Global Winds & Pressure Belts
Monday, June 22, 2026 9:30 AM
General Circulation of the Atmosphere
- Remember that b/c of axial inclination or “tilt” of the earth & the revolution around the sun, the tropical
areas receive the most direct rays from the sun & could be an energy surplus region. Similarly, the polar
areas receive rays at more oblique angle’s & are considered energy deficient regions. If left unaltered,
the tropics would burn up & polar areas freeze over. Fortunately, earth’s environment has an elaborate
system for redistributing this energy, primarily through:
1. Ocean currents that redistribute 20% of this imbalance
2. Winds which account for the remaining 80% of heat redistribution. Winds blow b/c of unequal
surface heating (air molecules flowing from higher pressure areas to lower pressure areas)
- If earth didn’t rotate, we could ignore the Coriolis & patterns of global circulation would be very
simple. Heated air would rise over equatorial latitudes, cold air would sink over the poles & winds would
circulate in one convection cell in each hemisphere, from poles to equator
- Thanks to the earth’s rotation, this simple one-celled model doesn’t happen, instead the global
circulation system is a 7-component system in each hemisphere. Wind & pressure belts can be
considered symmetrical to the equator w/ the Northern Hemisphere pattern, while being a mirror image
of the Southern Hemisphere pattern. Keep in mind that the general idea, these wind & pressure zones
are far from perfect in area’s they cover. Because of land & water differences daily, seasonal variations in
heating & cooling variations from the general pattern do exist.
Look at these 7 major belts of wind & pressure that experience a seasonal shift in latitude that follows the
annual latitudinal shift of the sun's vertical rays.
1) The Intertropical Convergence Zone (ITCZ) is centered on the equator & spans 5 degrees NS of it w/
- Along the equator w/ temps relatively high, the surface air is light & rises creating a zone of equatorial
low pressure (thermally induced by the high sun angle year round). As air rises it cools then clouds & rain
form. It shouldn't surprise you that this is where the world's rainforests are located!
○ In 1735, scientist George Hadley suggested air is heated strongly at the equator. The warm air rises
over the equator, moves toward the poles & cools at a very high altitude (about 30,000ft). Above the
surface at 30 degrees N+S, tropical air becomes cold & heavy, sinking to the surface
○ Early 16th century European sailing ships on trading voyages from Europe to the Caribbean in the
colonial period named the area the ITCZ "the doldrums," a name from an older English word meaning
foolish, b/c of the difficulty sailing ships encountered
1) The 3rd belt of wind/pressure (skipping the 2nd for a moment) is called the zone of Subtropical High
Pressure aka "Horse Latitudes." Referred to as the Horse Latitudes b/c old sailing crews of past centuries
frequently appeared when sailing through this calm region, as food & water supplies dwindled, they
threw horses overboard to lighten their loads
○ This belt of high pressure is a result of sinking tropical air, at about 25-30 N+S, cold air high above the
surface becomes so heavy it begins to sink. As it sinks, descending air is compressed to the surface,
warms & dries. Few clouds form in this zone & it remains hot & dry. It shouldn't surprise you that
most of the world's deserts are located in this belt of latitudes!
2) Trade Winds dominate the zones between 5 & 25 degrees N+S. Named "the trades" b/c early sailing ships
on trading voyages used these reliable winds through this belt of latitudes. These trades are produced by the
pressure gradient force between the Subtropical High Pressure Belt (H) & Intertropical Convergence Zone (L).
Then b/c the earth rotates, winds are deflected to the right in the Northern Hemisphere & the left in the
Southern Hemisphere. As the air sinks, it warms by compression near the surface, the winds blow from H to L
towards the equator. B/c of where they are blowing from (the Horse Latitudes) they are warming, drying
winds that are capable of large amounts of moisture that may be released when pushed up a topographic
barrier
- These first 2 pressure belts & 1 wind belt all complete a grand heat redistribution process called
convection, this global convection cell is called the Hadley Cell
○ Named after the meteorologist who 1st hypothesized its existence
4) The Westerlies are belts of winds that dominate the zones from 30-60 N+S. This is the great wind system
of the middle latitudes, the Westerlies are created by the air that spins out of the high pressure cells (Horse
Latitudes) & high altitude winds are deflected strongly by the Coriolis effect from the west. The Westerlies
are “geostrophic winds” which is wind that is affected by the pressure gradient & the Coriolis, but not
friction. These winds flow parallel to isobars, around centers of high & low pressure areas, following a
curving path.
