Transcript Understanding Weather and Climate Ch 9

Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 3. Distribution and
Movement of Air
Chapter 9
Air Masses and Fronts
Introduction
Air masses contain uniform temperature and humidity
characteristics
They affect vast areas
Fronts are boundaries between unlike air masses
Fronts are spatially limited
They are inherently linked to mid-latitude cyclones
Formation of Air Masses
Surface energy and moisture exchanges cause initial temperature,
pressure, and moisture characteristics in overlying air
Such exchanges are limited in spatial scope, leading to variation in
these parameters from place to place
Source regions
• Sites of air mass genesis
• Form when air stagnates over particularly large surface regions,
typically those which are topographically uniform
• Overlying air gains temperature and humidity characteristics of the
surface
• Air masses are characterized based on these properties
• Moisture characteristics are expressed first and in lower case in the
classification system
• Temperature is expressed next in upper case
• The wettest air is called maritime air, while the driest is called
continental
• Air deemed, from warmest to coolest, tropical, polar, or arctic
• Once formed, air masses migrate within the general circulation
• Upon movement, air masses displace residual air over locations thus
changing temperature and humidity characteristics
• Further, the air masses themselves moderate from surface influences
North American air masses and
air mass source regions
Continental Polar (cP) and Continental Arctic (cA) Air Masses
• Wintertime source regions for continental polar (cP) air include
northern Canada and Asia
• cP air takes on cold, dry characteristics and is inherently stable
• During summer, cP air is warmer and more humid by comparison but
still cool and dry as compared to other air masses
• Continental Arctic (cA) air represents extremely cold and dry
conditions as, due to its temperature, it contains very little water
vapor
• The boundary between cA and cP air is the arctic front
Migrations of cP air induce colder, drier
conditions over affected areas.
As cP air migrates toward lower latitudes
it warms from beneath. As it warms, moisture
capacity increases while stability decreases.
This sequence depicts the movement of cP air
equatorward.
Maritime Polar (mP) Air Masses
• Maritime polar air masses form over upper latitude oceanic regions
and are cool and moist
• Along the west coast of the U.S., mP air affects regions during winter
and may be present before mid-latitude cyclones advect over the
continent
• Along the east coast, mP air typically affects regions after cyclone
passage as the mP air wraps around the area of low pressure
– Referred to as a Nor’easter for the dominant northeasterly winds
Continental Tropical (cT) Air Masses
• Mainly a summertime phenomenon exclusive to the desert southwest
of the U.S. and northern Mexico
• Characteristically hot and very dry
• Very unstable, yet clear conditions predominate due to a lack of water
vapor
• Thunderstorms may occur when moisture advection occurs or when air
is forced orographically
Maritime Tropical (mT) Air Masses
•
•
•
•
Form over low latitude oceans and as such are very warm and humid
mT air is inherently unstable due to high temperature and humidity
The Gulf of Mexico is the primary source region for the eastern U.S.
As air advects over the warm continent in summer the high humidity
and high heat occasionally combine to dangerous levels
• Advection of mT air also promotes the so-called Arizona monsoon
Fronts
Fronts separate air masses and bring about changes in temperature
and humidity as one air mass is replaced by another
There are four general types of fronts associated with mid-latitude
cyclones with the name reflective of the advancing air mass
The four fronts
• Cold Fronts
– Cold fronts form when cold air displaces warm air
– Indicative of heavy precipitation events, rainfall or snow, combined with
rapid temperature drops
– Extreme precipitation stems from rapid vertical lifting associated with
the steep cold front boundary profile
– Because cold air is dense, it spills across the surface producing a
steeply inclined leading edge
– Warm moist air ahead of the front is forced aloft with great vertical
displacement
– This accounts for large vertical cumulonimbus clouds and heavy
precipitation
– Such sharp transitions between the colder, drier air behind the front and
the warmer, moisture air ahead of the front, can be easily detected on
satellite images and radar composites
Frontal development about a
low pressure system
Above: The vertical displacement of air along
a cold front boundary
Right: The sharp cold front boundary is evident
on both satellite pictures and radar composites
• Warm Fronts
– Created when warm air displaces colder air
– Even though the warmer air advances, it is displaced aloft
– This overrunning process places large amounts of warm, moist air over
cooler, drier air along extensive spatial areas
– Shallow horizontal stratus clouds dominate and bring light precipitation
to affected regions
– Stable regions above the warmer air aloft help propagate vertically
limited clouds and light precipitation
– Frontal fogs may occur as falling raindrops evaporate in the colder air
near the surface
– In a similar, but more extreme situation, sleet and freezing rain may
result
• Stationary Fronts
– When two unlike air masses remain side by side, with neither
encroaching upon the other, a stationary front exists
– Fronts may slowly migrate and warmer air is displaced above colder
– Fronts are zones of transition rather than sharp boundaries
Profile of a warm front
• Occluded Fronts
• When two fronts meet, the warm air mass between them is displaced
aloft resulting in an occluded front
• This typically occurs when a cold front closes on, and meets a warm
front as it circulates about the low pressure center of a mid-latitude
cyclone
• Cold air now occupies the surface completely around the low while
warmer air is displaced aloft
• A cold-type occlusion usually occurs in the eastern half of the
continent where a cold front associated with cP air meets a warm front
with mP air ahead
– This situation resembles a cold front in profile
• A warm-type occlusion is typical of the western edges of continents
where the cold front, associated with mP air, invades an area in which
colder cP air is entrenched
– This results in a vertical profile which resembles a warm front
Occlusion sequence
Some occlusions form when the surface low
elongates and moves away from the junction of
the cold and warm fronts
Some occlusions occur when the intersection
of the cold and warm fronts slides along
the warm front
• Drylines
– Because humidity is an important determinant of air density, air masses
with similar temperatures but strong humidity gradients will act as fronts
– Boundaries between dry and moister air are called drylines
– They frequently occur throughout the Great Plains and are an important
contributor to storm development
A dryline over
Texas
End of Chapter 9
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt