Chapter 10 Answers to Questions for Review
A thunderstorm is a convective storm that forms with rising air, containing lightning and thunder.
Sometimes a thunderstorm produces gusty surface winds with heavy rain and hail. The storm itself may
be a single cumulonimbus cloud, or several thunderstorms may form into a cluster. In some cases, a line
of thunderstorms will form that may extend for hundreds of kilometers.
The first stage is known as the cumulus stage, or growth stage. As a parcel of humid air rises, it
cools and condenses into a single cumulus cloud or a cluster of clouds. During the cumulus stage, there
normally is insufficient time for precipitation to form, and the updrafts keep water droplets and ice
crystals suspended within the cloud. Also, there is no lightning or thunder during this stage. The
appearance of the downdraft marks the beginning of the mature thunderstorm. The downdraft and updraft
within the mature thunderstorm constitute a cell. During its mature stage, the thunderstorm is most
intense. After the storm enters the mature stage, it begins to dissipate in about 15 to 30 minutes. The
dissipating stage occurs when the updrafts weaken as the gust front moves away from the storm and no
longer enhances the updrafts.
As the cloud builds well above the freezing level, the cloud particles grow larger. They also
become heavier. Eventually, the rising air is no longer able to keep them suspended, and they begin to
While this phenomenon is taking place, drier air from around the cloud is being drawn into it in a
process called entrainment.
The entrainment of drier air causes some of the raindrops to evaporate, which
chills the air. The air, now colder and heavier than the air around it, begins to descend as a downdraft.
The downdraft may be enhanced as falling precipitation drags some of the air along with it.
Because air near the ground is typically most unstable in the afternoon.
Ordinary cell thunderstorms or, simply, ordinary thunderstorms, tend to form in a region where
there is limited wind shear— that is, where the wind speed and wind direction do not abruptly change
with increasing height above the surface. Many ordinary thunderstorms appear to form as parcels of air
are lifted from the surface by turbulent overturning in the presence of wind. Moreover, ordinary storms
often form along shallow zones where surface winds converge. Such zones may be due to any number of
things, such as topographic irregularities, sea-breeze fronts, or the cold outflow of air from inside a
thunderstorm that reaches the ground and spreads horizontally.
The National Weather Service defines a severe thunderstorm as one having at least one of the
following: large hail with a diameter of at least three-quarters of an inch, and/or surface wind gusts of 50
knots (58 mi/hr) or greater, or produces a tornado.