The ITCZ, or InterTropical Convergence Zone, is a belt of low pressure girdling the globe at the equator. It is formed, as its name indicates, by the convergence of warm, moist air from the latitudes above and below the equator. This region is also known as Intertropical Front or the Equatorial Convergence Zone.

The air is drawn in to the ITCZ by the action of the Hadley cell, a mesoscale atmospheric feature which is part of the planet's heat and moisture distribution system. It is transported aloft by the convective activity of thunderstorms; regions in the ITCZ receive precipitation more than 200 days in a year.

The location of the ITCZ varies over time, as it moves back and forth across the equator in a semiannual pattern, following the sun's zenith point. There is also a diurnal cycle, with cumulus developing around midday and building to thunderstorms in mid to late afternoon.

The thunderstorms of the Intertropical Convergence Zone form a line across the eastern Pacific Ocean.

Variation in the location of the ITCZ drastically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. Longer term changes in the ITCZ can result in severe droughts or flooding in nearby areas.

Because of the strength of the Hadley cells on either side of it, weather systems familiar to mid-latitude dwellers do not have the chance to form, and as a result, there are no sustained winds. Advective (horizontal) motion is due entirely to air replacing that carried aloft by convection, a languorous process at best.

Early sailors named this belt of calm 'the doldrums' because of the low spirits they found themselves in after days of no wind. To find oneself becalmed in this region could mean death in the days when wind was the only motive force available, and it was certainly a depressing experience to be stuck in the middle of the ocean, in a hot, muggy climate, with not a breath of wind to flutter the sails or cool the brow.

External link

• Short NASA article with nice picture (http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=4028)