The Ascending Branch of the Hadley Cell

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If you're into "oldies" music, you might be familiar with the song "I'll Follow the Sun(opens in a new window)" by The Beatles. As it turns out, this hit song could be the anthem of the Intertropical Convergence Zone (ITCZ) and the ascending branch of the Hadley Cells. By way of review, the Hadley Cells are closed circulations of air rising over equatorial regions, flowing poleward at high altitudes, and sinking and returning equatorward via the low-level trade winds. The ITCZ marks the region where trade winds from each hemisphere converge. This zone of convergence, as well as the Hadley Cells themselves, are a product of strong solar heating at low latitudes. But, the ITCZ isn't located right at the equator (as you might think). Why is that?

Recall that over the tropics, there's a net gain in energy over the course of a year because incoming solar radiation dwarfs radiation emitted from the tropics. Consider this image created by NASA that shows the net radiation distribution over the earth(opens in a new window) during December 2001. The green shadings indicate surpluses in radiation, while blues indicate deficits. Clearly, the Southern Hemisphere (where it was summer) was running a surplus, while most of the Northern Hemisphere (where it was winter) ran a deficit. The tropics, however, run a surplus pretty much all year round, which you can get a feel for by watching this animation of net radiation distribution(opens in a new window) from December 2001 to December 2002. In the animation, the low latitude regions that mark the tropics are always shaded in green (indicating a net gain in radiation).

Given the continuously large energy surplus at low latitudes, there is a zone of maximum heating called the thermal (heat) equator that exists. The thermal equator connects all the points that have the highest annual mean temperatures compared to other locations at their longitude. For the record, the thermal equator bears no relationship to the geographical equator. That's because mountain ranges, ocean currents, and differences in heating between continents and oceans naturally prevent a smooth, latitudinal variation in temperature in equatorial regions. The thermal equator lies mostly in the Northern Hemisphere, as the plot of mean annual temperature below shows, primarily because the Northern Hemisphere has more land at low latitudes (which, of course, becomes hotter than surrounding oceans with strong solar heating).

Text description of the Northern Hemisphere plot of temperature graph.

The image is a colorful global map displaying long‑term annual mean surface temperatures, presented using a smooth gradient of hues that visually express how temperature varies across latitudes and continents. Bright purples and deep blues dominate the uppermost and lowermost parts of the map, marking the coldest regions near 50°N and 50°S latitude. Progressing toward the Equator, the colors transition through greens, yellows, and oranges, illustrating increasingly warm temperatures. The equatorial zone—spanning central Africa, northern South America, Southeast Asia, and the tropical Pacific—is highlighted in warm shades of yellow, orange, and red, reflecting consistently high average temperatures throughout the year. Notable regions of extreme warmth, shown in deep red, are visible over parts of northern Africa, the Arabian Peninsula, and areas along the western edges of South America and Australia. The oceans show broad, smoothly banded gradients, with equatorial waters appearing warmest and the cooler temperatures of mid‑latitude and polar waters depicted in blues and purples. Thin black lines of latitude and longitude segment the map, providing geographic reference points. Directly beneath the map, a color legend spans from purple to red and is labeled with temperature values from 0°C to 35°C, allowing viewers to interpret specific temperature ranges associated with each hue. The bottom text reads “SURFACE TEMPERATURES (C) 365‑DAY LONG TERM MEAN”, indicating that the displayed values represent averaged conditions over an entire year. The overall visualization conveys clear patterns of global temperature distribution, emphasizing the strong relationship between latitude and climate while also revealing regional variations across land and sea.

Credit: ESRL

Furthermore, the thermal equator marks the average annual position of the ITCZ. Given the link between the ITCZ and high surface temperatures, the ITCZ lies in a trough of low pressure because high temperatures in the lower troposphere cause the air density (and weight) to decrease in local air columns, which, in turn, helps to promote lower surface pressure. Lower surface pressure is further promoted by the fact that air spreads out near the tropopause and flows poleward at the top of the ascending branch of the Hadley Cells. This upper-level divergence also helps reduce the weight of local air columns. Thus, analyses of sea-level pressure aid in finding the position of the ITCZ in real time (or over a given time period). For example, I drew the mean positions of the ITCZ during January and July using patterns of sea-level pressure as a guide below.