- Keep in mind, wind direction varies b/c of topography (landforms), continentality, differential heating of
the earth's surface. However, prevailing winds which blow most consistently high above earth's surface
(30,000-50,000 feet high) blow from west to east. They oscillate up & down in waves called Rossby
Waves, which help bring cold air equator-ward & warm air pole-ward during severe weather changes to
the mid-latitudes
- These westerly winds primarily affect mid-latitudes by being the steering currents for smaller scale high
& low pressure areas; storms & non-storms, travel from west to east in the mid-latitudes.This is why
storms come from the west or northwest & travel eastward until they dissipate and re-form, but always
maintain an eastward path.
Jet Streams
• Jet streams are fast-moving bands of wind high in the atmosphere that form because of strong temperature
differences.
• The Polar Jet Stream is the most important because it steers weather systems like storms and high- and low-
pressure systems.
• It is located 35°–65° latitude, 6–9 miles high, blows west to east, and can reach 250 mph.
• During World War II, pilots discovered jet streams because they slowed westbound flights and sped up
eastbound flights. Today, airplanes use or avoid them depending on their direction of travel.
Subpolar Lows
• Found between 55°–75° latitude.
• Form where the Westerlies and Polar Easterlies meet.
• The air rises, creating low pressure and bringing cloudy, rainy, or snowy weather.
Polar Easterlies
• Cold, dry winds that blow east to west.
• They move from the Polar Highs toward the Subpolar Lows.
• They are strongest over Antarctica because there is very little surface friction.
Polar Highs
• Areas of high pressure over the North and South Poles.
• Form because cold, dense air sinks.
• They produce cold, dry, and stable weather.
https://riohondo.instructure.com/courses/40808/files/7609093/preview
Monday, June 22, 2026 9:30 AM
General Circulation of the Atmosphere
- Remember that b/c of axial inclination or “tilt” of the earth & the revolution around the sun, the tropical
areas receive the most direct rays from the sun & could be an energy surplus region. Similarly, the polar
areas receive rays at more oblique angle’s & are considered energy deficient regions. If left unaltered,
the tropics would burn up & polar areas freeze over. Fortunately, earth’s environment has an elaborate
system for redistributing this energy, primarily through:
1. Ocean currents that redistribute 20% of this imbalance
2. Winds which account for the remaining 80% of heat redistribution. Winds blow b/c of unequal
surface heating (air molecules flowing from higher pressure areas to lower pressure areas)
- If earth didn’t rotate, we could ignore the Coriolis & patterns of global circulation would be very
simple. Heated air would rise over equatorial latitudes, cold air would sink over the poles & winds would
circulate in one convection cell in each hemisphere, from poles to equator
- Thanks to the earth’s rotation, this simple one-celled model doesn’t happen, instead the global
circulation system is a 7-component system in each hemisphere. Wind & pressure belts can be
considered symmetrical to the equator w/ the Northern Hemisphere pattern, while being a mirror image
of the Southern Hemisphere pattern. Keep in mind that the general idea, these wind & pressure zones
are far from perfect in area’s they cover. Because of land & water differences daily, seasonal variations in
heating & cooling variations from the general pattern do exist.
Look at these 7 major belts of wind & pressure that experience a seasonal shift in latitude that follows the
annual latitudinal shift of the sun's vertical rays.
1) The Intertropical Convergence Zone (ITCZ) is centered on the equator & spans 5 degrees NS of it w/
- Along the equator w/ temps relatively high, the surface air is light & rises creating a zone of equatorial
low pressure (thermally induced by the high sun angle year round). As air rises it cools then clouds & rain
form. It shouldn't surprise you that this is where the world's rainforests are located!
