Global Hadley Circulation Cells Drive Weather Patterns

What is a Hadley Cell, Why is it Important?

Hadley Cells drive the Earth's general wind circulation patterns between the Equator and 30 degrees north and south latitudes. These three dimensional circulation loops transport energy as weather between Tropical and Subtropical regions. The motion of the energy transport as wind comes from various physical interactions that include unbalanced heating from the sun between the Equator and Arctic, the Pressure Gradient Force between high (clear skies) and low (stormy skies) pressure systems, and the Coriolis Effect caused by the continuous rotation of the Earth every day. Many additional factors come into play in the real-world, but this Hadley Cell model simplifies our understanding.

 

The image shows how the Hadley Cell Circulation would appear to someone centered at the equator looking to the west. The two big red L's indicate low pressure near the Equator where temperatures are hot (red arrows) and rainfall is plentiful. Air is colder (shown in blue arrows) above the clouds where the air flow splits into the Northern and Southern Hemispheres continuing toward polar latitudes (#2 in the diagram). At the 30 degree Subtropics, high pressure forces air to sink back towards the Earth's surface with clear skies (#3 in the diagram).

A More Detailed Application of the Earth's Hadley Cell Circulation

Now perceive the original diagram from above as a 90 degree rotation where vertical motion is now along a horizontal axis (light blue bubbles). To start the Hadley Circulation, surface convergence (air flowing towards a location from all directions) from the Northern and Southern Hemispheres near the Equator causes the formation of low pressure systems. The converging air rises vertically into the atmosphere where clouds regularly form, condense, and generate precipitation. Upper level atmospheric winds then diverge (air flowing away from a location in all directions) toward the Poleward upper atmosphere's lower pressure in both Hemispheres. This can be seen in the image of the introductory module above. Eventually the Coriolis Force that increases in strength towards the Poles takes over and directs air to the right of the initial air motion in the Northern Hemisphere and to the left of motion in the Southern Hemisphere.



Just to the north of the Hadley Cell in the Northern Hemisphere near the latitude of the southern United States the westerly winds or westerlies of the middle latitudes transport upper atmospheric air from west to east. Persistent anticyclonic clockwise motion around high pressure regions in the Northern Hemisphere Subtropics helps direct the middle latitude westerly winds eastward in the upper atmosphere. Here the Coriolis Force closely balances the Pressure Gradient Force that generally creates west to east motion around the globe in both the Northern and Southern Hemispheres. The Pressure Gradient Force wraps air flow around the subtropical High pressure system in a clockwise manner back towards the low pressure near the Equator where easterly trade winds dominate the air flow from east to west along ocean surface currents and the continuous Hadley Cell circulation begins again.

Rain, Rain Everywhere Along the Intertropical Convergence Zone

Intertropical Convergence Zone Highlighted in Red (July) and Blue (January) from Wikimedia Commons

Air that collides between the Northern and Southern Hemispheres and lifts upward creates an Intertropical Convergence Zone (ITCZ) where some of the rainiest cities on Earth can be found. From the Hadley Cell Circulation model, one would expect the highest precipitation areas to be located directly along the Equator. There is some truth to the basic Hadley Cell model, but differences in temperature, pressure, geographical boundaries, and elevation disrupt what would otherwise be uniform balance. The ITCZ swath of frequent precipitation instead meanders around the globe on a seasonal basis, occasionally crossing the Equator.

Dry as a Bone in the Subtropical Deserts

NVDI Global Vegetation Satellite Image from Wikimedia Commons

Sinking air along 30 degree north and south Subtropical latitudes generally leads to clear skies and and dry conditions. The Intertropical Convergence Zone from the above module rarely crosses into the arid Subtropics poleward of 30 degrees latitudes. It is no coincidence that major deserts lie within the Subtropical regions throughout the globe along the 30 degree latitude parallels, but there are exceptions. The southeast United States lies within the Subtropics, but the region receives some of the most abundant rainfall in the United States. Proximity to large water bodies makes all the difference in what would otherwise be desert land. Florida's proximity to the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea greatly contributes to producing much of the sunshine state's precipitation. This is also the case in southeast Asia, China for example, where the Indian and Pacific Oceans supply the region with high amounts of precipitation.

Times of Change: Seasonal Imbalance

Earth's Annual Average Temperatures from Wikimedia Commons

Seasons on Earth are caused by the 23.5 degree tilt axis angle of our planet that faces towards the Sun, also known as the Earth's obliquity. The Equator receives more direct perpendicular solar rays from the Sun than any other place on Earth in a given year while the Arctic regions receive the least heating from the sun. The Earth's surface is a theoretical example of a black-body that absorbs and emits heat as radiation. Temperatures above absolute 0 kelvin absorb and emit the same amount of energy as heat from a black-body. The energy that is absorbed increases the temperature and energy that is emitted decreases the temperature of a surface. The mathematical difference in the incoming short-wave energy from the sun, as heat absorbed, and emitted long-wave thermal infrared energy, as heat released towards outer space, determines the Earth's surface temperatures. An example of this calculation is measuring the temperature inside your oven using a thermal infrared thermometer. The oven produces heat that is released into the cooler kitchen until the heat is turned off and the oven eventually cools back down to room temperature. Diurnal temperature variations in the atmosphere between day and night oscillate in a similar manner.

The Sun emits radiation continuously as heat. Energy is absorbed through the process of conduction where heat easily travels through solid objects like the Earth's surface. An example of conduction is when you touch an oven door and it feels hot. Energy is released by the process of convection where heat travels slowly through fluids. The water in your soup will be cold when the burner is turned on and the pot will get hot first. Once the water in the soup becomes hot enough to boil, it will bubble, and water vapor molecules will rise into the air and become gas particulates. Another example of convection occurs when you feel a rush of some hot air when an oven door is opened.

Highs and Lows: The Pressure Gradient Force

Hadley Cell Circulation Model from Wikimedia Commons

I chose to use the main image again to show how the Pressure Gradient Force (PGF) works. Pressure is generally defined as a force over a particular area. Based on that simple definition, the highest pressure would be expected to have the greatest force applied over a given area. If we lower the pressure, the force would become weaker. High pressure in the atmosphere causes air to sink toward the Earth's surface (blue sinking arrows in the diagram). Because the air does not go into the ground, the flow must separate away from the force being applied.

Referring to the above seasonal imbalance module, convection currents in the air acting as a fluid will cause air to rise upward over converging surface low pressure, as seen by the rising red arrows. Like riding a bicycle up a hill, air particles must move upward when they hit a physical boundary such as a large mountain. The direction of air flow near the surface is typically opposite in the upper atmosphere. You can theoretically place an area of high pressure above the clouds in the diagram and add areas of low pressure on the upper left and right sides of the diagram.

When air rises over a mountain into the atmosphere, it does not keep going upward forever. Eventually the cooler sinking air from above the clouds will meet with the rising air from below and separate horizontally as the diagram shows on either the side of the cloud. The Pressure Gradient Force then causes air motion to flow from high to low pressure over a horizontal distance above the surface, as long as no other external force changes the direction of motion.