The Atmosphere | kissimmeehomeschool

Atmosphere

What is the sky? What is air? What is the atmosphere?

The atmosphere is a thin layer of gases that surrounds the Earth. It seals the planet and protects us from the vacuum of space. It protects us from electromagnetic radiation given off by the Sun and small objects flying through space such as meteoroids. Of course, it also holds the oxygen (O2) we all breathe to survive.

In the same way that there are layers inside of the Earth, there are also layers in the atmosphere. All of the layersinteract with each other as the gases circulate around the planet. The lowest layers interact with the surface of the Earth while the highest layers interact with space. On your level, you may feel the atmosphere as a cool breeze. Other times you will feel it as a hot or humid day that seems to push on you from all sides.

An Envelope of Gases

When compared to the diameter of the Earth, the atmosphere is very thin. The thickness of the atmosphere is a balance between the gravity of the Earth and energetic molecules that want to rise and move towards space. The molecules in the upper layers of the atmosphere become excited as energy from the Sun hits the Earth. The molecules in the lower layers are cooler and under greater pressure.

If the Earth were larger, the atmosphere would be denser. The increased mass and related gravity of a larger planet would pull those gas molecules closer to the surface and pressure would increase.

The atmosphere is more than just layers of gases surrounding the planet. It is also a moving source of life for every creature of the planet. While the majority of the atmosphere is composed of nitrogen (N2) molecules, there are also oxygen and carbon dioxide (CO2) which plants and animals need to survive. You will also find ozone (O3) higher in the atmosphere which helps filter harmful ultraviolet radiation from the Sun. The atmosphere also protects us from the vacuum and cold of space. Without our atmosphere, the Earth would be as barren and dead as the Moon or Mercury.

Connecting Climates

There is no single climate of the planet. Specialized climates are found in areas all over the planet and might include deserts, rainforests, or polar regions. The common trait of all of these climates is the atmosphere. The atmospherecirculates gases and particles between all of these regions.

The hot air from the equator eventually moves north or south to other climate regions. That warmer air combines with cooler air, mixing begins, and storms form. The constant mixing of the atmosphere maintains a stable system that helps organisms survive. Oxygen will never run out in one area of the planet and temperatures will not skyrocket in another. The atmosphere balances the possible extremes of the Earth and creates an overall stability.

A great example is seen in the way tropical cyclones (hurricanes) form over the Atlantic Ocean. Because of global atmospheric circulation, systems start over the Sahara Desert in Africa, move across the west coast of northern Africa, pick up large amounts of water as they pass over the warm Atlantic Ocean and Caribbean Sea, and finally dump all of the rain on the Caribbean or south eastern coast of the United States. In addition to the stormy weather, the atmosphere can also carry dust and particles from the Sahara to North America.

GEOMETRY OF OCEAN BASINS

Another theory explaining these changes in climate involves the opening and closing of gateways for the flow of ocean currents. This theory suggests that the redistribution of heat on the planet by changing ocean circulation can isolate polar regions, cause the growth of ice sheets and sea ice, and increase temperature differences between the equator and the poles.

Ocean modeling experiments suggest that the ocean could not have carried enough heat to the poles to maintain the early warm climates. But atmospheric climate modeling experiments show that even if the ocean did transport enough heat up to the coast of Antarctica to maintain sea surface temperatures at 10 to 15 degrees Celsius, the interior conditions would still be much colder—and this is contrary to the geologic record. It is possible, however, that changes in heat transport caused by variations in ocean gateways may have played a significant role in cooling trends over the last 60 million years, and, in particular, may help explain some of the relatively sudden cooling events.

ATMOSPHERIC CARBON DIOXIDE

Changes in the concentration of carbon dioxide in the atmosphere are a strong candidate to explain the overall pattern of climatic change. Carbon dioxide influences the mean global temperature through thegreenhouse effect. The globally averaged surface temperature for the Earth is approximately 15 degrees Celsius, and this is due largely to the greenhouse effect. Solar radiation entering earth's atmosphere is predominantly short wave, while heat radiated from the Earth's surface is long wave. Water vapor, carbon dioxide, methane, and other trace gases in the Earth's atmosphere absorb this long wave radiation. Because the Earth does not allow this long wave radiation to leave, the solar energy is trapped and the net effect is to warm the Earth. If not for the presence of an atmosphere, the surface temperature on earth would be well below the freezing point of water.

