Environmental Science: Atmospheric Circulation

westerlies

west to east winds that dominate the middle latitudes of both hemispheres on Earth

Albedo

Albedo is a measure of the amount of light that a surface reflects. Different types of surfaces will reflect different amounts of light. For instance, you may have heard that you should wear a white shirt when you go outside on a hot, sunny day. The reason why the white shirt will keep you cooler than a dark shirt is because the white shirts is able to reflect more light than the dark shirt. When you wear a dark shirt, more light energy is absorbed, and as a result, you feel much hotter. The white shirt has a higher albedo than the dark shirt. Here are some albedo figures for various types of surfaces on Earth. Notice how much more light is reflected by snow than by forest cover because snow has a higher albedo than vegetation. Surface % of Light Reflected Fresh snow 80-85 Old snow 50-60 Sand 25-30 Grass 20-25 Dry Soil 15-25 Forest 5-10

Ocean Circulation

An ocean current is a steady flow of surface ocean water in a definite path. Ocean currents are driven by wind, and there are several identifiable surface ocean currents, as seen on the map. A map of global ocean current patterns looks much like the maps you have seen already in this lesson on global atmospheric circulation. Ocean currents are very similar to air currents. Like the atmosphere, ocean currents are affected by the Coriolis effect; they turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. They are also influenced by contrasting amounts of solar radiation at various places on the globe; ocean currents that flow from the equator are warm, while those that flow from the poles are cold. Surface ocean currents travel along the edges of continents until they meet another current flowing in an opposite direction. The result is that surface currents travel in loops called gyres, which rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

The Coriolis Effect

Another factor that influences the movement of air between the poles and the tropic is the fact that the Earth spins on its axis. The axis is an imaginary line that runs through the North pole, the equator, and the South pole. The spinning of the axis gives rise to the Coriolis effect, which is the observation that air at Earth's poles tends to move due to the Earth's rotation. Air from the northern hemisphere appears to veer to the right, and air in the southern hemisphere appears to veer to the left. The What is the Coriolis Effect? video will help you visualize the Coriolis effect. As you watch, think about how the merry-go-round represents Earth and the ball represents moving air.

Jet Streams

Another type of atmospheric circulation is the jet stream, which is a high-speed current of air that forms when there are great temperature differences between adjacent atmospheric cells. Jet streams can travel faster than 115 miles per hour and can be thousands of miles long. Airplane travel times between the same destinations can be different depending on the direction they are going. In one direction, the plane is traveling against the jet stream, which makes the travel time longer, and in the other direction the plane is going with the jet stream, which makes the travel time shorter. Each hemisphere has a subtropical jet stream and a polar jet stream. At the transitions of jet streams, the weather is often stormy. Thunderstorms and tornadoes can also develop as a result of the effects of jet streams. A polar jet stream pushes cold, dry air south, while a subtropical jet stream pushes warm, moist air to the north. The collision of the two air masses is a recipe for disaster because of the weather conditions that result from different temperature air masses like major storms, so weather forecasters play close attention to moving jet streams to help them predict the weather.

What You Should Know

Before you begin, you should: understand how light interacts with objects; understand reflection; understand refraction; understand absorption; understand the properties and components of the Electromagnetic Spectrum.

OBJECTIVES

Describe the relationship between latitude and the amount of solar energy that strikes an area. Explain how the imbalance of heat between low and high latitude areas drives atmospheric circulation and ocean currents. Describe how the Coriolis effect causes a change in the direction of motion. Examine the relationship between global wind belts, jet streams, and the global patterns of ocean currents. Describe El Niño and some weather events associated with it.

Latitude

Earth's radiation budget as a whole is well-balanced. However, the radiation budget does vary by location on Earth. Places closest to the equator receive more solar radiation than the areas near the poles. As discussed in lesson on weather and climate, the distance north or south of the equator is measured by latitude. For reference, the equator has latitude of zero and the poles are at 90° North and 90° South latitudes. The three main reasons that solar radiation varies between the poles and the equator can be seen in the illustration above.

FAQs

I've heard that the Coriolis effect causes toilets to spin counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Is this true? No. This is a common myth about the Coriolis effect. The Coriolis effect is a weak force that can only affect large-scale systems like wind patterns or ocean currents. The force of the flushing toilet determines which direction the toilet will flush, NOT the Coriolis effect.

We're Not in Kansas Anymore

If you ever watch the news in the late summer and early fall, you often hear of extreme weather conditions, such as tornadoes and hurricanes. These atmospheric conditions highlight the fact that the atmosphere is in constant motion, which can lead to catastrophic weather patterns. This lesson is dedicated to learning what factors contribute to the Earth's atmosphere.

Radiation Balance

Imagine you have a money jar on your desk. Every morning, you add 100 pennies to the jar (energy absorbed), and every day you later take out 53 pennies to spend (energy reflected back into space). So that leaves you with 47 pennies (or 47 percent of your original incoming money) at the end of every day. If you did this every day for one year, all those left-behind pennies would really add up. You would have 17,155 pennies (or $171.55). If you did this everyday for ten years, you would have 171,550 pennies (or $1715.50). Fortunately, Earth's radiation balance works to keep all that absorbed solar radiation from building up. Most of it is converted to heat and emitted back to space eventually. This keeps Earth's radiation balance close to equilibrium. As long as incoming solar energy equals the amount of energy leaving the Earth, things stay essentially unchanged—or balanced.

LET'S REVIEW!

