[<< wikibooks] Planet Earth/4i. Global Atmospheric Circulation.
== Hadley Cells ==
George Hadley grew up in the shadow of his older brother, John. Both born to a wealthy English family, and educated in mathematics and the sciences during the late 1600s. George’s older brother John had at a young age discovered a new scientific tool— the reflecting telescope. The reflecting telescope uses mirrors to enhance the image of stars and greatly advanced astronomical tools used to study astronomy, leading to the eventual discovery of the sexton. For his inventions, and astronomical and mathematic work John was elected to the Royal Society in 1717, and became an early active member of the oldest scientific society in England. 
George Hadley, the younger brother worked as a lawyer, but continued to dabble in science in his free time and was wealthy enough to do so. He was particularly interested in working on scientific instruments to determine latitude, but was frequently overshadowed by his older brother’s scientific work. In 1735 he finally was elected to the Royal Society himself, and given access to observations made by meteorological logs made by scientists and sailors from around the world. These meteorological records included locations, dates, temperatures, barometric readings, and weather reports. In 1735 this was a huge amount of information as Britain was quickly expanding its international trade, with colonies and ports in the American Colonies, Canada, Belize, and in India.  George Hadley noticed that barometric readings around the equatorial regions were significantly lower than barometric readings around 30 degrees latitude north and south of the Equator, which exhibited much higher barometric readings. In 1735, he wrote a quick explanation of why this occurs, due to a global atmospheric circulation pattern that now bears his name. 

Hadley circulation is a result of the temperature gradient that is caused by the spherical nature of Earth. The region around the Earth’s equator receives the greatest amount of sunlight, resulting in warmer temperatures. This warm zone results in rising air masses that form a zone of low pressure at the surface of the Earth. This equatorial belt is referred to as the Intertropical Convergence Zone (ITCZ). Air masses along this zone will rise, some of this rise is powered by the centrifugal force of the Earth’s spin resulting in upward movement, and bulge in the atmosphere around the equator. As air masses rise, the air will expand and cool, resulting in a zone or belt across the Earth of intense rain storms and cloud cover. The Earth’s tropical rainforests fall within the ITCZ, including portions of Central America, Northern tip of South America, West Africa and the Congo Basin, Southern India, South Eastern Asia, Indonesia, and New Guinea. These regions account for dense jungles and rainforests, and watersheds for nearly continuous wet climates year-round.

Since the Earth is tilted 23.5 degrees from the plane of its orbit around the sun, the location of the ITCZ shifts from the summer solstice to the winter solstice. This annual shift in the ITCZ greatly expands the region or belt around the Earth’s equator which is within the low-pressure atmospheric zone. Near the summer solstice each year in the Northern Hemisphere the ITCZ shifts northward bringing this rainy zone to these northern regions, while during the winter solstice in the Northern Hemisphere the ITCZ shifts southward bringing this rainy zone southward. These seasonal rains are called monsoons.  Monsoon comes from the Arabic word mawsim, which means season. A monsoon is any seasonal period of abundance of rainfall, but monsoons associated with the seasonal motion of the ITCZ can alternate regions between major dry and wet weather patterns. The most famous monsoon associated with the motion of the ITCZ occurs in India. 