Through a million year period, the average amount of carbon dioxide in the atmosphere is affected by four fluxes: flux of carbon due to (1) metamorphic degassing, (2) weathering of organic carbon, (3) weathering of silicates, (4) burial of organic carbon. Degassing reactions associated with volcanic activity and the combining of organic carbon with oxygen release carbon dioxide into the atmosphere. Conversely, the burial of organic matter removes carbon dioxide from the atmosphere.

A Cozy Blanket Around The Earth

The atmosphere looks like a blanket of gas when you look at it from space or the ground. When scientists started to examine the atmosphere, they noticed that there were different parts and different layers. There are layers of different molecules, temperatures, and pressures. Overall, the atmosphere is made up of a few main molecules. The air above you is made of 78% nitrogen (N2), 21% oxygen (O2), 0.9% argon (Ar) and 0.04% carbon dioxide (CO2). That's it. The rest of it is made of things called trace elements. Those trace elements include water vapor, ozone, and other particles and molecules floating around.

Thermosphere

The thermosphere is the layer closest to space. There is a huge amount of energy in this layer. The source of that energy is the solar radiation from space hitting the thermosphere. There are very high temperatures because of all the excited atoms zipping around. Something interesting you should know is that even though the temperature is very high (very excited atoms), there is actually very little heat.

Heat happens when energy is transferred from one atom to another. In the thermosphere there is such a low pressure (the molecules are spread out) that there is very little heat transfer. The mesosphere is directly under the thermosphere. The mesosphere has a lower temperature and is the coldest of all the layers in the entire atmosphere.

Stratosphere

The next layer down is the stratosphere. This is a layer with a very large temperature change. It changes from cold to warm, almost to 0 degrees Celsius (which is warm for the atmosphere).

The real importance of the stratosphere is the ozone layer. Scientists call it theozonosphere. Those ozone (O3) molecules absorb large amount of UV (ultra-violet) radiation from the Sun. A chemical reaction takes place when an ozone molecule absorbs the UV radiation. The energy is then radiated as IR (infra-red) radiation, and that is what heats up the layer. Without the ozone, UV light would flood the surface of the Earth and the temperature of the stratosphere would be much cooler.

Troposphere

At the bottom of the atmosphere, where most of the life on the surface exists, is the troposphere. The troposphere is the only atmospheric layer that can support life. The higher layers have filtered out the harmful radiation, and there are large amounts of water vapor.

This is the layer where clouds develop, birds fly, and pollution collects. Yes, the troposphere is where humans most pollute the atmosphere. It's right where we live. The pollution goes into the troposphere and rarely leaves until it falls to the ground or is mixed into the oceans. Some pollutants called CFC's make it into the stratosphere and break down the ozone layer.

Carbon and Oxygen Cyles

What is Caron Dioxide

Measuring The Temperature

The temperature of the air is caused by the combination of air, water, and land working together. Those three factors heat and cool areas at different rates. Temperature also varies by the time of day, the season, or even the year.

When weather people talk about temperature, they are talking about amean temperature. That's an average of a bunch of different measurements. You are usually told an average temperature for a day. That value is the average of all the measurements made at each moment during the day. There is also a temperature range. That range is the difference between the hottest and coldest temperatures in a specific amount of time.

Several factors affect the temperature you are feeling right now. Where are you? Think about it. Chances are it's going to be colder in Alaska than it will be in the Amazon Jungle. How high up are you? Temperatures are different if you're on top of Mount Everest as opposed to right next to the ocean. You know what? It even matters whether you're over the ocean or on land. There are bigger temperature ranges over the land because of the way soil and rock absorb heat.

Local Changes

Look up in the air. What is the weather like? Those clouds make a big difference in the local temperatures. They can block the Sun's heat and trap other heat between the ground and the clouds. Weather is a short term, day-to-day pattern. Don't confuse weather with climate. Scientists may study temperature ranges over years when they look at climates. The temperature you are feeling right now is directly affected by today's weather. Did you know that local areas heat up when it rains? If you think about the water vapor turning into a liquid, you will know a lot of energy is released. Some of that energy goes into increasing the local temperatures.

Ocean Currents

One more factor affects temperatures. You don't see this influence, but the currents in the ocean affect the temperatures. An ocean current change called El Niño affects the world's temperatures. It carries warm water across the Pacific Ocean and up towards the west coast of North America. The weather changes all over the world when El Niño starts up. Local temperatures in the western United States increase and rainfall increases. More rain in one area can also mean higher temperatures and less rain in other areas of the planet. The entire world is connected.

More Effects On Temperatures

We were just talking about temperature differences over land and oceans. To be specific, it can get much hotter and much cooler over the land. The bigger change in temperature is because of how much heat is absorbed by the land (compared to water). Water is constantly moving around, swirling, and mixing in the ocean.

On land the dirt just kind of sits there. When the Sun hits the dirt, it heats up the top layer. Not much heat moves into the lower levels of the ground. The heat that the ocean absorbs is mixed with the lower water quickly. That mixing spreads the heat around. At night, while the land cools off quickly, the water at the surface is kept warmer because the water is mixed around with the warmer water underneath. All of this mixing keeps the temperature in the area more constant, not like the land that goes from hot to cold. Water also absorbs more energy, because it is semi-transparent. The ground is called opaque, because you can't see through it.

What about that altitude thing? It gets colder when you're higher. That makes sense, but why? At higher altitudes, there is a greater gain and loss of heat. When you're up high there is also a lower pressure. There are actually fewer molecules in the air. Because there are not as many molecules, there is less matter to absorb heat. That fact means the temperatures will change more wildly, from hot to cold. There is less matter to absorb the heat.

Clouds affect the temperature too. It's like wearing a windbreaker jacket on a windy day. The jacket traps the heat you give off from your body, keeping it warmer between you and the jacket. That jacket is like cloud cover. It reflects the heat given off by the land. The jacket also reflects heat away from you in the same way clouds reflect incoming heat. The thing to know... Just like a jacket where some heat gets in and some escapes, the clouds don't reflect and keep all the heat, some of it still passes through.

Ocean Effect

Your distance from the ocean also matters. Southern California is a good example. Near the beach is much cooler than it is just over a small set of mountains. The sea air keeps things warmer at night and cooler during the day. This is because of the atmospheric mixing and local winds that are created.

The last idea is one of big ocean currents. Large amounts of heat and energy move every day by the force of the wind acting on the ocean. First you get a long steady wind, like a trade wind. This wind pushes on the surface of the water and the water starts to move... Currents are created. The ocean currents start to swirl and the water circulates across the planet. Generally the ocean is warmer at lower latitudes and colder at higher latitudes. Higher latitudes mean you are closer to the poles. This heat difference also causes the oceans to swirl. Eventually the warmer water from the area near the equator moves up to the poles. As the water moves North or South it gives off heat and warms the atmosphere.

Pushy Pressure

Pressure is the force of one object pushing on another. In the case of the air around you, it is the force of all the air molecules hitting your body. When you are standing on the ground, the pressure is the weight of all the air above you (all the way to the edge of the atmosphere) pushing down on you. Mind you, it doesn't just push down. All of those molecules are pushing sideways, up, diagonally, and every way imaginable.

Think about the desk your computer is on. If the force of the air were just down, it would probably collapse. Because the air also pushes up, the desk is able to stay in one piece and not collapse under the weight of your books. Think about when you go swimming. There is pressure all around you from the water. That pressure is from the water molecules around you and the air above the surface. The deeper you go, the more pressure, and you have to pop your ears.

A Little About Gases

The key to understanding pressure in the atmosphere is to understand how gases work. You can read about gases and how pressure can increase and decrease when forces change (like temperature and volume). We're also adding on the idea of density. Density is the amount of a substance in a specific area. Water has a greater density than ice, which has a greater density than water vapor. When you decrease the volume of a container (and keep the same amount of matter) you will increase the pressure. If you increase the temperature of a container, you will increase the pressure.

Water is a special case where the solid is actually less dense than the liquid form. Ice floats at the top of your soda because it is less dense than the surrounding liquid. The solid version of most compounds is more dense than the liquid version. Liquid states are always more dense that the gas state (under normal conditions).

Real World Explanations

So you have a hot day. Chances are the pressure will rise when it gets hotter. The molecules are getting more excited and have nowhere to go. They wind up pushing on everything with a greater force. Let's say you're up in the sky. There is less pressure because there are fewer molecules above you pushing on you.

That idea explains why the pressure is lower in Colorado than it is on a beach in California. Colorado has a higher altitude. When do you get the greatest pressure? On a hot day? No. Really cold days actually have a higher atmospheric pressure. Why? As the temperature drops, the molecules of the air around you begin to condense and are less excited. These compressed molecules actually create a greater pressure than the excited and hot ones on a warm day. You will be able to prove this fact if you visit Siberia, Russia. Go figure.

Up Up And Away

Let's talk about the very top of the atmosphere. As you move higher above the ground, the temperature will drop and then begin to rise again. The pressure continually decreases. How? There is a lot more energy hitting the Earth from the Sun as you move closer to the outer edge of the atmosphere. This extra energy causes the molecules to get excited and the temperature goes up. As the temperature increases, there is no pressure pushing on the molecules (like on the surface of the Earth) so they can spread out as much as they want. Only gravity pulls on them. They spread out so much that there is actually less pressure than on a place the same temperature lower in the atmosphere.

Atmospheric Circulation

There are both global and local circulations of the air around us. Scientists have different terms for the circulation based on how large the air movements are. They say macroscale to describe wind currents that are on a global scale. Mesoscale describes storms like thunderstorms or blizzards. There are also winds and small circulations that only last for a few seconds. These smaller circulations are described with the term microscale.

Around The Neighborhood

Let's talk about local winds first. Sometimes you're outside of your house and you feel a breeze. There are very fast winds high in the atmosphere, sometimes moving at hundreds of miles per hour. The unequal heating of air masses creates those winds. Those air masses are actually a big chunk of warm air and a big chunk of cold air.

The unequal heating and temperature differences also create a pressure difference, and the warmer gases spread out because the molecules need more room. All of these differences cause the molecules of air to move from one area to another. That air movement is the wind. When you open a soda can you are hearing wind coming out (so to speak). The gas rushes out because of the difference in pressure.

Around The World

Let's look at the larger winds of the Earth called global winds. What about the huge, monstrous winds that circle the globe? What about the trade winds that helped sailors cross the Atlantic and Pacific Oceans? Scientists use the term cells. There are enormous cells of wind that wrap around the Earth. The winds that blow in the cells are created by temperature and pressure differences but also because of the spin of the Earth. The effect of the spinning Earth is called the Coriolis Force.

Building Cells

A big part of circulation is due to temperature differences. Think about the Earth. It is warmer in the middle than on the top and bottom. The poles are colder than the equator. When warm winds want to move north, the cold winds need to move south and fill the empty space. Can you picture the cell being built? It's a big rotation of the gas molecules in the atmosphere.

Changing The Atmosphere

All of the factors in this section affect the air around you. As the atmosphere circulates, it is influenced by many factors. We have discussed ways that the atmosphere affects life around the planet. What happens when life changes the atmosphere? Most animals don't affect the world on a global level. Animals need to adapt to what the world gives them, and they try to survive. Humans are special. Our struggle for survival has actually changed our environment, and now we must react to those changes.

Interacting With The Surf And Turf

Of course, there are interactions between the ocean and land. As the tides go higher, more land is covered with water and local atmospheric changes occur. It may get warmer in the morning and cooler at night. There are also times when a volcano might explode and shoot dust into the atmosphere. The dust moves around the world on the wind currents. A result of the increased amount of particles is less radiation getting to the Earth. Overall, the atmosphere may become cooler.

Here We Go Again

Then there are the humans. We have factories and cars going all the time. Each of these machines is tossing small particles and molecules into the atmosphere. They change the way the Earth works. We make huge amounts of carbon dioxide (CO2). While that carbon dioxide may be good for plants, an excess can slowly heat up an atmosphere. People also use aerosol cans that send out compounds. Those compounds (CFC's) break down ozone in the atmosphere. Less ozone in the atmosphere allows a larger amount of UV radiation to hit the surface of the Earth.

Then It Gets Ugly

You are probably from a country in the western world. Did you know that in China, most of the country still burns coal for energy? They even burn it in the cities. There are many nasty compounds created when you burn coal, not to mention black soot that goes into the air and all over the buildings. Read about how much money China spent to clean up Beijing for the 2008 Olympic Games (just one city). That pollution and uncontrolled industry is what happens when you don't have as many environmental controls as we have in the U.S. Even though people protest about the pollution in the U.S., when you compare it to places like Mexico and China, the U.S. is a very clean country for all the industry and cars we have here.

Coriolis Spinning Around The Earth

The Coriolis Force. It's sometimes also called the Coriolis Effect. Really really simply, it affects everything that moves through the air and it makes everything turn a little bit. Now we'll go into the details. When you look at a satellite picture of the Earth you see all these storms and clouds swirling around. The Coriolis Force causes a lot of that swirling action. It's a force acting on winds because the Earth is spinning. Objects normally move in a straight line when you're on a non-spinning world. However, in a spinning world, if you move in a straight line, you really wind up curving and never get to the place you want to go.

Altitude Differences

The amount of force depends on where you are on the planet. What do we mean? Is it like being in Los Angeles, or in Moscow? No. We mean whether you are on the ground or in the air. Let's say you're a bird or in a plane. If you fly in a straight line, you'll watch the world pass beneath you. If you're on the ground, you watch the plane fly away and you stay in one place. In reality, if you are that bird flying across the United States, you need to change course every now and then. You need to turn a little bit to make sure you wind up where you want to go.

Some Things Don't Matter

Your direction does not matter. Here are the basics: if you are in the Southern Hemisphere you will always wind up curving to the left, no matter what direction you go. If you are in the Northern Hemisphere, you curve to the right. There is an old story, not true, that says that the water in a toilet drains to the right (clockwise) in the Northern Hemisphere and to the left in the Southern. It may not true, but it is a good way to remember the direction the force will move you. Water spinning in a toilet or sink is usually affected of the shape of the sink.

Starting To Spiral

Let's say you have some air. The day heats up and so does the air. The warm air begins to rise. As it moves higher, there is less drag (a force that slows down gas molecules) from the surface of the Earth. The air is able to accelerate and move faster, and as anything goes faster in the atmosphere, the Coriolis Force has a greater force on it. As the air begins to move faster, it starts to turn (right in the North, left in the South). The combination of the air moving from high to low pressure PLUS the Coriolis Force starts a spiral (like a hurricane spinning) set of winds. The winds are called geostrophic winds (strophic means "to turn").

Greenhouse Effect - What Is It?

So you're thinking its nice, comfortable and smells good in a greenhouse. It's hundreds of happy plants and a bit humid. Don't start thinking that thegreenhouse effect for Earth would make such a nice place. It's a term that scientists use to describe a slow increase in atmospheric temperature. That increase could be natural or accelerated by humans. It's not always bad, but it does bring change. As with everything on Earth, there are cycles. Cold periods changing to warmer periods just happen in nature. For our discussion, the current greenhouse effect is changing the world we live in and it might not lead to happy plant times.

It Just Happens

As we mentioned, many things cause the greenhouse effect in our atmosphere. Our story starts with energy from the Sun. High energy radiation hits the Earth and the shorter (more energetic) wavelength energy makes it to the surface. As all of that electromagnetic radiation hits the surface, the land and water heat up. When something heats up, it means they are releasing long wavelength radiation (infrared). When that new form of energy is radiated, it all doesn't leave the Earth. Most of it bounces around our atmosphere. The result is an increase in the atmospheric temperature.

Water vapor also plays a part in trapping that reflected heat energy. More water vapor in the atmosphere allows for more absorption. With that said, think about the larger picture of the planet. As water vapor levels increase, the temperature rises. As the temperature rises, the polar ice caps begin to melt. A portion of that additional water moves to the atmosphere and allows for more absorption. The higher temperatures also lead to more extreme weather including winter storms and tropical cyclones.

All of that natural absorption, radiation, and reflection is supposed to happen. Without it, the temperature of our atmosphere would be much lower and life would have a harder time surviving. But lately, that temperature has begun to increase. and only part of it is natural.

