In figure 8a (page 19), a simplified description of the general circulation of the atmos-
p h e re in the Nort h e rn Hemisphere is given. The area of interest for this section is the
n o rt h e rn latitudes where the nort h w a rd branch of the Ferrel cell (point 6) interacts
with polar air moving south (point 7). Instabilities associated with the coexistence of
these warm and cold air masses are responsible for the wave motion that is character-
istic of the weather in mid-latitudes. The general circulation shown in figure 8a has
several distinct circulation regions, or cells. The horizontal air motion associated with
these cells, however, is not directly north-to-south (meridional flow)because the air
is flowing over a rotating sphere (see figure 8b).
Because the Earth is rotating, our point of view about local motions—our frame of ref-
erence—is rotating as well (figure 9, page 21). Although this motion is imperceptible to
us, if we observe Earth from a vantage point in space, the Earth rotates beneath us
from right to left (counterclockwise). As an example of the effect of the Earth’s rotation
on relative motion, figure 9 shows a baseball (or parcel of air) moving northward at
high speed from point A to B, If the length of the trip is long enough, the Earth will
figure 8b.Hadley cell
Single-cell model of circulation that assumes Earth is uniformly covered with
water, that the Sun is always directly over the equator, and that the Earth does
not rotate. Circulation consists of a closed loop with rising air over the equator
and sinking air over the poles. Named after 18th century meteorologist,
George Hadley.Ferrel cell
Each hemisphere of the rotating Earth has three cells to redistribute energ y. The
middle cell, named for American meteorologist William Ferrel, is completed
when surface air from the horse latitudes flows poleward toward the polar fro n t.Surface high pressure is located at the poles and near 30° latitude, low pres-
sure exists over the equator and 60° latitude.