
Cold Fronts
• A cold front is a mass of cold air advancing towards warm air.
• Typically associated with heavy precipitation, rain or snow, combined
with rapid temperature drops.
• Since friction decreases with height, winds move faster at higher
altitude. Then the surface of cold front becomes more steeper
through time, leading to a narrow belt of precipitation.
• Moving speed 0-30mph
Satellite and radar images of cold fronts
(narrow belt of clouds/precipitation)
Warm Fronts
• Warm fronts are warm air moving towards cold air.
• This overrunning process produces large amounts of warm, moist air
over cooler, drier air.
• Shallow stratus clouds dominate and bring light precipitation to
affected regions. Stable regions above the warmer air create
vertically limited clouds and light precipitation. Frontal fogs may occur
as rain evaporates in the colder air near the surface.
• Moving speed about 12 mph
Slope of warm Fronts
• Friction decreases with
height, so winds move
faster at higher altitude
• This causes the surface of
the front to become less
steep through time. Then
clouds will be spread to a
wider region.
Satellite and radar images of warm fronts
(wide region of clouds/precipitation)
Stationary Fronts
• Stationary fronts do not move. They do not advance.
They are two unlike air masses side by side.
• They may slowly migrate and warmer air is displaced
above colder.
From Environment
Canada
Occluded Fronts
• Occluded fronts occur when two fronts meet, the warm air
mass between them is displaced aloft.
• This typically occurs when a cold front meets a warm front as
it circulates the low pressure center of a mid-latitude cyclone.
• The cold and warm fronts curve naturally poleward into the
point of occlusion, which is also known as the triple point.
Formation of
Occluded
Fronts
Different types of occluded fronts
• A cold-type occlusion usually
occurs in the eastern half of the
continent where a cold front
associated with continental/Polar
air meets a warm front with
maritime/Polar air ahead.
• A warm-type occlusion is typical
of the western edges of continents
where the cold front, associated
with maritime/Polar air, migrates to
an area that is occupied by
continental/Polar air.
Drylines
• Drylines are boundaries between lighter humid air and denser
dry air.
• Air masses with similar temperatures but strong humidity
gradients will act as fronts.
• They frequently occur throughout the Great Plains, and are a
favored location for thunderstorm development.
Summary
1. Definition of airmasses. Bergeron classification of air
masses (3 letters)
2. Surface weather analysis: Station model, wind speed
code, present weather
3. Fronts: 6 types.
4. What is a cold front? Steep, narrow, fast
5. What is a warm front? Less steep, wide, slow
6. What is an occluded front? Two types