○ In 1735, scientist George Hadley suggested air is heated strongly at the equator. The warm air rises
over the equator, moves toward the poles & cools at a very high altitude (about 30,000ft). Above the
surface at 30 degrees N+S, tropical air becomes cold & heavy, sinking to the surface
○ Early 16th century European sailing ships on trading voyages from Europe to the Caribbean in the
colonial period named the area the ITCZ "the doldrums," a name from an older English word meaning
foolish, b/c of the difficulty sailing ships encountered
1) The 3rd belt of wind/pressure (skipping the 2nd for a moment) is called the zone of Subtropical High
Pressure aka "Horse Latitudes." Referred to as the Horse Latitudes b/c old sailing crews of past centuries
frequently appeared when sailing through this calm region, as food & water supplies dwindled, they
threw horses overboard to lighten their loads
○ This belt of high pressure is a result of sinking tropical air, at about 25-30 N+S, cold air high above the
surface becomes so heavy it begins to sink. As it sinks, descending air is compressed to the surface,
warms & dries. Few clouds form in this zone & it remains hot & dry. It shouldn't surprise you that
most of the world's deserts are located in this belt of latitudes!
2) Trade Winds dominate the zones between 5 & 25 degrees N+S. Named "the trades" b/c early sailing ships
on trading voyages used these reliable winds through this belt of latitudes. These trades are produced by the
pressure gradient force between the Subtropical High Pressure Belt (H) & Intertropical Convergence Zone (L).
Then b/c the earth rotates, winds are deflected to the right in the Northern Hemisphere & the left in the
Southern Hemisphere. As the air sinks, it warms by compression near the surface, the winds blow from H to L
towards the equator. B/c of where they are blowing from (the Horse Latitudes) they are warming, drying
winds that are capable of large amounts of moisture that may be released when pushed up a topographic
barrier
- These first 2 pressure belts & 1 wind belt all complete a grand heat redistribution process called
convection, this global convection cell is called the Hadley Cell
○ Named after the meteorologist who 1st hypothesized its existence
4) The Westerlies are belts of winds that dominate the zones from 30-60 N+S. This is the great wind system
of the middle latitudes, the Westerlies are created by the air that spins out of the high pressure cells (Horse
Latitudes) & high altitude winds are deflected strongly by the Coriolis effect from the west. The Westerlies
are “geostrophic winds” which is wind that is affected by the pressure gradient & the Coriolis, but not
friction. These winds flow parallel to isobars, around centers of high & low pressure areas, following a
curving path.
- Keep in mind, wind direction varies b/c of topography (landforms), continentality, differential heating of
the earth's surface. However, prevailing winds which blow most consistently high above earth's surface
(30,000-50,000 feet high) blow from west to east. They oscillate up & down in waves called Rossby
Waves, which help bring cold air equator-ward & warm air pole-ward during severe weather changes to
the mid-latitudes
- These westerly winds primarily affect mid-latitudes by being the steering currents for smaller scale high
& low pressure areas; storms & non-storms, travel from west to east in the mid-latitudes.This is why
storms come from the west or northwest & travel eastward until they dissipate and re-form, but always
maintain an eastward path.
Jet Streams
• Jet streams are fast-moving bands of wind high in the atmosphere that form because of strong temperature
differences.
• The Polar Jet Stream is the most important because it steers weather systems like storms and high- and low-
pressure systems.
• It is located 35°–65° latitude, 6–9 miles high, blows west to east, and can reach 250 mph.
• During World War II, pilots discovered jet streams because they slowed westbound flights and sped up
eastbound flights. Today, airplanes use or avoid them depending on their direction of travel.
Subpolar Lows
• Found between 55°–75° latitude.
• Form where the Westerlies and Polar Easterlies meet.
• The air rises, creating low pressure and bringing cloudy, rainy, or snowy weather.
Polar Easterlies
• Cold, dry winds that blow east to west.
• They move from the Polar Highs toward the Subpolar Lows.
• They are strongest over Antarctica because there is very little surface friction.
Polar Highs
• Areas of high pressure over the North and South Poles.
• Form because cold, dense air sinks.
• They produce cold, dry, and stable weather.
https://riohondo.instructure.com/courses/40808/files/7609093/preview
