Transcript Lecture 7, Airmasses, 25 October 2006

Air mass: an extremely large body of air (thousands of square
kilometers) whose properties of temperature and moisture are fairly
similar in any horizontal direction at any given altitude.
• The properties of an air mass are determined by the type
of surface over which it develops – known as the source
region.
• The source region should be generally flat and of uniform
composition, with light surface winds; such as ice- and
snow-covered arctic plains in winter and subtropical oceans
and desert regions in summer.
• Air masses are characterized by TEMPERATURE and
MOISTURE
• To have a uniform air
mass in temperature and
moisture, the air mass must
stay in its source region for
several days or weeks.
Photo courtesy of NOAA
Air Mass Temperature Classifications:
Temperature of an air
mass is classified by the
general characteristics
of its source region
A = Arctic
P = Polar
T = Tropical
Airmass moisture characteristics:
m = maritime (ocean) surfaces
c = continental (land) surfaces
The combination of
temperature and
moisture gives us
five basic air mass
types that affect
Europe
A
Air masses
that affect
weather in
Europe
cP
mP
mT
cT
Continental tropical, cT
Continental polar, cP
Maritime tropical, mT
Maritime polar, mP
Arctic
Outside of Europe there are a couple of other types of air
masses that affect regional weather.
In terms of temperature, there are two other types of air
masses that are not seen Europe:
• Equatorial (E)
• Antarctic (AA)
We still use the same designations for humidity
• Maritime - forming over ocean surfaces- (m), which
create relatively humid air masses
• Continental – forming over land surfaces - (c), which
create relatively dry air masses
Arctic (A, or cA)
• Extremely low temperatures and very little moisture (often don’t bother
with the “c” designation since these are so cold they are always “dry”)
• usually originate north of the Arctic Circle, where days of 24 hour
darkness allow the air to cool to sometimes record-breaking low
temperatures
• form over ice- and snow-covered regions of Siberia, the Arctic Basin,
Greenland, and northern North America.
•. these air masses often plunge south across Europe during winter, but
very rarely form during the summer because the sun warms the Arctic.
Photo courtesy of NOAA
• Arctic air masses move about as a shallow area
of high pressure, commonly known as an "Arctic
High".
•Clear skies, extremely low humidity, and very
high atmospheric pressure prevail.
• Northerly surface winds transport the colder air
southward. Even though the Alps provide a
significant topographical barrier, arctic air masses
entering Europe can still move all the way to North
Africa.
• An arctic outbreak that spreads southward can
bring record-breaking cold temperatures to the
continent.
•Austria’s record low: -36.6C, 11 Feb 1929 at
Zwettl (source: http://www.dandantheweatherman.com/
wortrivfeb.html )
•Arctic air masses are responsible for bringing
below zero Fahrenheit (below -18C) weather to
every state in the U.S., including Florida.
Continental Polar (cP)
 very cold, having developed over sub-polar regions (not as
cold as Arctic air masses)
 very dry, due to cold and developing over land
 form farther south in the subpolar Siberia and eastern
continental Europe
 common across continent during winter
 do form in summer,
but mostly only in
interior Siberia and
Russia
 typically bring clear
and pleasant weather
during the summer
Maritime Polar
(mP)
very cool and moist
 typically bring
cloudy, damp weather
 form over northern
Pacific and northern
Atlantic Oceans
 most often influence
the British Isles and
Scandinavia.
 Maritime polar air masses can
form any time of the year and are
usually not as cold as continental
polar air masses.
Maritime Tropical (mT)
• very warm and very humid – develop in tropical and subtropical latitudes
• originate over warm waters of southern north Atlantic
Ocean
•can form year around, but are most common in summer
• important source of moisture feeding storms all year round
• responsible
for hot,
humid days of summer
across southern
Europe and much of
the south and eastern
half of North America
Continental Tropical (cT)
 very warm; develop in lower
sub-tropical latitudes
 very dry because of formation
over land
 form over the deserts of
North Africa and southwest Asia
 bring heat to southern
Europe during summer
 as air mass moves eastward,
moisture is evaporated into it,
making it more mT
 these air masses rarely form
in winter
Austria’s highest
temperature: 39.7C, on
27 July 1893 in Dellach
im Drautal
 boundary between cT and mT is often called the dryline, and is a
favored location for thunderstorm initiation
Typical dryline thunderstorm setup
Air moving off an elevated
mountain plateau is “wellmixed”: (a) its potential
temperature, θ, is nearly
constant with height [so its
stability borders between
neutral and absolutely
unstable], and (b) its mixing
ratio is also nearly constant
with height
This dry, warm, well-mixed
layer moves out over a surface
layer of warm and very moist
air. At the interface is a
temperature inversion. Moist,
conditionally unstable air from
this surface layer must break
through the inversion and rise
through the EML
Typical “synoptic” setup for a
US, southern plains dryline
Rocky mountains (4500 m
elevation) stretch from central
Colorado, through New
Mexico, and into Mexico
Gulf of Mexico provides mT
air with high (> 20 g/kg)
mixing ratios
Surface low (cyclone) forms on
lee side of mountains, and
circulation around this cyclone
brings dry cT air into contact
with moist mT air. Interface
can be violent!
Case study: 29 May 2004
DDC sounding
location
Dryline over
eastern India
Mean position of
mesoscale features
during Indian /
Bangladeshi severe
events
Air masses do not remain in their source region indefinitely
but move from place to place changing their temperature,
humidity, and/or stability. The modification in air mass
occurs primarily by
i) exchanging heat or moisture, or both, with the surface
over which the air mass travels
ii) radiative cooling, and
iii) processes associated with large-scale vertical motion.
Photo courtesy of NOAA