High School Earth Science/Air Movement
Knowing a few basic principles can give a person a good understanding of how and why air moves. Warm air rises, creating a low pressure region, and cool air sinks, creating a high pressure zone. Air flowing from areas of high pressure to low pressure creates winds. Air moving at the bases of the three major convection cells in each hemisphere north and south of the equator creates the global wind belts.
- List the parts of an atmospheric convection cell and the properties of the air currents within it.
- Describe how high and low pressure cells create local winds and explain how several types of local winds form.
- Ask each other how global convection cells lead to the global wind belts.
Air Pressure and Winds
Think back to what you learned about convection cells in the previous lesson. Warm air rises, creating an upward-flowing limb of a convection cell (Figure 15.19). Upward flowing air lowers the air pressure of the area, forming a low pressure zone. The rising air sucks in air from the surrounding area, creating wind.
At the top of the troposphere, the air travels horizontally from a low pressure zone to a high pressure zone. Since it is at the top of the troposphere, the air cools as it moves. This cold, dense air creates the downward flowing limb of the convection cell. Where the sinking air strikes the ground, air pressure is relatively high. This creates a high pressure zone. The sinking air is relatively cool, since it has traveled across the tropopause.
Air that moves horizontally between high and low pressure cells makes wind. The winds will race from the high to low zones if the pressure difference between them is large. If the difference is smaller, the winds will be slower.
Convection in the atmosphere creates the planet's weather. It's important to know that warm air can hold more moisture than cold air. When warm air near the ground rises in a low pressure zone, it cools. If the air is humid, it may not be able to hold all the water it contains as vapor. Some water vapor may condense to form clouds or even precipitation. Where cooler air descends at a high pressure zone, it warms. Since it can then hold more moisture, the descending air will evaporate water on the ground.
Air moving between large high and low pressure systems creates the global wind belts that profoundly affect regional climate. Smaller pressure systems create localized winds that affect the weather and climate of a local area.
Local winds are created when air moves from small high pressure systems to small low pressure systems. High and low pressure cells are created by a variety of conditions. Some of these winds have very important effects on the weather and climate of certain regions.
Land and Sea Breezes
You learned that water has a very high specific heat: it maintains its temperature well. This means that water heats and cools more slowly than land. Sometimes there is a large temperature difference between the surface of the sea (or a large lake) and the land next to it. This temperature difference causes small high and low pressure regions to form, which creates local winds.
In the summer, and to a lesser degree in the day, a low pressure cell forms over the warm land and a high pressure cell forms over the cooler ocean. During warm summer afternoons, winds called sea breezes blow from the cooler ocean over the warmer land (Figure 15.20). Sea breezes often have a speed of about 10 to 20 km (6 to 12 miles) per hour and can lower air temperature much as 5 to 10°C (9 to 18°F). The effect of land and sea breezes is felt only about 50 to 100 km (30 to 60 miles) inland.
The opposite occurs in the winter, the land is colder than the nearby water due to its lower specific heat. The cold land cools the air above it. This causes the air to become dense and sink, which creates a high pressure cell. Meanwhile, the warmer ocean warms the air above it and creates a low pressure cell. This occurs to a smaller degree at night, since land cools off faster than the ocean. Winds called land breezes blow from the high to the low pressure cell. These local winds blow from the cooler land to the warmer ocean. Some warmer air from the ocean rises and then sinks on land, causing the temperature over the land to become warmer.
Land and sea breezes are very important because they moderate coastal climates. In the hot summer, sea breezes cool the coastal area. In the cold winter, land breezes blow cold air seaward. These breezes moderate coastal temperatures. Land and sea breezes create the pleasant climate for which Southern California is known.
Monsoon winds are larger scale versions of land and sea breezes; they blow from the sea onto the land in summer and from the land onto the sea in winter. Monsoon winds are incredibly strong because they occur in coastal areas with extremely high summer temperatures. Monsoons are common wherever very hot summer lands are next to the sea. The southwestern United States has summer monsoon rains when relatively cool moist air sucked in from the Gulf of Mexico and the Gulf of California meets air that has been heated by scorching desert temperatures (Figure 15.21).
