From: Subject: Thursday Weather Topic in Greater Depth Date: Sun, 20 Sep 2009 10:56:05 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/last_week/r_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3350 Thursday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Thursday, 3 = September 2009=20

AIR MASSES, THEIR SOURCE REGIONS and AIR MASS = MODIFICATION=20

Various weather elements, such as temperature and moisture, can be = used to=20 identify air masses. An "air mass" is a large body of air (hundreds of = miles in=20 extent) in which these weather elements are relatively uniform in their=20 horizontal extent at each particular altitude. Consequently, the density = is=20 essentially uniform in the horizontal. While horizontal uniformity is an = ideal=20 condition, variations do exist, but they are usually gradual with no = abrupt=20 discontinuities within the same air mass.

AIR MASS SOURCE REGIONS

In a general sense, the temperature of the air mass is determined by = the=20 latitude of the source region, while its moisture content is determined = by the=20 type of surface over which the air mass developed. When air stagnates = over the=20 snow-covered polar latitude continents in winter, the near surface air = rapidly=20 loses heat through radiation during the long winter nights. Although the = entire=20 air column can cool, the greatest cooling occurs near the surface. In = the=20 tropics and subtropics, the intense sunlight heats the surface during = the day.=20 The air immediately above the surface is heated upon contact.

AIR MASS CLASSIFICATION

Air masses are usually classified using the following scheme devised = during=20 the early 20th century that contains a one or two letter = code:

AIR MASS TYPE

SOURCE REGIONS

THERMAL & MOISTURE
PROPERTIES AT=20 SOURCE

Maritime Polar (mP)

Polar and subpolar oceans poleward of approximately 50 degrees=20 latitude

Prominent in all seasons.
Cool, moist (damp).

Continental Polar (cP)

Continents in vicinity of Arctic and Antarctic Circles or=20 Antarctica

Prominent in winter.
Winter: Very cold and dry
Summer: = cool and=20 dry.

Maritime Tropical (mT)

Tropical, Trade wind belt and major subtropical ocean = basins.

Prominent in all seasons.
Moist and warm.

Continental Tropical (cT)

Tropical continents, especially tropical and subtropical = deserts,=20 chiefly Sahara and Australian deserts

Prominent in summer.
Hot and very dry.

Arctic (A)

Arctic or Antarctic
especially the Polar anticyclone =

Found all seasons, prominent in winter.
Bitterly cold and = very dry=20 in winter.

Equatorial (E)

Equatorial belt

Prominent in all seasons.
Hot and high=20 humidity.

AIR MASS MODIFICATION

When the air mass travels away from the source region, the air mass = will=20 remain for a time as a recognizable entity, retaining thermal and = moisture=20 properties. However with time and new underlying surface conditions, the = air=20 mass does undergo a slow process called "airmass modification". The = amount of=20 modification depends upon the route that the air mass traveled, the = nature of=20 the underlying surface and the time spent enroute. Usually, polar air = masses=20 moving over warmer surfaces are modified more rapidly than tropical air = moving=20 over cold ground. Heating of the polar air mass from below enhances = vertical air=20 motions that mix the air mass, and increase the likelihood that clouds = with a=20 puffy appearance (called cumulus type clouds) will form. Conversely, = cooling of=20 the air by contact with a colder surface suppresses vertical motions = restricting=20 cloud formation.

Return to the Thurs= day Daily=20 Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =