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The USGS Water Science School

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The sun is what makes the water cycle work. The sun provides what almost everything on Earth needs to go-energy, or heat. Heat causes liquid and frozen water to evaporate into water vapor gas, which rises high in the sky to form clouds...clouds that move over the globe and drop rain and snow. This process is a large part of the water cycle.

Even in a dry desert environment, the water cycle is taking place. If you look at a picture of a real desert-no, the Sahara Desert will do, but Antarctica at the bottom of the world is even more of a desert. This place actually gets less precipitation than the Sahara does! The inner regions of Antarctica gets only about 2 inches of precipitation per year. The winds here blow up snow from the land and put it into the atmosphere, which is part of the water cycle. And the sun helps out, too, causing sublimation to occur, which causes snow to evaporate directly into water vapor gas.

Credit: NASA

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The sun and the water cycle, from the USGS Water Science School

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The USGS Water Science School [an error occurred while processing this directive] Back to previous page The sun is what makes the water cycle work. The sun provides what almost everything on Earth needs to go-energy, or heat.

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The water cycle shows the continuous movement of water within the Earth and atmosphere. It is a complex system that includes many different processes. Liquid water evaporates into water vapor, condenses to form clouds, and precipitates back to earth in the form of rain and snow.

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Note: Our information only covers the natural water cycle, which does not take human activities into account. In today's world, humans have a major impact on many components of the water cycle.
Access in-depth information about each component of the water cycle:
A (very) quick summary of the water cycle

Where does all the Earth's water come from? Primordial Earth was an incandescent globe made of magma, but all magmas contain water. Water set free by magma began to cool down the Earth's atmosphere, until it could stay on the surface as a . Volcanic activity kept and still keeps introducing water in the atmosphere, thus increasing the surface- and groundwater volume of the Earth.

The water cycle has no starting point. But, we'll begin in the , since that is where most of Earth's water exists. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as into the air. Ice and snow can directly into water vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds.

Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt.

Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff, and groundwater seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for long periods of time.

Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge, and some groundwater finds openings in the land surface and emerges as freshwater springs. Over time, though, all of this water keeps moving, some to reenter the ocean, where the water cycle "ends" ... oops - I mean, where it "begins."

Global water distribution

For an estimated explanation of where Earth's water exists, look at the chart below. By now, you know that the water cycle describes the movement of Earth's water, so realize that the chart and table below represent the presence of Earth's water at a single point in time. If you check back in a thousand or million years, no doubt these numbers will be different!

Notice how of the world's total water supply of about 333 million cubic miles (1,386 million cubic kilometers) of water, over 96 percent is . And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Thus, rivers and lakes that supply surface water for human uses only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 0.007 percent of total water, yet rivers are the source of most of the water people use.
One estimate of global water distribution Source: Igor Shiklomanov's chapter "World fresh water resources" in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water Resources (Oxford University Press, New York).
(Percents are rounded, so will not add to 100)

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Note: Our information only covers the natural water cycle, which does not take human activities into account. In today's world, humans have a major impact on many components of the water cycle. Access in-depth information about each component of the water cycle: Where does all the Earth's water come from?

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Global Wind Patterns

wind belts of the general circulation

The global wind pattern is also known as the "general circulation" and the surface winds of each hemisphere are divided into three wind belts:

Polar Easterlies: From 60-90 degrees latitude.

Prevailing Westerlies: From 30-60 degrees latitude (aka Westerlies).

Tropical Easterlies: From 0-30 degrees latitude (aka Trade Winds).

The easterly trade winds of both hemispheres converge at an area near the equator called the "Intertropical Convergence Zone (ITCZ)", producing a narrow band of clouds and thunderstorms that encircle portions of the globe.

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Global Wind Patterns: wind belts of the general circulation

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Global Wind Patterns wind belts of the general circulation The global wind pattern is also known as the "general circulation" and the surface winds of each hemisphere are divided into three wind belts: Polar Easterlies: From 60-90 degrees latitude. Prevailing Westerlies: From 30-60 degrees latitude (aka Westerlies).

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Global Wind Explained

The illustration below portrays the global wind belts, three in each hemisphere. Note that the U.S. lies primarily in the Westerly Wind Belt with prevailing winds from the west. Each of these wind belts represents a "cell" that circulates air through the atmosphere from the surface to high altitudes and back again. The cells on either side of the Equator are called Hadley cells and give rise to the Trade Winds at Earth's surface. How do we explain this pattern of global winds and how does it influence precipitation?

We'll start at Earth's equator, where solar radiation is the highest year around. Air near the equator is warmed and rises because it is less dense (mass/unit volume) than the air around it as shown in Figure 21 below.

The rising air creates a circulation cell, called a Hadley Cell, in which the air rises and cools at high altitudes moves outward (towards the poles) and, eventually, descends back to the surface. The continual heating and rise of air at the equator create low pressure there, which causes air to move (wind) towards the equator to take the place of the air that rises. On the other hand, sinking air creates high pressure at the surface where it descends. A gradient of pressure (high to low) is formed that causes air to flow away from the high and towards the low pressure at the surface.

The Earth would have two large Hadley cells if it did not rotate. But, because it does rotate, the rotation of the Earth leads to the Coriolis effect. You should view the short video on this so-called "effect" or "force." ( The Coriolis Effect). Without going into detail as to why rotation creates this apparent force, the Coriolis effect causes winds (and all moving objects) to be deflected:

to the right in the Northern Hemisphere

to the left in the Southern Hemisphere

The Coriolis effect causes winds to deflect as they travel within circulation cells and results in the two large hypothetical Hadley cells breaking into six smaller cells, which looks something like the diagram below (and the first figure in this series).

Ok, so, we now have some idea about the origin of global wind systems that result from pressure gradients at Earth's surface. How does this produce precipitation, and where? Precipitation occurs where moisture-laden air rises, either by heating at the equator or by running up and over a more dense air mass. As the rising air cools its capacity to hold water decreases (relative humidity increases) and, at some point, saturation with respect to water vapor is reached. Then, condensation--clouds and rain!

The diagrams above and below portray just the Hadley cell circulation, that is driven by heating in the equatorial region. On the surface, wind moves away from high pressure (High) and toward low pressure (Low). Convergence occurs near the equator (winds blow in towards one another) and Divergence occurs under the descending air that forms high-pressure belts. The final figure (Figure 26) shows all six cells diagrammatically, along with the pressure variations at the surface of the Earth and zones of typical wet and dry belts. Note particularly the dry belts near 30 degrees North and South.

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The illustration below portrays the global wind belts, three in each hemisphere. Note that the U.S. lies primarily in the Westerly Wind Belt with prevailing winds from the west. Each of these wind belts represents a "cell" that circulates air through the atmosphere from the surface to high altitudes and back again.

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Large global wind systems are created by the uneven heating of the Earth's surface. These global wind systems, in turn, drive the oceans' surface currents. To understand how global winds form and drive the major ocean currents, you need to know that wind is the basically the movement of air from an area of high pressure to an area of low pressure. Pressure is force per unit area, and air pressure is simply the weight (force) of the column of air above a particular location, per unit area. Air pressure therefore depends on elevation or altitude (higher up means less air above), the average temperature of the air above the particular location (hot air is lighter than cold air), and what the air's composition is. For example, air with a large amount of water vapor is less dense than dry air because the water molecule has less mass than either an individual nitrogen or oxygen molecule. Also as elevation or altitude increases, air becomes less dense.

Unequal heating of the Earth's surface also forms large global wind patterns. In area near the equator, the sun is almost directly overhead for most of the year. Warm air rises at the equator and moves toward the poles. At the poles, the cooler air sinks and moves back toward the equator. However, it is not this simple. Global winds do not move directly from north to south or south to north because the Earth rotates. All winds in the Northern Hemisphere appear to curve to right as they move. In the southern hemisphere, winds appear to curve to the left. This is known as the Coriolis effect, which is the apparent shift in the path of any fluid or object moving about the surface of the Earth due to the rotation of the Earth.