India sits just north of the equator, which runs through the Indian Ocean, during the late summer months, August-September, the ITCZ shifts northward and brings heavy rains to the Indian subcontinent. During the late winter months, February-March the ITCZ shifts far southward bringing dry weather to the Indian subcontinent. These seasonal monsoonal rains are felt across east Africa, northern Madagascar, Brazil, and northern Australia in February and March, while in August and September these seasonal monsoonal rains are felt in southern Mexico, the Niger Delta of Africa, India, Bangladesh and Southeastern Asia.
The shift of the ITCZ northward in August and September also effects rain and other weather patterns in the United States. These monsoonal weather patterns result in the appearance of hurricanes in the Atlantic Ocean and Gulf of Mexico in August and September, that bring intense rainfall and storms to southeastern coastal states during this time of year.
The ITCZ is only the beginning of a global circulation pattern in the atmosphere. As the air rises and water is lost from the rising air mass, new air rushes in to the low-pressure region. The motion of the Earth’s spin and the Coriolis effect result in this incoming flow of air to blow from the east. These strong prevailing winds are called the trade winds, and are useful for sailing ships crossing oceans. The northeasterly trade winds in the Northern Hemisphere blow from the northeast into the low-pressure ITCZ near the equator, while in the Southern Hemisphere, the southeasterly trade winds blow in from the southeast. These trade winds result in fairly continuous wind from the east toward the west just north or south of the equator, and are useful for sailing ships crossing large sections of the world’s oceans. 
The rising warm air mass above the ITCZ hits the top of the troposphere and moves northward and southward at high altitudes. This warm air mass at high elevation is now dry, and undersaturated with low water vapor pressure. This high flow of air moves toward the poles causing the air to cool. As the air mass cools, it sinks, and as the air sinks it compresses and warms due to the adiabatic lapse rate. This results in a high-pressure subtropical zone. This high-pressure subtropical zone occurs at around 30 degrees north and south of the equator. This high-pressure subtropical zone is north of the Tropic of Cancer (23.5 degrees north) and south of the Tropic of Capricorn (23.5 degrees south). Like the ITCZ, this high-pressure subtropical zone various with the seasons, given the tilt in Earth’s orbit. The high-pressure subtropical zone results in Earth’s great hot deserts. In North Africa in this zone of high pressure is the Sahara Desert and in South Africa the Kalahari, the middle eastern hot Arabian Desert also sits in this high-pressure subtropical zone, as does the Great Victoria Desert in Australia. In the Americas, the Patagonian Desert in South America, and the Chihuahuan Desert of northern Mexico, Sonoran Desert of Arizona, and Mojave Desert of California lay within the high-pressure atmospheric zone. These deserts are all found in the high-pressure zone caused by the global circulation pattern of Earth’s atmosphere. 

The atmospheric circulation patterns of the low-pressure equatorial rainforest and jungle regions of Earth and the high-pressure subtropical hot desert zones of Earth are collectively called the Hadley Atmospheric Circulation, or Hadley Cells after George Hadley who first described them nearly 300 years ago.

== Hadley Atmospheric Circulation and Climate Change ==

In the hills above the city of Paradise California wildfires spread around the hilled woodlands of a once idyllic community. A caravan of cars led by a firetruck tried to pave a path through the burning landscape. Never before had such wildfires enveloped so many homes and buildings in northern California. In 2018 wildfires would extend over 1.5 million acres in California, making it the worst fire season in recorded history for the state, until 2020 when over 3.0 million acres burned. Why were fires becoming so common in the once wet northern pine dominated forests of northern California? California wildfires between 2000 and 2020 have peaked over 1.0 million acres each year, back in the 1960s, wildfires rarely exceeded 0.4 million acres for the state. What was causing the rise in fires?
California, and much of the western United States sit in a precarious latitude in response to recent changes to the Earth’s Hadley Atmospheric Circulation pattern. The increasingly higher concentration of carbon dioxide in the atmosphere, allows air masses to retain more of the sun’s heat, resulting in an expansion of the Hadley circulation into new geographic regions. In the enriched carbon dioxide atmosphere, warm air high in the troposphere takes a longer time to cool and sink, and hence can travel further northward. Above the Pacific Ocean this northward expansion of the high-pressure subtropical zone pushes dry winds into Northern California. These dry warm winds from the high-pressure subtropical zone result in wildfires in forests and regions that typically would receive more rainfall. The increased frequency of wildfires in California and other western states is a result of the thermal expansion of the Hadley Circulation. Latitudes as high as 40 degrees north are now falling more frequently within the high-pressure subtropical zones. The atmospheric circulation patterns that determine the distribution of Earth’s deserts, are now exerting an effect further north. This has led to especially dry and warm late summer months in August and September for these regions. California is not the only region to be affected by these changes. In the southeastern United States these changes have resulted in warmer and drier August and September months, while in Europe the more northern motion of the Hadley Circulation has led to increasingly hot summer temperatures north of the Mediterranean Sea, with annual heat-waves in France, Spain, Italy, and Greece. It also has led to increasingly hot December-January temperatures in Australia, in the Southern Hemisphere, leading to massive forest fires during the 2018-19 and 2019-2020 fire seasons.

