The study of air masses, storms,
and fronts falls in the discipline known as synoptic meteorology.
Synoptic meteorology includes the study of weather systems that
are on the order of 1500 kilometers (or about 1000 miles) across.
Thus, they are much smaller than the global circulations that
govern world climate but larger than thunderstorms, snow flurries,
and individual clouds.
When air lingers over a region for a number of days, it acquires
characteristics associated with the underlying surface, and is
called an air mass. Boundaries between air masses are called fronts,
and low pressure areas tend to form along them.
If the surface is warm and moist, the air in the region becomes
warm and moist also. If the air then moves to a different region,
it takes these properties along, gradually changing in response
to the new area of residence or passage. Here we study the nature
of the large air masses found around the world, air masses on
the order of 1500 kilometers in diameter.
Cold air masses originate over the polar and near-polar regions,
whereas warm air masses form in the tropics and subtropics. Air
masses acquire moisture through evaporation, which depends on
the temperature of the evaporating surface, as well as on available
moisture. Thus, air masses originating over tropical oceans are
the moistest, whereas air masses that form over cold, dry land
are the driest.
Cold air masses are called polar to represent air masses that
originate in the northern parts of the northern hemisphere. Those
that originate over the very coldest part of the polar regions
are called Arctic. Warm air masses originate in low latitudes
and are called tropical air masses. An air mass that originates over a landmass is called continental;
if it originates over water, it is called maritime. The names
of the major air masses are derived from various combinations
of the designations polar, tropical, maritime and continental:
thus, a warm moist air mass is maritime tropical (mT), and so
on, as shown in this table.
Table 8.1 Air Mass Designations Origin | Polar (P) | Tropical (T) | Continental (c) | cP (cold, dry) | cT (hot, dry) | Maritime (m) | mP (cool, moist) | mT (hot, humid) |
Fronts are boundaries between air
masses possessing different temperature characteristics. Other
contrasts exist in the vicinity of fronts, including wind shifts,
pressure troughs, moisture (dew point) differences, cloudiness,
and areas of precipitation with varying characteristics. On weather
charts, cold fronts and warm fronts are placed at the warm edge
of the temperature transition zone. Thus, the change in temperature
occurs on the colder side of the front.
The movement of the air on the cold side of any front determines
the front's movement. If cold air advances, the front is called
a cold front. If cold air retreats and allows warm air to advance,
it is a warm front. If the cold air is neither advancing nor retreating,
the front is stationary. When a cold front overtakes a warm front,
an occluded front results. Occlusions are the most complex fronts
to study because the weather at the ground may be caused by hidden
frontal locations aloft.
Cold fronts have relatively steep slopes and commonly cause convective
precipitation as cold air pushes retreating warm air aloft. In
contrast, warm fronts typically have gentler slopes, which fosters
development of layered clouds that often produce steady rain or
snow. Stationary fronts are often associated with clouds and precipitation
due to overrunning, even though there is little or no frontal
motion. While fronts may remain stationary for an extended period,
they eventually move or dissipate.
Frontal cyclones are large low pressure systems that form on the
polar front and are major producers of clouds and precipitation
in middle latitudes. The Norwegian cyclone model describes the
life cycle of these storms from formation and development through
occlusion and dissipation. The conveyor belt model depicts the
three-dimensional motion of air streams through frontal cyclones.
The development stage is one of special interest because of its
relevance to weather forecasting. Divergence and vorticity changes
related to upper-level long wave and short wave flow patterns
are important indicators of storm development, intensification,
and dissipation. Introduction:
We start with a look at major air mass source regions, and take
a snapshot view of the properties of air masses as they existed
on a sample day in late spring. We'll look at this weather situation
from several directions: through plots of temperature and dewpoint,
by looking at moisture as seen from a geostationary satellite,
then by examining fronts, low and high pressure areas and the
weather associated with these features. After this we'll focus
on a summary review of the major players on the weather maps we
see on television. Finally, we'll see how those features are influenced
by various systems higher in the atmosphere. We will begin to
assess how future weather will unfold by looking at how the three
dimensional atmosphere is depicted and predicted by some of the
numerical meteorological models.
Simulation08_01 (15800.0K)
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