The most important monsoon in the world occurs each year over the Indian subcontinent. More than two billion residents of India and southeastern Asia depend on monsoon rains for their drinking and irrigation water. In the summer, air over the Indian subcontinent becomes extremely hot, so it rises. Warm, humid air from the northern Indian Ocean enters the region, and it too is heated and rises. As the rising wet air cools, it drops heavy monsoon rains. In the winter, cool air from over the land moves seaward. Back in the days of sailing ships, seasonal shifts in the monsoon winds carried goods back and forth between India and Africa.
Mountain and Valley Breezes
Temperature differences between mountains and valleys create mountain and valley breezes. During the day, air on mountain slopes is heated more than air at the same elevation over an adjacent valley. As the day progresses, warm air rises off the slopes and draws the cool air up from the valley. This uphill airflow is called a valley breeze. When the Sun goes down, the mountain slopes cool more quickly than the air in the nearby valley. This cool air sinks, which causes a mountain breeze to flow downhill.
Katabatic winds also move up and down slopes, but they are stronger mountain and valley breezes. Katabatic winds form over a high land area, such as on a high plateau. The plateau is usually surrounded on almost all sides by mountains. In winter, the plateau grows cold, making the air above it extremely cold. This dense air sinks down from the plateau through gaps in the mountains. Wind speeds depend on the difference in air pressure over the plateau and over the surroundings. If a storm, which has low pressure, forms outside the plateau, there is a big difference in wind pressure and the winds will race rapidly downslope. Katabatic winds form over many continental areas. Extremely cold katabatic winds blow over Antarctica and Greenland.
Foehn Wind (Chinook Winds)
Foehn winds or Chinook winds develop when air is forced up over a mountain range. This takes place, for example, when the westerly winds bring air from the Pacific Ocean over the Sierra Nevada Mountains in California. As the relatively warm, moist air rises over the windward side of the mountains, it cools and contracts. If the air is humid, it may form clouds and drop rain or snow. When the air sinks on the leeward side of the mountains, it forms a high pressure zone. The windward side of a mountain range is the side that receives the wind; the leeward side is the side where air sinks.
The descending air warms and creates very strong, dry winds. Foehn winds can raise temperatures more than 20°C (36°F) in an hour and cause humidity to decrease. If there is snow on the leeward side of the mountain, it may disappear by quickly melting and evaporating in the dry winds. If precipitation falls as the air rises over the mountains, the air will be very dry as it sinks on the leeward size of the mountains. This dry, sinking air causes a rainshadow effect (Figure 15.22). Many deserts are found on the leeward side of mountains due to rainshadow effect.
The name of these winds is a bit confusing. Some people refer to all of these winds as Foehn winds, others as Chinook winds, and still others as orogenic winds. The names Foehn and Chinook are sometimes used for any of these types of winds, but are also used regionally. Foehn winds are found in the European Alps, and Chinook winds are found in the Rocky Mountains of North America. Although the description is apt, Chinook does not mean "snow eater".
Santa Ana Winds
"Deadly" is a term often used to describe the Santa Ana winds in Southern California (Figure 15.23). These winds are created in the late fall and winter when the Great Basin east of the Sierra Nevada cools. The high pressure is created when the Great Basin cools forces winds downhill and in a clockwise direction. The air sinks rapidly, so that its pressure rises. At the same time, the air's temperature rises and its humidity falls. The winds blow across the Southwestern deserts and then race downhill and westward toward the ocean. Air is forced through canyons cutting the San Gabriel and San Bernadino mountains. The winds are especially fast through Santa Ana Canyon, which gives them their name.
The Santa Ana winds often arrive at the end of California's long summer drought season. The hot dry winds dry out the landscape even more. If a fire starts, it can spread quickly, causing large-scale devastation. In late October 2007, Santa Ana winds fueled many fires that together burned 426,000 acres of wild land and more than 1500 homes (Figure 15.24). The 2003 Santa Ana winds contributed to the loss of 721,791 acres to wild fires.
High summer temperatures on the desert create high winds, which are often associated with monsoon storms. A haboob forms in the downdrafts on the front of a thunderstorm (Figure 15.25). Air spins and lifts dust and sand into a cloud of dirt that may include dust devils or tornadoes. Haboobs cause many sandstorms.