== Mid-latitude to High-latitude Atmospheric Circulation (Ferrel Cells) ==
A century after George Hadley had developed his ideas of atmospheric motion, a young American named William Ferrel was refining the idea in 1856. In particular, Hadley’s ideas of atmospheric motion was based on the idea that air masses conserve their linear motion, while Ferrel proposed a more correct assessment, that the motion of air masses followed angular momentum with respect to Earth's Axis. In other words, they were affected by the spin of Earth’s motion. This causes air masses north of the high-pressure subtropical zone to blow out of the west toward the east (westerly winds). These higher latitude patterns of air flow are collectively called the Ferrel Cell, after William Ferrel. The combined motion of the Hadley Cells which blows winds out of the east into the low-pressure ITCZ and the motion of the Ferrel Cells which blows winds out of west away from the high-pressure subtropical zone, generates spiraling or a complex circular motion. These westerly winds blow into the coldest air mass near the poles. The air mass that encircles the polar regions sinks, resulting in the top of the troposphere to be lower above the poles. These westerly winds blow strongly against this cap of cold air that rotates around the top of poles. This thermal gradient results in a powerful path of wind called the jet stream.  

The jet stream provides a nearly continuous high wind in the atmosphere near the boundary with the troposphere and stratosphere, near the tropopause. These altitudes are within the cruising altitudes of most commercial aircraft, and hence is called the jet stream. These westerly winds can affect airplanes, significantly speed up airplanes traveling with this strong wind to the east, and slow down airplanes traveling against the jet stream to the west.

== Out of Control Jet Streams and the Polar Vortex ==
The increase in atmospheric carbon dioxide also affects the Ferrel Atmospheric Circulation Patterns near the Earth’s poles and results in changes to the strength of the jet stream. Normally, the jet stream gets its strong wind velocity from the strong thermal gradient between the very cold polar air in the Arctic, and the warmer air encircling the Earth to the south, around 50 degrees latitude. The cold dense air above the poles prevents the airmasses from the south to move northward, however as the Earth retains more of the sun’s energy, the polar air has warmed significantly.  A warmer atmosphere over the poles, particularly the North Pole, results in a weaker jet stream. This weaker jet stream allows warm air masses to push up into the arctic, displacing the cold air southward.
Sometimes the polar arctic air will drift southward, resulting in cold air masses to drift to latitudes of 50 degrees, leaving warmer air over the arctic. These weather events are called a Polar Vortex. 

During the winter, a polar vortex can move cold Arctic air southward, especially in the eastern portions of North America and northern Asia as it interacts with the westerly jet stream. These cold spells bring cold weather to the Northeastern portions of the United States, Eastern Canada, and the Great Lakes Region, while warm weather predominates along west coast, such as Alaska, the Yukon and British Colombia.

== Rossby Waves ==

A century after Ferrel, and two centuries after Hadley, a Swedish scientist named Carl-Gustaf Rossby described the complex nature of the motion of the jet stream in relation to the westerly winds. Rossby identified atmospheric waves in the jet stream in 1939 and went on to explain the science of its motion. These waves are today are called Rossby waves. Atmospheric Rossby waves result from the conservation of the potential spinning forces stemming from the motion of the Earth, the Coriolis force and the pressure gradient, which results from differences in temperature with latitude. These waves result in polar cold air cresting like a wave with the tip pointing toward the southwest, and the broad polar front of cold air moving toward the east. Rossby waves in the Earth's atmosphere cause between 4 to 6 large-scale meanders in the Earth’s jet stream. When these deviations become very pronounced, masses of cold air detach, and become low-strength storms and are responsible for many of the day-to-day weather patterns at mid-latitudes (40 to 50 latitudes). The action of Rossby waves helps to explains why the eastern continental coast of the Northern Hemisphere, such as the Northeast United States and Eastern Canada, are much colder than Western Europe at the same latitudes. They also explain the recent weather phenomenon of warming along the northwestern coast of North America, while the northeastern coast of North America is hit with more frequent cold winter storms. In the Southern Hemisphere, the action of Rossby waves helps to explain recent weather variation in Antarctica Ice Sheets, with the recent warming and melting of the West Antarctic ice shelf in the Amundsen Sea, while the Eastern Antarctica ice sheet remains more stable with cooler weather.