T/G Layout 1

284

ackground

William F. Ryan, University of Maryland, College Park, Department of Meteorology

The general circulation of the atmosphere—the average motion of the winds aro u n d

the globe—is driven by the diff e rential heating of the Earth. In the simplest terms, excess

heating near the equator causes the air to expand or swell over the equatorial re g i o n s.

U p w a rd motion associated with this heating is typically concentrated in a relatively nar-

row band named the I n t e r- Tropical Convergence Zone (ITCZ). The satellite signature

of the ITCZ is a band of clouds, usually tall thunderstorms (cumulonimbus), that circ l e s

the globe near the equator. The position of the ITCZ varies seasonally, moving nort h-

w a rd during the nort h e rn summer and moving south during the nort h e rn winter.

The ITCZ forms as a result of moist air rising under the influence of strong surface heat-

ing. Upward motion along the ITCZ is limited to approximately 15 kilometers by the

presence of the stratosphere.

figure 99.

TH E IN T E RT R O P I C A L

CO N V E R G E N C E ZO N E ( I T C Z )

B

Z 1

Z 2

differential heating (latitudinal)

A 1

A 2

Earth

Z = the optical path through

Earth’s atmosphere

A = surface area

Z 2 >> Z 1

A 2 >> A 1

Result:

• longer optical path at pole


• more reflection,
  absorption, scattering


• larger area per unit of
  insolation at pole