Weather 101 - the ITCZ and Global Wind and Pressure Zones

Take a look a full disk weather satellite image, especially during the summer months (Fig. 1) and you will likely notice a more or less continuous band of cloudiness extending east-west along or near the Equator. This is the Inter-Tropical Convergence Zone or the ITCZ.

The ITCZ is the region where the northeast low-level trade winds (Northern Hemisphere) meet up with the southeast low-level trade winds (Southern Hemisphere). The zone of intersection is actually a bit to the north of the Equator, but for the initial introduction here, let's not go into that slight displacement.

At higher altitudes above this low-level convergence zone (where winds come together and are forced upwards), there is often an upper level divergence region (Fig. 2). This sets up 3 parts of a complete circulation cell. The fourth part, sinking air, outside the convergence-divergence couplet, takes place further poleward, typically in the region between about 20 degrees to 40 degrees latitude away from the Equator. On satellite images, this shows up as a region, mostly devoid of middle and high-level cloudiness. There may still be occasional weather fronts and storm systems in this 20-degree wide band; but, for the most part, it's a zone of sinking air. This type of vertical circulation pattern is also associated with the sea breeze front (Fig. 3), cold fronts and thunderstorms.

High-pressure systems dominate this global latitude band, especially over ocean areas. Equatorward of the centers of the highs, trade winds blow. Toward the poles, southwesterlies rule north of the Equator and northwesterlies south of the Equator.

Another region of convergence in low-level winds can be found from 45 degrees latitude to each pole. Here an ongoing series of storms pock-marks the landscape. Each storm has its own low-level convergence, upper-level divergence pattern and it's own complete vertical circulation cell. Note that the actual latitudinal banding shifts toward or away from the poles based upon time of year. Here, winds match the direction of the trade winds in their respective hemisphere.

Finally, at or near the poles and over high latitude land masses, high-pressure rules.

The result is that on a global scale, the Northern and Southern Hemispheres have well-defined, almost mirror-image, global wind flow patterns (Fig. 4) and associated pressure bands (Fig. 5).

On any given day, you won't necessarily find these simplified patterns globally or even in a particular region. But when averaged out, or when one looks at the overall cloud distribution (as we have done here), the patterns are unmistakable.

© 2010 H. Michael Mogil