In this lesson, you have covered the following points: Much of the sunlight that comes to Earth is reflected back to space or absorbed by clouds. That which is absorbed by land and air is eventually converted to heat and emitted back to space. This keeps Earth's radiation budget in balance as a whole. Equatorial regions have a surplus of solar energy; the polar regions have a deficit of polar energy. Air moves globally in atmospheric cells in response to temperature and pressure differences. In general, air circulates from low pressure, warm equatorial areas to high pressure, cool polar areas. The Coriolis effect describes the impact of Earth's rotation on the movement of air. Jet streams are large, fast-moving currents of air that often bring extreme weather. El Niño is a weather phenomenon that occurs in the Pacific Ocean. It results in weaker winds and warmer sea temperatures.

El Niño:

Our final example of atmospheric circulation is El Niño. El Niño, also known as the El Niño Southern Oscillation (ENSO), is an atmospheric and climatic event where lower air pressure differences across the Pacific region cause a weakening of the trade winds, which normally move seawater westward. As a result of less movement of ocean currents, the water warms and begins to pile up along the eastern side of the Pacific. This reduces upwelling, which generally brings cold water as well as nutrients to the surface for fish and other marine organisms. Fish depend on these nutrients from the deep waters for food. Without these nutrients, the fisheries collapse. El Niño, however, has even greater impacts globally as weather patterns are altered. As the warm water evaporates and rises in the Pacific, it is transferred in global air currents around the world. In some areas of the world, there is torrential rain, flooding, and violent weather activity such as tornadoes. In other areas, there is severe drought and little rain, which makes land ripe for wildfires. This change in atmospheric circulation in the Pacific impacts weather events all around the globe, and scientists are still trying to study its causes and impacts. Learn more about how and why El Niño occurs and how it affects life on Earth by reading the feature article at NASA's Earth Observatory website. As you read the article, look for information about how the trade winds change to bring about an El Niño event. Be sure to click at the bottom of the first page, so you don't miss page two.

EARTH'S RADIATION BALANCE

Solar energy travels through space and enters the top of Earth's atmosphere as visible and ultra violet (UV) light. The diagram shows you the fate of incoming solar energy. Further explanation of the diagram can be seen below. About 50 percent of the solar energy reaches Earth's surface, while the remaining 50 percent is absorbed by clouds, reflected back to space by clouds, or scattered in the atmosphere. Of the 50 percent of solar energy that reaches Earth's surface, approximately 3 percent of it is reflected back to the atmosphere, while the remaining 47 percent is absorbed by water and land. The amount of solar radiation that the Earth commonly receives is called its energy budget. Scientists hypothesize that if the polar ice caps melt due to a surplus of the Earth's energy budget, less of the sun's energy will be reflected back into space. The melting of the ice caps will cause an increase in the amount of solar energy the Earth can absorb.

El Niño

a weather event that occurs when unusually warm waters from the Pacific Ocean disrupt typical global weather patterns

ATMOSPHERIC CIRCULATION

The sun is responsible for heating our earth, but it does so unevenly. As you have learned, the equator receives the most direct sunlight and has a surplus of heat, and the poles receive very little direct sunlight and have a deficit of heat. As a result of this uneven heating of the Earth and the pressure differences that arise, the atmosphere is put into motion. An air cell is simply a parcel of air that moves in a circular motion as it warms and rises followed by its cooling and sinking. This cycle of air rising and sinking occurs continuously throughout the day and across the globe. On Earth, air cells, such as the Hadley cell, occur when warm air from the equatorial regions rises and floats toward the poles. This air has a lower air pressure than the cold air that moves in under it. As the warm low-pressure air travels, it cools and eventually becomes dense enough to sink. When it reaches Earth's surface, it is pulled back toward the equator and warms up once again. Once at the equator, the cycle starts all over. This movement of air between warm and cold areas is also known as convection current.

Currents

When the equatorial current hits the Americas, it turns north and becomes the Gulf Stream current. The Gulf Stream current is large and quick-moving. It is made of warm water and raises air temperatures as it moves northward along the east coast of the United States. Along southeastern Canada, the Gulf Stream swings away from North America and heads east toward Europe. Europe acts as a barrier to the current. Because it can't go through the land, it divides in half. One part of it continues northward; the other half heads back toward the equator. The part that heads north takes relatively warm water to the northern latitudes along Britain and Norway. This gives northern Europe a milder climate than some other parts of the world with similar latitudes. If England did not have the influence of the Gulf Stream's warm waters, it would have much more snow than rain and would be a generally colder place.

Global Wind Belts

You have already learned that the temperature and pressure differences and the Coriolis effect modify the global circulation of the atmosphere. They create global wind patterns that are recognizable and predictable. This map shows the locations of trade winds and westerlies. Trade winds are the movements of air from subtropical areas just north and south of the equator back toward the equator. They move in response to pressure differences. It is important to recognize that the trade winds are named for the direction from which they blow. The warm equatorial air has a lower air pressure than the subtropical air, and air molecules will always move from higher pressure to lower pressure areas. The westerlies are winds that blow from high pressure areas at about North and South 30° latitudes to low pressure areas at about North and South 60° latitudes. Their name comes from the fact that they always blow from the west.

latitude

a measure of distance from the equator, either north or south, on the globe

albedo

a measure of the amount of light that a surface reflects

convection current

any air current that is caused by the transfer of heat in the atmosphere

jet streams

fast-moving currents of air

ocean currents

movement of surface ocean water in predictable paths

coriolis effect

observation that air and ocean currents appear to be deflected in their course of direction due to the spin of the earth on its axis

trade winds

persistent winds that blow from the subtropics toward the equator