Dust devils, also called whirlwinds, may also form on hot, clear desert days. The ground becomes so hot that the air above it heats and rises. Air flows into the low pressure and begins to spin. Dust devils are small and short-lived but they may cause damage.
You have already learned that more solar energy hits the equator than the polar areas. The excess heat forms a low pressure cell at the equator. Warm air rises to the top of the troposphere where half of the warmed air moves toward the North Pole and half toward the South Pole. The air cools as it rises and moves along the top of the troposphere. When the cooled air reaches a high pressure zone, it sinks. Back on the ground, the air then travels toward the low pressure at the equator. The air rising at the low pressure zone at the equator and sinking at a high pressure in the direction of the North or South Pole creates a convection cell.
If the Earth was just a ball in space and did not rotate, there would be only one low pressure zone and it would be at the equator. There would also be one high pressure at each pole. This would create one convection cell in the northern hemisphere and one in the southern. But because the planet does rotate, the situation is more complicated. The planet's rotation means that the Coriolis Effect must be taken into account.
The Coriolis Effect causes freely moving objects to appear to move right in the Northern Hemisphere and to the left in the Southern Hemisphere. The objects themselves are actually moving straight, but the Earth is rotating beneath them, so they seem to bend or curve. An example might make the Coriolis Effect easier to visualize. If an airplane flies 500 miles due north, it will not arrive at the city that was due north of it when it began its journey. Over the time it takes for the airplane to fly 500 miles, that city moved, along with the Earth it sits on. The airplane will therefore arrive at a city to the west of the original city (in the Northern Hemisphere), unless the pilot has compensated for the change.
A common misconception of the Coriolis Effect is that water going down a drain rotates one way in the Northern Hemisphere and the other way in the Southern Hemisphere. This is not true because in a small container like a toilet bowl, other factors are more important. These factors include the shape of the bowl and the direction the water was moving when it first entered the bowl.
But on the scale of the atmosphere and oceans, the Coriolis Effect is very important. Let's look at atmospheric circulation in the Northern Hemisphere as a result of the Coriolis Effect (Figure 15.26). Air rises at the equator as described above. But as the air moves toward the pole at the top of the troposphere, it deflects to the right. (Remember that it just appears to deflect to the right because the ground beneath it moves.) At about 30°N latitude, the air from the equator meets relatively cool air flowing toward the equator from the higher latitudes. This air is cool because it has come from higher latitudes. Both batches of air descend, creating a high pressure cell. Once on the ground, the air returns to the equator. This convection cell is called the Hadley Cell and is found between 0° and 30°N.
There are two more convection cells in the Northern Hemisphere. The Ferrell cell is between 30°N and 50° to 60°N. This cell shares its southern, descending side with the Hadley cell to its south. Its northern rising limb is shared with the Polar cell located between 50°N to 60°N and the North Pole, where cold air descends.
There are three mirror image circulation cells in the Southern Hemisphere. In that hemisphere, the Coriolis effect makes objects appear to deflect to the left.
Global Wind Belts
Global winds blow in belts encircling the planet. The global wind belts are enormous and the winds are relatively steady (Figure 15.27). We will be able to figure out how the wind in these belts blows using the information you just learned about atmospheric circulation.
In between each convection cell, where air moves vertically, there is little wind. But where air moves horizontally along the ground between the high and low pressure zones, steady winds form. The air movement of each large circulation cell creates the major wind belts. The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. Some names remain from the days when sailing ships depended on wind for their power.
Let's look at the global wind belts at the Earth's surface in the Northern Hemisphere. In the Hadley cell, air moves north to south, but is deflected to the right by the Coriolis Effect. These winds therefore blow from the northeast to the southwest. They are called the trade winds because at the time of sailing ships they were good for trade. Winds in the Ferrel cell blow from the southwest and are called the westerly winds or westerlies. The westerlies are the reason a flight across the United States from San Francisco to New York City takes less time than the reverse trip. On the outbound flight, the airplane is being pushed along by the westerlies, but on the reverse trip the airplane must fight against the air currents. In the Polar cell, the winds travel from the northeast and are called the polar easterlies. These names hold for the winds in the wind belts of the Southern Hemisphere as well.
