Air Pressure and Wind Basics
Air pressure depends on the density and temperature of the air in addition to the altitude.
Air pressure was first measured by Torricelli (1600s), Galileo's pupil.
Barometer (Gk: baros = weight)
- mercury barometer: the weight of the air pushes mercury up an evacuated glass tube
- anneroid barometer (anneroid = without fluid): weight of air pushes in on partially evacuated chamber
Atmospheric pressure is commonly measured in inches or milimeters of mercury, or as milibars or kilopascals of pressure.
Winds are produced by pressure differences between two locations. Wind speed is measured by an anemometer. Wind speed is recorded in miles per hour (mph), kilometers per hour (kmph), or knots (1 knot = 1.15 mph). Wind direction is measured by a wind vane. Wind direction is the direction that the wind is coming from (not the direction it is moving toward).
The Beaufort Wind Scale was developed in 1806 to estimate wind speed from visual cues at sea. It was modified for using visual cues on land by Simpson in 1926.
Driving Forces of the Wind
Gravitational Force - compresses the atmosphere
Pressure Gradient Force - air moves from high pressure (more compressed) toward low pressure (less compressed)
Coriolis Force - air in motion appears to be deflected as Earth spins west to east
Friction Force - drag against the Earth's surface slows the wind; slows surface winds the most, high level winds the least
- Pressure Gradient Air pressure differences are the result of uneven heating of the atmosphere. In some areas, stronger heating leads to expansion of air, making it less dense (fewer molecules and less weight per cubic foot or cubic meter of air). Hot, rising air is associated with low pressure. Cold, dense, sinking air is associated with high pressure.
Pressure gradient refers to the pressure difference (inches or mm of mercury or milibars) divided by the distance over which the pressure drop occurs. Since wind (moving air) is caused by pressure differences, the greater the pressure gradient the stronger the wind.
Air pressure maps plot lines of equal pressure called isobars. Isobars are drawn at equal increments of air pressure. The more closely spaced the isobars are, the greater the pressure gradient and the stronger the winds.
The pressure gradient force, if it acted by itself (which it doesn't), would produce winds that moved at very high speeds at right angles to the isobars, the shortest path from high to low pressure. But...(see the next two forces)
- Coriolis Force As winds move north or south they are deflected due to the rotation of the Earth. As the Earth spins on its axis, a person standing on the equator moves from west to east at around 1000 mile per hour. At the poles, on the other hand, that person would not move at all, just spin around in place. So, the equator and anything on it moves west to east faster than any other place on Earth. The west to east motion decreases from the equator to the pole.
As winds move away from the equator, their west to east momentum carries them to the east of a true poleward trajectory. In the northern hemisphere they are deflected to the right. In the southern hemisphere they are deflected to the left. For the opposite case, as air masses move toward the equator, their west to east momentum lags behind the west to east motion of the Earth at lower latitude and they curve to the west. In the northern hemisphere moving air (wind) is deflected to the right. In the southern hemisphere winds are deflected to the left.
The strength of the coriolis force is zero at the equator, half its maximum strength at 30° latitude, and maximum at the poles. Fast winds and winds covering the greatest distances are deflected the most. In the absence of friction (approximated in the upper atmosphere), the coriolis force would cause the winds to blow parallel to isobars, in circles, clockwise around high pressure and counterclockwise around lows in the northern hemisphere. In the southern hemisphere the effect is the opposite, counterclockwise around highs and clockwise around lows. These isobar parallel circular winds, geostrophic winds, only occur in the upper atmosphere, away from friction with the Earth's surface.
- Friction Force Friction with the Earth's surface only affects wind speed up to altitudes around 500 m (1640 ft). Friction prevents geostrophic winds at low altitude. Rather, low level winds move at an angle across isobars.
The net effect of these forces is that near surface winds spiral outward away from high pressure centers and inward toward low pressure centers.
In the northern hemisphere, where winds are deflected to the right,
winds spiral clockwise around high pressure
and counterclockwise around low pressure
- In the southern hemisphere, where winds are deflected to the left,
winds spiral counterclockwise around high pressure
and clockwise around low pressure
Low pressure centers are zones of convergence, with winds spiralling inward. These are called cyclones.