Human Influence

As with everything on our planet, we affect the environment. Related to the greenhouse effect, we are releasing many chemical compounds into the atmosphere that trap that longer wavelength energy (heat). When it is trapped and reflected back, the temperature increases. Fossil fuels are the easy culprit. All of our cars and power plants are spitting out emissions that spread through the atmosphere changing the way the Earth works. There are also many pollutants that are released into the air. It doesn't happen as much in developed countries because of laws that are in place. Developing nations across the world need to generate income, so they often don't think about the pollution. The problem is that the atmosphere of the world is shared and all of their pollution becomes our pollution after a few weeks. Chemical compounds such as carbon dioxide, methane, and ozone are big players in the greenhouse effect.

Other Planets

Astronomers studying our solar system believe that many other planets show signs of the greenhouse effect. Planets such as Venus have many gases in the atmosphere that permanently trap energy received from the Sun, further increasing their temperatures. If it can happen in our solar system, there is also reason to believe we will discover planets in other systems that show the same activity.

Planet Earth Atmosphere

In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide. Plants use carbon dioxide and sunlight to make their own food and grow. The carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like coal and oil over millions of years. When humans burn fossil fuels, most of the carbon quickly enters the atmosphere as carbon dioxide.

Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place. In fact, ice cores show us that there is now more carbon dioxide in the atmosphere than there has been in the last 420,000 years.

Photosynthesis

Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar. This process occurs in plants and some algae (Kingdom Protista). Plants need only light energy, CO2, and H2O to make sugar.

The process of photosynthesis takes place in the chloroplasts, specifically usingchlorophyll, the green pigment involved in photosynthesis.

Go to Versal Overview of Photosynthesis to learn more.

Plants and other producers use carbon dioxide in photosynthesis. They produce oxygen as a waste product. Carbon dioxide moves from the air into the leaves of plants. Oxygen moves from the plant into the air through the leaves. Almost all living things, including plants, get energy from cellular respiration. This process releases energy from the sugar molecules in food. Oxygen is used in cellular respiration. Carbon dioxide is produced as a waste product. The oxygen produced during photosynthesis is used in cellular respiration. The carbon dioxide produced in cellular respiration is used in photosynthesis. This is the oxygen-carbon dioxide cycle.

All living things are made of carbon. Carbon is also a part of the ocean, air, and even rocks. Because the Earth is a dynamic place, carbon does not stay still. It is on the move!

Leaf Cross-Section Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems, etc. The parts of a typical leaf include the upper and lower epidermis, the mesophyll, the vascular bundle(s) (veins), and the stomates. The upper and lower epidermal cells do not have chloroplasts, thus photosynthesis does not occur there. They serve primarily as protection for the rest of the leaf. The stomates are holes which occur primarily in the lower epidermis and are for air exchange: they let CO2 in and O2 out. The vascular bundles or veins in a leaf are part of the plant’s transportation system, moving water and nutrients around the plant as needed. The mesophyll cells have chloroplasts and this is where photosynthesis occurs.

Chlorplast as you hopefully recall, the parts of a chloroplast include the outer and inner membranes, intermembrane space,stroma, and thylakoids stacked in grana. The chlorophyll is built into the membranes of the thylakoids. Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light which is NOT absorbed that finally reaches our eyes, making chlorophyll appear green. However, it is the energy from the absorbed red and blue light that is, thereby, able to be used to do photosynthesis. The green light we can see is not/cannot be absorbed by the plant, and thus cannot be used to do photosynthesis.

The light reaction happens in the thylakoid membrane and converts light energy to chemical energy. This chemical reaction must, therefore, take place in the light. Chlorophyll and several other pigments such as beta-carotene are organized in clusters in the thylakoid membrane and are involved in the light reaction. Each of these differently-colored pigments can absorb a slightly different color of light and pass its energy to the central chlorphyll molecule to do photosynthesis. The central part of the chemical structure of a chlorophyll molecule is a porphyrin ring, which consists of several fused rings of carbon and nitrogen with a magnesium ion in the center.

The Water Cycle

The water cycle has no starting point, but we'll begin in the oceans, 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 vapor into the air; a relatively smaller amount of moisture is added as ice and snow sublimate directly from the solid state into 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, and 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 of the water infiltrates 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 freshwatersprings. Yet more groundwater is absorbed by plant roots to end up as evapotranspiration from the leaves. 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."

