Geoscience Reference
In-Depth Information
Arctic
cA
maritime Polar
(Pacific)
mP
continental Polar
cP
Cold,
dry in
winter
maritime Polar
(Atlantic)
mP
Cool
moist
Cool
moist
Dry
Dry
hot
Warm
moist
Warm
moist
maritime Tropical
(Atlantic)
mT
continental
Tropical
cT
maritime Tropical
(Gulf)
mT
maritime Tropical
(Pacific)
mT
Figure 8.1 Principal air masses in North America. Five major types of air masses periodically flow into the con-
tinent. Note the character of these air masses and the geographical regions in which they originate.
with each other. The logical place to begin is by investigating
the boundaries of these air masses on the Earth's surface.
These boundaries are called fronts . The dominant front in the
Northern Hemisphere midlatitudes is the polar front, which,
if you recall, is the boundary between cold, dry (cP) air to the
north and warmer, moist (mT) air to the south. These contrast-
ing air masses flow parallel to one another along a stationary
front most of the time. Sometimes, however, atmospheric
conditions arise that cause one particular air mass to advance
into another along distinct frontal boundaries. This part of
the chapter focuses on these atmospheric boundaries and the
types of precipitation that occur along them if air is uplifted
sufficiently. Recall from Chapter 7 that this type of uplift is
called frontal uplift .
Warm Fronts Warm fronts occur in places where warm air
advances into relatively cool air. Because the warm air has the
lesser density of the two bodies, it slowly slides over the top of
the underlying cooler air (Figure 8.2). This process begins in
the upper troposphere when warm air at this altitude gradually
overrides the cooler air ahead of the front and causes the lifting
mT air to cool adiabatically. As the air cools, clouds form, be-
ginning with high-level cirrus clouds at the top of the upwardly
moving air mass. As the surface warm front approaches, these
clouds change to progressively lower stratus clouds, culminat-
ing in rain-producing nimbostratus clouds at the surface front.
When these conditions evolve, the sky may be overcast with
a slow but steady rate of precipitation that may last a day or
two. This precipitation can be in the form of drizzle, light rain,
Stationary front A boundary where contrasting air masses
are flowing parallel to one another.
Warm front A frontal boundary where warm air is advancing
into relatively cool air. This front is typically associated with
slow, steady precipitation.
 
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