Tornadoes and hurricanes are strong cyclonic storms.
High pressure centers are zones of diveregence, with winds spiralling outward. These are called anticyclones.
Primary Gobal Pressure Systems, Atmospheric Circultion, and Climate Belts
Equatorial Low-Pressure - Intertropical Convergence Zone (ITCZ) The equatorial region is the most strongly heated area on the Earth. It is there that we find the most vigorous upward convection. Low pressure is found all along the equator. Winds converge on the intertropical convergence zone from the northeast (northeast tradewinds) and the southeast (southeast tradewinds). The trades are fairly strong and consistent. Right at the ITCZ winds are weak and variable. Sailors in the days of sailing ships called this the doldrums.
Subtropical Highs At latitudes around 25° to 30° north and south of the equator there are several more-or-less continuous and stationary centers of high pressure. For example the Bermuda High (or Azores High) in the north Atlantic Ocean, the Pacific High (or Hawaiian High) in the north Pacific, and highs over the south Atlantic, south Pacific, and south Indian oceans. Winds diverging from these highs toward the equator form the tradewinds. Winds diverging from the highs towards the poles are deflected to the east in northern and southern hemispheres forming the prevailing westerlies in midlatitudes.
Subpolar Lows A series of low pressure centers encircle Antarctica summer and winter. In the northern hemisphere, the Icelandic Low in the north Atlantic and Aleutian Low in the north Pacific spawn cyclonic storms in winter but weaken or die out in summer as the subtropical highs strengthen in the north Atlantic and north Pacific.
Polar Highs High pressure dominates in the polar regions because the air is very cold and dense. Antarctica is the coldest place on Earth because it lies over the south pole, because it is continental, and because the ice sheet is very thick and so the surface elevation is also high. High pressure dominates Antarctica year-round. The north pole, however, lies in the Arctic Ocean. The ocean has a moderating effect on the arctic. High pressure is less well developed in the north polar summer, but devlops over land as the Canadian and Siberian Highs in winter.
Climate Belts Simplified
It is commonly known that when materials (such as atmospheric gases) are heated they expand thereby becoming less dense or "lighter." In a room with a radiator or other such heater, as the air around the radiator becomes heated it expands and rises to the ceiling and is replaced by cooler denser air. This is an example of convection. A similar process occurs near the coast in summer. Land heats up faster during the day than the water does. Air over the land is heated, expands, and rises. It is replaced by cooler, denser air from over the water forming a cool sea breeze. Convection also occurs on a global scale driven by the uneven heating of the Earth.
equatorial: hot & wet
The equatorial regions are the most strongly heated areas on the Earth's surface. It is there that we find the most vigorous upward convection. Hot air is capable of holding much water vapor. Hot, humid air rises over the equator. As it rises to high altitude it expands because the air pressure decreases (there is less mass of air above it). As air expands due to this decreasing pressure, it also cools. Since cool air is able to hold less water vapor than warm air, condensation occurs. This is why the equatorial regions normally have very high rainfall. It is here that we find tropical rainforests such as those in the Amazon, Congo, and Indonesia. Areas of upward convection are dominated by low atmospheric pressure.
desert belts: hot & dry
The rising air at the equator is replaced by low-level air from higher latitudes north and south of the equator. To balance the air moving toward the equator at low altitude, the convecting air moves away from the equator, toward the north and south, at high altitude. It is now cool because of expansion and dry because it has dropped off excess moisture. To complete the convection loop, in the regions around 25 degrees north and 25 degrees south of the equator, this cool and dry air descends back to the surface (subtropical highs). As it descends, the pressure increases (because there is now more air overhead). The increased pressure increases the temperature of the air and therefore increases the capacity of the air to hold water vapor. Now the air is very dry and has the capacity to soak up much evaporation. Consequently, these latitudes are very dry with high evaporation and low rainfall. These are the desert belts, including the Sahara, Mojave and Sonoran deserts of the U.S. southwest and Mexico, the Kalahari and Namib in southern Africa, the Australian desert, and the Atacama Desert on the west coast of South America. Areas of descending air are dominated by high atmospheric pressure.
