satellite signature of the ITCZ is a band of clouds, usually tall thunderstorms (cumulonim-
bus), that circles the oceans near the equator (figure 6). The position of the ITCZ varies
s e a s o n a l l y, 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
s t rong surface heating. Upward motion along the ITCZ is limited to approximately 15
kilometers by the presence of the stratosphere. The stratosphere, which is kept very
w a rm by its abundance of ozone efficiently absorbing solar radiation, acts as a lid on
the lowest portion of the atmosphere—the tro p o s p h e re (figure 7, page 19). For practi-
cal purposes, all the weather that we experience occurs in the tro p o s p h e re.
figure 6. ITCZ: Full disc GOES image with 10°N-10°S indicated.
image courtesy of the SSEC: University of Wisconsin-Madison
The air that rises in the vicinity of the ITCZ must spread out, or diverge, at the top of
the troposphere. In the simplest case (figure 8b, page 20), we could assume that the
Earth has a one-cell circulation in which the air lifted at the ITCZ travels north until it