<P> In some cases, the ITCZ may become narrow, especially when it moves away from the equator; the ITCZ can then be interpreted as a front along the leading edge of the equatorial air . There appears to be a 15 - to 25 - day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of the Madden--Julian oscillation (MJO). </P> <P> Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed . Early sailors named this belt of calm the doldrums because of the inactivity and stagnation they found themselves in after days of no wind . To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only effective way to propel ships across the ocean . Even today leisure and competitive sailors attempt to cross the zone as quickly as possible as the erratic weather and wind patterns may cause unexpected delays . </P> <P> Tropical cyclogenesis depends upon low - level vorticity as one of its six requirements, and the ITCZ fills this role as it is a zone of wind change and speed, otherwise known as horizontal wind shear . As the ITCZ migrates more than 500 kilometres (300 mi) from the equator during the respective hemisphere's summer season, increasing Coriolis force makes the formation of tropical cyclones within this zone more possible . Surges of higher pressure from high latitudes can enhance tropical disturbances along its axis . In the north Atlantic and the northeastern Pacific oceans, tropical waves move along the axis of the ITCZ causing an increase in thunderstorm activity, and under weak vertical wind shear, these clusters of thunderstorms can become tropical cyclones . </P> <P> Thunderstorms along the Intertropical Convergence Zone played a role in the loss of Air France Flight 447, which left Rio de Janeiro--Galeão International Airport on Sunday, May 31, 2009, at about 7: 00 p.m. local time (6: 00 p.m. EDT or 10: 00 p.m. UTC) and had been expected to land at Charles de Gaulle Airport near Paris on Monday, June 1, 2009, at 11: 15 a.m. (5: 15 a.m. EDT or 9: 15 a.m. UTC) The aircraft crashed with no survivors while flying through a series of large ITCZ thunderstorms, and ice forming rapidly on airspeed sensors was the precipitating cause for a cascade of human errors which ultimately doomed the flight . Most aircraft flying these routes are able to avoid the larger convective cells without incident . </P>

The itcz shifts north during the northern hemisphere summer and south during the winter due to