Credit: David Babb @ Penn State is licensed under CC BY-NC 4.0(opens in a new window)

Note that in January, the ITCZ is mostly located in the Southern Hemisphere, where summer is occurring. But, the ITCZ drifts northward as the seasons change and is mostly located in the Northern Hemisphere in July (when it's summer in the Northern Hemisphere and solar heating is stronger there). On any given day, in response to maximum heating and low-level convergence, a ragged belt of cumulonimbus clouds fed by relative strong upward motion usually hangs like a necklace around the globe(opens in a new window), marking the ITCZ.

I point out that, on any given day, the prevailing pattern of clouds associated with the ITCZ may not reflect a continuous belt of convection over equatorial latitudes, but tracking the rain that falls from showers and thunderstorms in the ITCZ can help us see how its position varies throughout the year. To see what I mean, check out this loop of monthly averages of rainfall(opens in a new window) (estimated from satellites) in millimeters per day, that fell from January 1999, to January 2003. The strong signal of rainfall associated with the ITCZ and the ascending branch of the Hadley Cells should be apparent to you. Clearly, the ITCZ "follows the sun" as it drifts north and south along with the belt of maximum solar heating throughout the year.

Since the ITCZ coincides with a belt of low sea-level pressure, low-level air flows horizontally and converges toward the thermal equator as the atmosphere attempts to equalize the weights of air columns. Those converging winds are indeed the trades. Check out this cross section schematic(opens in a new window) and note that the ITCZ corresponds with the ascending branch of the two Hadley Cells (one in each hemisphere). In case you're wondering, the background image was created by a down-looking LIDAR (a "light-equivalent" of radar that detects clouds) aboard the space shuttle Discovery. By the way, the zone where the opposing trade winds converge generally has light and variable winds. For this reason, this east-west belt is called the doldrums, which means "much rain and light winds".

How much rain falls in the doldrums thanks to the ascending branch of the Hadley Cells? Let's focus in on a particular area to see. If you recall the figure showing the January and July positions of the ITCZ(opens in a new window), you can see that the there's not much of a seasonal shift over northwestern South America near the Amazon River Basin. Thus, a recurrent dose of rising currents of humid air characterizes this region. Indeed, check out the annual mean precipitation(opens in a new window) over northern South America, which shows average rainfalls up to 3,500 millimeters (almost 140 inches) over the Amazon River Basin.

In regions where the position of the ITCZ varies more dramatically during the year (unlike the Amazon River Basin), a clear-cut wet and dry season emerge. Take, for example, the Brazilian city of Fortaleza(opens in a new window), which is located on the northeast coast. In the animation below, you can review the monthly averages of rainfall and track the seasonal migration of the ITCZ with respect to Fortaleza on the inset map of global monthly precipitation. Note that the heavy rainfall associated with the ITCZ dips southward to Fortaleza's latitude and then retreats northward. This annual variation dictates that there are rainy and dry seasons at Fortaleza, as you can see from the bar graph of average monthly precipitation (late summer and fall mark Forteleza's dry season).

Text description of the average rainfall in Foraleza animation.

The animation presents a rainfall climatology chart for Fortaleza, Brazil, combining a bar graph with a small inset map for geographic context. The main chart shows monthly mean rainfall (mm) on the vertical axis and the months January through December on the horizontal axis. Each month is represented by a bright green vertical bar, with heights varying dramatically across the year. Rainfall begins around 120 mm in January and increases sharply through February and March, reaching a pronounced peak in April, where the bar rises to approximately 350 mm, making it the wettest month by a wide margin. Rainfall remains high in May but declines noticeably from June onward, with the dry season becoming evident between August and November, where monthly totals fall below 50 mm and nearly approach zero in some months. Rainfall begins to rise again slightly in December, signaling the transition back toward the wet season. In the upper right corner of the image, a small inset graphic shows a global rainfall map with colorful shading representing precipitation intensity; a pink arrow points specifically to the location of Fortaleza, Brazil, which is also labeled in pink text. This inset helps contextualize Fortaleza within global precipitation patterns by displaying its position along the northeastern coast of South America. Together, the bar chart and inset provide a clear and visually accessible summary of the region’s highly seasonal rainfall pattern, characterized by a pronounced wet season in early to mid‑year and an extended dry season in the latter part of the year.

Credit: David Babb @ Penn State is licensed under CC BY-NC 4.0(opens in a new window)

If you revisit the loop of monthly average rainfall from 1999 to 2003(opens in a new window), the blotches of white (indicating very little precipitation) that line up at latitudes near 30-degrees South and North also stick out as curious features. Ultimately, what goes up in the ascending branches of the Hadley Cells, must come down, and these dry regions correspond to the sinking branches of the Hadley Cells. We'll explore them in the next section. Read on!