The usual pattern of atmospheric circulation cells and the global wind belts determine normal global climate, but many other factors come into play locally. The high and low pressure areas created by the six atmospheric circulation cells generally determine the amount of precipitation a region receives. In low pressure regions, where air is rising, rain is common. In high pressure cells, the sinking air causes evaporation and the region is usually dry. More specific climate affects will be described in the chapter about climate.
The junction between the Ferrell and Polar cells is a low pressure zone. At this location, relatively warm, moist air that has circulated from the equator meets relatively cold, dry air that has come from the pole. The result is a place of extremely variable weather, known as the polar front. This weather is typical of much of North America and Europe.
The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast-flowing river of air at the boundary between the troposphere and the stratosphere. A jet stream can flow faster than 185 km/hr (115 mi/hr) and be thousands of kilometers long and a few hundred kilometers in width, but only a few kilometers thick. Jet streams form where there is a large temperature difference between two air masses. This explains why the polar jet stream is the world’s most powerful.
Jet streams move seasonally as the angle of the Sun in the sky moves north and south. The polar jet stream moves south in the winter and north in the summer between about 30°N and 50° to 75°N. The location of the jet stream determines the weather a location on the ground will experience. Cities to the south of the polar jet stream will be under warmer, moister air than cities to its north. Directly beneath the jet stream, the weather is often stormy and there may be thunderstorms and tornadoes.
- Winds blow from high pressure zones to low pressure zones. The pressure zones are created when air near the ground becomes warmer or colder than the air nearby.
- Local winds may be found in a mountain valley or near a coast.
- Global wind patterns are long term, steady winds that prevail around a large portion of the planet.
- The location of the global wind belts has a great deal of influence on the weather and climate of an area.
- Draw a picture of a convection cell in the atmosphere. Label the low and high pressure zones and where the wind is.
- Under what circumstances will winds be very strong?
- Given what you know about global-scale convection cells, where would you travel if you were interested in experiencing warm, plentiful rain?
- Describe the atmospheric circulation for two places where you are likely to find deserts, and explain why these regions are relatively warm and dry.
- How could the Indian and southeast Asian monsoons be reduced in magnitude? What effect would a reduction in these important monsoons have on that part of the world?
- Why is the name "snow eater" an apt description of Chinook winds?
- Why does the Coriolis Effect cause air (or water) to appear to move clockwise in the Northern Hemisphere? When would the Coriolis Effect cause air to appear to move counterclockwise?
- Sailors once referred to a portion of the ocean as the 'doldrums'. This is a region where there is frequently no wind, so ships would become becalmed for days or even weeks. Given what you know about atmospheric circulation, where do you think the doldrums might be in terms of latitude?
- Imagine that the jet stream is located further south than usual for the summer. What will the weather be like in regions just north of the jet stream, as compared to a normal summer?
- Give a general description of how winds form.
- Coriolis Effect
- The tendency of a freely moving object to appear to move right in the Northern Hemisphere and left in the Southern Hemisphere.
- Foehn winds (Chinook winds)
- Winds that form when low pressure draws air over a mountain range.
- Desert sandstorms that form in the downdrafts of a thunderstorm.
- high pressure zone
- A region where relatively cool, dense air is sinking.
- jet stream
- A fast-flowing river of air high in the atmosphere, where air masses with two very different sets of temperature and humidity characteristics move past each other.
- katabatic winds
- Winds that move down a slope.
- land breeze
- A wind that blows from land to sea in winter when the ocean is warmer than the land.
- low pressure zone
- A region where relatively warm, less dense air is rising.
- mountain breeze
- A wind that blows from up on a mountain down to the valley below in the late afternoon or at night when mountain air is cooler.
- polar front
- The meeting zone between cold continental air and warmer subtropical air at around 50°N and 50°S.
- Santa Ana winds
- Hot winds that blow east to west into Southern California in fall and winter.
- sea breeze
- A wind that blows from sea to land in summer when the land is warmer than the ocean.
- valley breeze
- An uphill airflow.
Points to Consider
- How do local winds affect the weather in an area?
- How do the global wind belts affect the climate in an area?
- What are the main principles that control how the atmosphere circulates?