Climate Science

Climate Science

As we grapple with the challenges of climate change, many of us would like to improve our understanding of climate science and planetary weather systems. Climate patterns are affected both by local geographical factors and by the global circulation of atmospheric, terrestrial and oceanic convection currents.

What is Atmospheric Pressure?

Atmospheric pressure relates to the mass of air contained or pressing down in the earth’s atmosphere. Like when you pump more air into a bike tyre to increase its psi, or pressure per square inch, the compression of air in the air column above the earth’s surface under gravity generates atmospheric pressure.

When air in the atmosphere cools and sinks, it creates high pressure. Meteorologists call this a high.

Highs bring calm, clear, sunny weather. The dew point of high pressure air is too low for water vapour to condense into dew, clouds or rain.

Air flows upward in low-pressure zones, also known as as lows.

Lows bring cold, rainy weather. As air rises into colder altitudes, it cools and reaches its dew point. Water vapour condenses into clouds and rain.

The Hadley Cell

The Hadley Cell, named after the amateur English meteorologist George Hadley, is the area of the globe between 30 degrees of latitude south and 30 degrees of latitude north.

In this region, the prevailing (regular) winds are observed to blow from the subtropics toward the equator and from east to west.

At the equator, the angle of incidence of the sun’s rays is near 90 degrees. This direct sunlight heats air at the equator. This hot, moisture-filled air rises into the atmosphere. The equator is therefore a low pressure zone. The water vapour in the air cools and condenses into rain when it reaches higher, colder altitudes. The air which has lost its water content is driven by the rising air off to both sides of the equator.

As this dry air blows away from the equator in the upper atmosphere, it receives less direct sunlight. At around 30 degrees of latitude, this dry air cools and sinks toward the earth. As a result, the subtropics are a high pressure zone.

In the subtropical high pressure zone, where cool, dry air descends, there are clear skies and dry conditions.

Dry subtropical terrestrial regions include Australia, the Sahara, the Kalahari Desert, the Atacama Desert, Mexico, Texas and southern California, northern India, and parts of China and Arabia.

The equator on the other hand is a low pressure belt characterised by large amounts of rain. Rainy countries and tropical regions on the equatorial belt include the Congo, the Amazon and the Caribbean, Indonesia and Southeast Asia, southern India and Sri Lanka.

At the surface, the difference between the high atmospheric pressure in the subtropics and the low atmospheric pressure at the equator causes the prevailing air flow toward the equator.

The Earth is rotating on its axis from west to east. The circle that a point on the equator traces in space in one rotation of the Earth has a greater circumference than a point anywhere else in the globe. Therefore, as surface air flows from the subtropics toward the equator, the speed (distance/time) at which the earth under it is rotating increases. Since the eastward momentum of the air flowing toward the equator falls behind the speed of the eastward rotation of the earth, the winds blow from the east.

These easterlies are also known as the trade winds, because early European explorers took advantage of them to sail east across the Atlantic to the New World, as well as across the Pacific to Asia, for the purposes of trade and exchange.

The latitude of the Intertropical Convergence Zone (ITCZ), where the northeasterlies and southeasterlies meet, shifts back and forth across the equator, tracking the latitude of highest insolation (concentration of the sun’s rays), as the earth orbits the sun through the year. This seasonal oscillation of the low-pressure ITCZ causes seasonal monsoons in the tropics.

The high pressure 30 degree latitudes are also known as the “horse latitudes” because sailing ships often became becalmed here and sailors were known to throw their horses overboard to save water.

The part of the ocean that lies on the equator is known as the doldrums by sailors. Where the northeast and southeast trade winds converge at the ITCZ, the air rises, also resulting in light surface winds leaving sailing ships becalmed.

The Ferrel Cell

The middle latitudes in both hemispheres, between 30 degrees and 60 degrees, are described by meteorologists as the Ferrel Cell, named after American meteorologist William Ferrel.

The polar highs are found at the poles, as the freezing cold makes air sink. This creates a convection current, where surface polar fronts flow away from the polar highs toward the subpolar lows found at around 60 degrees. This weather system is called the polar cell.

In the Ferrel Cell, the surface wind blows from the subtropical high zone at 30 degrees toward the subpolar low at 60 degrees, and from west to east.

In the Ferrel cell, surface winds are flowing from a lower latitude, which is a relatively fast-spinning zone, to a higher latitude, a relatively slow-spinning zone. Thus, contrary to easterlies, which fall behind the earth as it spins from west to east, as these winds blow poleward, their initial eastward momentum carries them east faster than the earth they flow over. Therefore, in the middle latitudes, the prevailing winds are westerlies.

Where the polar fronts meet the warm, subtropical air masses flowing toward the poles, there is a ramp effect. The warm air rises up over the cold, dense polar air. These sharp temperature boundaries create jet streams. Jet streams are fast-flowing, narrow bands of strong winds located in the upper atmosphere.

In the northern hemisphere, the circulation of the Ferrel cell causes the Gulf Stream, which carries water northeast from the Gulf of Mexico up the eastern coast of North America through the Atlantic towards the Arctic.

New Zealand mainly experiences westerlies because it lies between 34 degrees and 47 degrees south. Our weather comes from the west. A high or a low pressure system (different to the planetary-scale high and low pressure zones) located in the Tasman Sea will reach New Zealand, and also move off the eastern side of the country into the South Pacific.

Sailors call the area of the Southern Ocean between 40 and 50 degrees of latitude south the Roaring Forties because of its famously strong westerlies. The historic clipper route relied on this wind belt during the Age of Sail, charting a southerly route around the globe under Africa, Asia, Australia and South America, to avoid the doldrums. Westerlies here are fiercer than in the Northern Hemisphere because of the large expanses of uninterrupted ocean in which they can build and grow in strength. Some captains were willing to risk the even more intense westerly gales, as well as icebergs, further south in the Furious Fifties and Shrieking Sixties, to shorten their route.

 

The Earth’s climate is carefully balanced by a self-regulating system of atmospheric circulation. Therefore, it is crucial we consider the impacts of our human actions before we act in any way that can upset the balance of this delicate, precious and dynamic system.