Near the equator, the trade winds converge into a broad east to west area of light winds. The area is known as the doldrums because there are light winds. This belt of air around the equator receives much of the sun's radiant energy. This area is known as the intertropical convergence zone (ITCZ), and is the area with the most active weather. The latitude where Earth's mean annual surface temperature is highest is located at 10°N. As you learned the Northern Hemisphere has more landmass and is relatively warmer than the Southern Hemisphere. Also at the equator, warmer, moist air rises and produces a low-pressure area extending many kilometers north and south of the equator.

Trade Winds - About 30° north and south of the equator, the warm, moist air that rose vertically cools and begins to sink. Here the sky is clear. There are few clouds and little rainfall. Winds are calm. These are called the horse latitudes, because when food ran out, sailors had to throw horses overboard. Deserts, such as the Sahara in Africa, are also common at 30°N and 30°S. At the horse latitudes some of the sinking air travels back toward the equator. The air moving back toward the equator forms warm, steady winds, known as the trade winds.

The rising air at the equatorial regions and the sinking air at about 30°N and 30°S form huge convection current, known as a Hadley cell for the English meteorologist who first proposed their existence to explain the trade winds.

Prevailing Westerlies - Some of the cool, sinking air continues to move toward the North and South. These winds are called the westerlies and are located between 40°to 60° latitude in both hemispheres.

Polar Easterlies - In both hemispheres, the westerlies start rising and cooling between 50° and 60° latitude as they approach the poles. They meet extremely cold air flowing toward the equator from the poles and form the polar easterlies.

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Large global wind systems are created by the uneven heating of the Earth's surface. These global wind systems, in turn, drive the oceans' surface currents. To understand how global winds form and drive the major ocean currents, you need to know that wind is the basically the movement of air from an area of high pressure to an area of low pressure.

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The Thermohaline Circulation - The Great Ocean Conveyor Belt | Precipitation Education

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The oceans are mostly composed of warm salty water near the surface over cold, less salty water in the ocean depths.

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Since the age of exploration, mariners have needed to know the speed and direction (velocity) of ocean currents to steer their ships within harbors and along trade and exploration routes. A mariner needs to be able to measure the velocity of currents by observing distance, time, and direction.

The simplest method of determining the velocity of a current involves an observer, a floating object or drifter, and a timing device. The observer stands on an anchored ship with a timer. He or she then places the drifter (such as a piece of wood) into the water and measures the amount of time the drifter takes to move along the length of the ship. He or she then stops the timer after the object has traveled some distance, and measures that distance, noting the direction in which the object moved.

The observer then divides the distance the object traveled by the time it took the object to travel that distance, which equals the speed of the current. By combining the speed of the object with the direction in which it moved, the observer can then determine the current's velocity. Ocean currents typically are measured in knots.

Although they still follow the same essential concept to measure ocean currents, mariners today use more accurate and sophisticated instruments. Today, drifters are often elaborate buoys equipped with multiple oceanographic instruments. Some are equipped with global positioning system technology and satellite communications to relay their position in the ocean back to observers on land. Other drifters submerge for long periods of time to measure the ocean currents at depth. The drifter occasionally rises to the surface to send a signal that relays its position.

All drifter measurements are termed "Lagrangian measurements," named after mathematician Joseph Louis Lagrange (1736-1813), who first described the path followed by fluids. But current velocities can be measured another way as well-using "Eulerian measurements." Named after Swiss mathematician Leonhard Euler (1707-1783), Eulerian measurements involve describing fluid flow by measuring the speed and direction of the fluid at one point only. In this method, an instrument is anchored in the ocean at a given location, and the water movement is measured as it flows past the instrument.

Measuring currents by Eulerian methods is becoming increasingly more common. One reason is that it is easier to retrieve these expensive but stationary instruments than it is to locate floating drifters.

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national oceanic and atmospheric administration

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Since the age of exploration, mariners have needed to know the speed and direction (velocity) of ocean currents to steer their ships within harbors and along trade and exploration routes. A mariner needs to be able to measure the velocity of currents by observing distance, time, and direction.

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How do ocean currents work? - Jennifer Verduin

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national oceanic and atmospheric administration

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The global ocean conveyor belt is a constantly moving system of deep-ocean circulation driven by temperature and salinity.