Geoscience Reference
In-Depth Information
Likely cloud sequence
30,000 ft
Cirrus
km
20,000 ft
9
8
Cirrostratus
7
10,000 ft
Advancing
mT air mass
Altostratus
6
5
Passive cP
air mass
Stratus
4
Nimbostratus
3
Slow lifting
2
1
Surface front
Rain at base of front
0
1,500 km
Figure 8.2 Uplift of air at a warm front.
At a warm front, the advancing warm air gradually slides over the top of underly-
ing, passive cooler air, causing the formation of stratiform clouds as the warm air slowly cools.
and even snow. These kinds of conditions are common in the
Midwest during spring when slow-moving mT air from the
Gulf of Mexico interacts with the cooler air to the north. When
this kind of weather settles in, it is sometimes hard to imagine
that it will ever end.
air rises rapidly, it quickly cools further, which means that
large amounts of latent heat energy are rapidly released as
the vapor condenses. As a result, rainfall is intense and of
short duration. If sufficient moisture is present and enough
latent heat is released, an intense
thunderstorm
can form,
with the latent heat being the fuel for its development. Al-
though thunderstorms can develop through the simple pro-
cess of convection, they are prone to be more severe and
widespread along a cold front because of the rapid cooling
and associated release of latent heat release that occur. For
example, Figure 8.4 shows how widespread frontal precipita-
tion can be. This radar image shows a portion of a cold front
that stretched from Canada southwest into Oklahoma. A line
of thunderstorms extends along this section of the front from
southwestern Lower Michigan into southeastern Kansas. We
will describe the evolution of thunderstorms in more detail
later in the chapter.
Cold Fronts
Cold fronts
occur when cool air moves into a
region that was previously dominated by warmer air. Because
the cold air is denser and heavier than the warm air ahead of it,
the warm air is forced to rise. In this fashion, a cold front is sig-
nificantly different from a warm front because the cold air hugs
the surface along a cold front and vigorously drives the warm
air ahead of it aloft. Notice in Figure 8.3 that the edge of a cold
front is very steep when compared to the angular nature of the
warm front in Figure 8.2.
Once the warm air begins to lift along a cold front, cool-
ing starts and vapor condenses, forming clouds. Because the
30,000 ft
Cirrus
km
20,000 ft
9
8
Cirrostratus
Cumulonimbus
7
10,000 ft
mT air mass
6
mT air forced
rapidly upward
5
4
Advancing
cP air mass
3
Warm
winds at
surface
2
Surface front
1
Heavy rain along front
0
800 km
Figure 8.3 Uplift of air along a cold front.
In contrast to a warm front, warm air ahead of a cold front is abruptly lifted and
quickly cooled, causing cumulonimbus clouds to form.
Cold front
A frontal boundary where cold air is advancing
into relatively warm air. This front is typically associated with
intense rain of short duration.
Thunderstorm
A brief but intense storm that contains strong
winds, lightning, thunder, and perhaps hail.
