San José State University
Thayer Watkins
Silicon Valley
& Tornado Alley

Seasonal Variation of Latitudinal Profiles of Zonally
Averaged Temperature and Precipitation as a Guide to
the Latitudinal Impact of Global Warming on Precipitation

One of the major concerns about global warming is its possible impact on rainfall. The impact of global warming on temperatures may be greater in the Polar Regions than in the equatorial regions. Forecasts of the impact of this temperature increase can be generated by computerized climate models, but there is some substantial amount of uncertainty concerning their predictions, particularly for regions. The range of errors for even the best of those models ranges from 20% to 50%. An alternative is to look at how the Earth's climate responds to seasonal variations in temperature.

Below are the latitudinal profiles of precipitation for land only for December-January-February and for June-July-August.

The ocean-only profiles are a bit simpler.

These profiles expressed in degrees of separation from the ITCZ are shown below:

Northern Hemisphere

Southern Hemisphere

It would be of interest to plot the curves for the two hemispheres in the same graph. However, because the Intertropical Convergence Zone (ITCZ), on average is ten degrees south of the equator the range for two hemispheres are different, 80° versus 100°. The smaller range for the southern hemisphere means that the temperature gradient for the southern hemisphere is greater than that of the northern hemisphere. In the plot below the ranges of both hemispheres have been normalized to 90°.

Both Hemispheres

These profiles are associated with latitudinal temperature profiles. Unfortunately the corresponding land-only profiles are not currently available but the general shape will be of the zonally averaged data shown below:

The latitudinal precipitation profile falls from a peak at the equator to a valley at about 30 degrees of latitude and rises to another peak before dropping off to desert levels in the Polar Regions. This profile is produced by the general circulation cells of the atmosphere.

Warm, moist air rises at the equator, cools and spreads to the 30 degree latitudes where it descends drier. From 30 degrees it returns to its original latitude. This circulation is called the Hadley cell. There is another cell operating between 30 degrees and about 50 degrees called the Ferrell cell. A much weak version, called the Polar cell, operates between 50 degrees and 90 degrees.

Most of the equatorial air's moisture is lost on its ascent and when it descends at 30 degrees latitude, north and south, it is dry and warm and there is not much rain that comes from it. On the other hand the clear skies promote evaporation of whatever moisture there is at that region. This is what produces the deserts near the 30 degree latitude zones.

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