Geoscience Reference
In-Depth Information
Cloud Classification
Like many other things in nature, it is possible to clas-
sify clouds based on particular characteristics, specifically
with respect to altitude and form (Figure 7.18). The in-
ternational cloud classification scheme recognizes three
categories based on their form.
Cirrus clouds
are thin and
wispy clouds composed of ice crystals rather than water
droplets.
Cumulus clouds
consist of individual, puffy
clouds with a flat, horizontal base.
Stratus clouds
consist
of layer-like, grayish sheets that cover most or all of the
sky. As you can see in Figure 7.18, these clouds occur in
various combinations. For example, stratocumulus clouds
have characteristics of both stratus and cumulus clouds be-
cause they are individual clouds that spread out more than
regular cumulus clouds.
Clouds can be further classified based on their alti-
tude, with three categories present that you can see in Fig-
ure 7.18.
High clouds
are usually found at altitudes greater
than 6 km (20,000 ft) and generally consist of cirrus clouds
because very little water vapor is present and the tempera-
tures are cold.
Middle clouds
range between approximately
2 km and 6 km (6500 ft and 20,000 ft) and include cumulus
and stratus clouds that form during intervals of stable and
changing weather, respectively.
Low clouds
typically oc-
cur below 2 km (6500 ft), usually in association with stra-
tus and cumulus clouds. These low clouds are the source
of most precipitation, with
nimbostratus clouds
affiliated
with long-term rain or snow events and
cumulonimbus
clouds
growing to great heights during short-term, severe
storms.
With a fundamental understanding of clouds and their for-
mation, it is possible to determine atmospheric conditions and
even to predict weather a few days in advance. For example,
if the sky is filled with individual puffy, cumulous clouds, it
means that the atmosphere is fairly stable and fair weather will
probably last a little while. If you happen to notice that high-
level cirrus clouds are becoming more numerous and dense,
it probably means that a storm system is approaching. These
clouds often give way to altocumulus clouds, which indicate
that the storm system is closing in and that moisture is flow-
ing into the region from hundreds of kilometers away. If the
clouds begin to thicken and expand into nimbostratus clouds,
it means that some form of precipitation will most likely oc-
cur soon.
Fog
Although condensation of water vapor usually occurs at fairly
high levels of the atmosphere, sometimes conditions are right
for it to happen very close to the ground. When this relation-
ship occurs, the hazy, moist air is known as
fog
and is one of
the more distinctive atmospheric phenomena people encounter.
As you probably know, foggy conditions can be rather eerie
because visibility is poor and the Sun and street lights have an
unusual glow. For these same reasons, fog is a travel hazard
because drivers have difficulty seeing oncoming cars and sharp
bends in the road.
Fog can form in several different ways. Perhaps the most
common type of fog on autumn or spring days is
radiation
fog
, which develops at night when a temperature inversion
exists in the lower troposphere. Recall from Chapter 5 that
temperature generally cools at a consistent rate (the environ-
mental lapse rate) from the surface up into the higher levels
of the troposphere. Occasionally, a temperature inversion
develops whereby a body of cooler air lies beneath warmer
air. If this cooler air reaches the dew-point temperature, ra-
diation fog develops. This kind of scenario sometimes devel-
ops in deep valleys when cool air collects at the bottom of
the valley (Figure 7.19a). You can see this type of fog when
you are driving and popping in and out of fog while passing
through hilly country.
In addition to radiation fog, another type of fog is
advection fog
. This type of fog develops when warm air
flows over a cooler surface, such as snow or a body of wa-
ter. When this process occurs, the warmer air cools to the
dew-point temperature. Yet a third type of fog is
sea fog
,
which develops when cool marine air comes into direct
contact with the colder ocean water. In the United States,
sea fog frequently develops on the coast of California,
along which the cool California current flows. If you hap-
pen to live near the Bay area, or have visited in the sum-
mer, you know that San Francisco can be socked in by sea
fog (Figure 7.19b).
Regardless of the type of fog that develops, you have
probably noticed that it typically dissipates or
burns off
dur-
ing the day. This process occurs because the cool air in which
the fog forms warms above the dew-point temperature as the
day progresses. Next time you experience morning fog, notice
that by the late morning or middle of the day it will probably
be gone.
Thin, wispy clouds that develop high in the
Fog that develops at night when a temperature
Cirrus clouds
Radiation fog
troposphere.
inversion exists.
Individual puffy clouds that develop due to
Fog that develops when warm air flows over
Cumulus clouds
Advection fog
convection.
cooler air.
Layered sheets of clouds that have a thick and
Sea fog
Fog that develops when cool, marine air comes into
direct contact with colder ocean water.
Stratus clouds
dark appearance.
