T/G Layout 1

Because wave motion is so important to weather prediction, meteorologists have
devised standard terminology for discussing wave structure. An idealized wave is
shown in figure 3. Waves tend to be quasi-horizontal. The top/northern-most exten-
sion of the wave is a ridge, the jagged line in figure 3 is the ridge axis. In general
terms, weather conditions beneath the ridge axis are dry and storm free. The bottom/
southern-most extension of the wave is the trough, it has a trough axisrepresented
by the dashed line. As will be shown in section 3, the area just ahead (east) of the
trough axis is the preferred location for storm development. The area to the west of the
trough is usually cool and dry.

figure 3. common mid-latitude weather pattern: comma cloud

Weather disturbances in the vicinity of atmospheric waves, like ocean waves near the
beach, have a life cycle in which they initiate, amplify, break, and then dissipate. As a
mid-latitude cyclone moves through its life cycle, certain characteristic cloud shapes
develop that can be observed from space. At the mature stage, when the weather
associated with the wave is most intense, the satellite signature is the spiral-shaped
comma cloudand the weather system associated with it is a cyclone or cyclonic
disturbance (figure 4a).

There is often confusion associated with the term cyclone. Cyclonerefers to large-scale
closed circulations in the atmosphere whose direction of rotation is counter clockwise
in the Nort h e rn Hemisphere. Cyclones in the tropics, such as hurricanes, are re f e rre d
to as tropical cyclones. Cyclones in the upper latitudes are called e x t r a t ro p i c a l, or

45°

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surface

low

pressure

cold

front

trough

axis

ridge

axis

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