Hadley Cells: A Crucial Cog in Earth’s Climate Machine

Atmospheric circulation is an important component of Earth’s climate. Without it, the transport of heat and water across the globe would be virtually nonexistent, which would be bad news for life in most areas of the world.

Currently, the average temperature difference between the equator and the poles is about 40°C, or 104°F. Without atmospheric circulation, that difference would be about 100°C, or 180°F. Most life needs what we consider to be mild temperatures. Increasing temperature extremes at the equator and poles would diminish areas with mild temperatures in which most of Earth’s life forms thrive. Luckily, Earth has configured a pretty fantastic climate system that continuously transports heat and water all over the world, allowing creatures both like and unlike us to survive at almost any latitude.

Image by Dwindrim

One major component of our climate responsible for atmospheric circulation is the Hadley cell. Hadley cells are essentially a pattern of air movement. At the equator, hot air rises from the surface to the top of the troposphere (the lowest layer of our atmosphere). Once it reaches the tropopause (the barrier between Earth’s troposphere and stratosphere) at about 15 km (9 miles), air is pushed north or south, depending on which hemisphere the air resides in. As air moves towards the poles, it begins to cool and sink. Once an air mass reaches about 30° North or South latitude, it is redirected back to the equator to begin this process again.

Image by Cory Spruit

The mechanics of a Hadley cell are pretty simple, but this simple atmospheric pattern has massive implications for Earth’s climate system and life on Earth. The most notable impact is its influence on precipitation distribution.

When the sun heats air at the equator, it also encourages evaporation. When a humid mass of air cools down, as it does when it rises, moisture in the air will condense into clouds, which then precipitate large amounts of rain over the equator. This forms what’s known as the Intertropical Convergence Zone (ITCZ), doldrums, or horse latitudes. The large volume of rain that these 10 degrees of latitude around the equator receive supports numerous tropical rainforests, including the Amazon rainforest, Congo rainforest, and Indonesian rainforest.

Further north at the edge of these cells, the Hadley circulation is having the complete opposite effect. After depositing huge amounts of rain over the equator, air continues to rise until it reaches the tropopause. From there, it is diverted towards the subtropics.

At about 30 degrees North or South of the equator, the air masses begin to sink. This air is very dry, as it has already been depleted of moisture at the equator. As the air sinks, an increase in pressure causes it to warm in a process called adiabatic heating. This, in turn, further decreases the air’s relative humidity.

This low relative humidity coupled with an already dry mass of air results in extremely dry conditions at the edge of Hadley cells. This leads to very little precipitation to these regions and, consequently, atmospheric conditions that provide us with most of the world’s hot deserts (as opposed to cold deserts, like Antarctica). If you look at a satellite image of Earth and note the locations of the planet’s hot deserts, such as the Saharan desert, Arabian desert, Australian deserts, and Kalahari desert, you’ll see that many of them are located at similar latitudes. The reason for this isn’t simply because these regions are hot, but because they are all located at the edge of Hadley cells.

When Christopher Columbus journeyed across the Atlantic, he used the Trade Winds to carry his ships from Spain to San Salvador. It’s very possible that he may have been the very first European traveler to use the Trade Winds to cross the Atlantic Ocean."

Because of high pressure air masses in the subtropics and low pressure air masses at the equator, a pressure gradient forms at the Earth’s surface. This causes air to move from the high pressure system to the low pressure system, which creates a very regular pattern of surface winds. Due to the Coriolis Effect, these winds move both east to west as well as north to south in the Northern Hemisphere and south to north in the Southern Hemisphere. These winds are commonly called the tropical easterlies (winds are named after which direction they come from, not the direction in which they’re moving) or the trade winds.

Though the original meaning behind the term “trade” in the trade winds came from a Middle English word meaning “path” or “track”, by the 18th century these winds became hugely important for foreign commerce, our modern understanding of the word “trade”. Before the invention of motorized boats or airplanes to transport people and cargo across the oceans, sailors utilized the Trade Winds to carry their ships across the Atlantic to exchange goods and explore new lands.  

A crucial characteristic of the Hadley circulation is that the wind patterns that make up these cells are consistent. It’s through this consistency that a region’s climate is established, allowing life to adapt to its environment. Recently though, scientists have discovered that the Hadley circulation is changing, which may put many lives at risk. Several recent studies have found that over the past few decades, the tropics have expanded by about 2 degrees of latitude. Though 2 degrees doesn’t seem very significant, the implications of such a change (and of a larger expansion, which is likely to occur) certainly are.

Within Earth’s climate system, almost everything is connected. When one aspect of the climate system changes, it can affect the entire world. A shift in the tropics will affect lives in the tropical region, of course, but it could also affect regions directly outside of the subtropics if they also expand.

On a larger scale, changes in the Hadley circulation could also lead to changes in the position of jet streams, storm tracks, and high and low pressure systems that affect climates completely outside of the Hadley circulation. All of this is to say, there are a lot of consequences that even a small change to our climate can have. For now, it’s unclear what exactly those consequences are. If one thing is clear, though, it’s that with this change and so many others taking place in our climate currently, it’s more important now than ever to continue to study the climate in an effort to expand our understanding of this complex system.

- Haley Royer, Guest Writer