midlatitude: temperate - cool and moist
The midlatitudes are a battleground between very cold, dense polar air and warm air moving poleward from the subtropical highs. The boundary between them is called the polar front. The polar front is an undulating boundary. The undulations are called Rossby Waves. In the midlatitudes, these undulations are sites where cold air pushes equatorward and warm air pushes poleward. Cyclonic circulation (convergence, counterclockwise in northern hemisphere) develops around the southward bulges of the polar front. As these undulations of the polar front sweep southward and eastward (northern hemisphere) cold dense air pushes under warmer, less dense air. As the warmer air rises, it expands, cools, and condenses some of its water vapor. That is why cold fronts bring clouds and rain. During the summer the polar front lies farther north and we seldom see summer cold fronts on Long Island.
polar: cold & dry
The polar regions are the coldest on Earth. The air is very cold, dense, and dry. High pressure dominates. Much of Antarctica is essentially a desert because so little precipitation falls (though what does fall remains frozen).
Wind drives both waves and surface currents in the oceans. Warm surface waters at the equator are driven westward by the easterly trade winds. When the equatorial currents reach the western edge of the ocean basin (east coast of some continent) they are diverted to the north and south along the continents. In the central Atlantic this northward flowing branch is called the Gulf Stream. It carries warm water from the equator, Caribbean and Gulf of Mexico, northward along the east coast of North America, across the North Atlantic, and to arctic Ocean off northwestern Europe. The westerlies help to drive the Gulf Stream eastward across the Atlantic toward Europe. The warmth of the Gulf Stream moderates the climate of northwestern Europe. The waters in the North Atlantic cool. The cooled waters then flow southward along the coast of Europe and Africa. This southern current is called the Canary Current. It brings cool waters down to northwest Africa and keep the coast here relatively cool. Eventually this current approaches the equator where the waters warm again and the easterly winds drive them westward across the Atlantic again to start another clockwise loop. Such large surface current loops, called subtropical gyres, are found in all the open ocean basins. They generally flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
deep ocean currents
As water chills in the North Atlantic it becomes more dense. Also, when the cold water begins to freeze to form sea ice the ice that forms is from pure water; the salt is left behind in the remaining sea water. The sea water gets saltier. The saltier the water the denser it becomes. These cold, salty, dense surface waters sink down to the bottom of the Atlantic. The sinking waters are replaced by less dense surface waters from the south. The sinking waters flow southward along the bottom of the ocean as surface waters flow northward to replace them. The North Atlantic Deep Water continues south until they meet a northward flowing bottom current of even denser waters that formed off the coast of Antarctica. The North Atlantic Deep Water then rides up above the Antarctic Bottom Water and continues southward at intermediate depths until they eventually rise to the surface near Antarctica. From there they follow other currents that carry them throughout the oceans.
ocean conveyor belt There is an ocean conveyor belt that mixes waters through all the ocean basins from the sea bottom to the sea surface, connecting the surface, intermediate, and bottom water currents. This mixing moves heat, dissolved gases, and nutrients through the oceans in one grand cycle. Breakdown of this conveyor belt may have been responsible for sudden changes in the Earth's climate in the past.
global warming and sudden cooling in Europe? The Earth is warming, largely due to the release of greenhouse gases from industrial and agricultural activity. As a result of the warming, the rate of melting and release of icebergs from Greenland into the far north Atlantic and Arctic Ocean is increasing. The increased influx of fresh water into the sea will make these surface waters less dense which could slow or stop the formation of deep water (sinking of surface water). This should slow the northward movement of the Gulf Stream. Because heat carried into the far north Atlantic helps to moderate the climate of densely populated northwestern Europe, a weakening or cessation of the Gulf Stream would cause major social, agricultural, and economic problems. Geologic evidence has shown that this has happened very rapidly in the past yielding a sudden cooling of northwestern Europe within about two decades time.