Main components of the water cycle

Water storage in oceans
Evaporation
Sublimation
Evapotranspiration
Water in the atmosphere
Condensation
Precipitation
Water storage in ice and snow

Snowmelt runoff to streams
Surface runoff
Streamflow
Freshwater storage
Infiltration
Groundwater storage
Groundwater discharge

Springs

Water storage in oceans: Saline water existing in oceans and inland seas

The ocean as a storehouse of water

The water cycle sounds like it is describing how water moves above, on, and through the Earth ... and it does. But, in fact, much more water is "in storage" for long periods of time than is actually moving through the cycle. The storehouses for the vast majority of all water on Earth are the oceans. It is estimated that of the 332,600,000 cubic miles (mi3) (1,386,000,000 cubic kilometers (km3)) of the world's water supply, about 321,000,000 mi3 (1,338,000,000 km3) is stored in oceans. That is about 96.5 percent. It is also estimated that the oceans supply about 90 percent of the evaporated water that goes into the water cycle.

During colder climatic periods more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age glaciers covered almost one-third of Earth's land mass, with the result being that the oceans were about 400 feet (122 meters) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 18 feet (5.5. meters) higher than they are now. About three million years ago the oceans could have been up to 165 feet (50 meters) higher.

Oceans in movement

If you have ever been seasick (we hope not), then you know how the ocean is never still. You might think that the water in the oceans moves around because of waves, which are driven by winds. But, actually, there are currents and "rivers" in the oceans that move massive amounts of water around the world. These movements have a great deal of influence on the water cycle. The Kuroshio Current, off the shores of Japan, is the largest current. It can travel between 25 and 75 miles (40 and 121 kilometers) a day, 1-3 miles (1.4-4.8 kilometers) per hour, and extends some 3,300 feet (1,000 meters) deep. The Gulf Stream is a well known stream of warm water in the Atlantic Ocean, moving water from the Gulf of Mexico across the Atlantic Ocean towards Great Britain. At a speed of 60 miles (97 kilometers) per day, the Gulf stream moves 100 times as much water as all the rivers on Earth. Coming from warm climates, the Gulf Stream moves warmer water to the North Atlantic.

Evaporation

Evaporation and why it occurs

Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Studies have shown that the oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in our atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration.

Heat (energy) is necessary for evaporation to occur. Energy is used to break the bonds that hold water molecules together, which is why water easily evaporates at the boiling point (212° F, 100° C) but evaporates much more slowly at the freezing point. Net evaporation occurs when the rate of evaporation exceeds the rate of condensation. A state of saturation exists when these two process rates are equal, at which point, the relative humidity of the air is 100 percent. Condensation, the opposite of evaporation, occurs when saturated air is cooled below the dew point (the temperature to which air must be cooled at a constant pressure for it to become fully saturated with water), such as on the outside of a glass of ice water. In fact, the process of evaporation removes heat from the environment, which is why water evaporating from your skin cools you.

Evaporation drives the water cycle

Evaporation from the oceans is the primary mechanism supporting the surface-to-atmosphere portion of the water cycle. After all, the large surface area of the oceans (over 70 percent of the Earth's surface is covered by the oceans) provides the opportunity for such large-scale evaporation to occur. On a global scale, the amount of water evaporating is about the same as the amount of water delivered to the Earth as precipitation. This does vary geographically, though. Evaporation is more prevalent over the oceans than precipitation, while over the land, precipitation routinely exceeds evaporation. Most of the water that evaporates from the oceans falls back into the oceans as precipitation. Only about 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation. Once evaporated, a water molecule spends about 10 days in the air.

Water storage in the atmosphere: Water stored in the atmosphere as vapor, such as clouds and humidity

The atmosphere is full of water

The water cycle is all about storing water and moving water on, in, and above the Earth. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe. There is always water in the atmosphere. Clouds are, of course, the most visible manifestation of atmospheric water, but even clear air contains water—water in particles that are too small to be seen. One estimate of the volume of water in the atmosphere at any one time is about 3,100 cubic miles (mi3) or 12,900 cubic kilometers (km3). That may sound like a lot, but it is only about 0.001 percent of the total Earth's water volume. If all of the water in the atmosphere rained down at once, it would only cover the ground to a depth of 2.5 centimeters, about 1 inch.

Condensation

Condensation: The process by which water is changed from vapor to liquid

Condensation is the process in which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation.

You don't have to look at something as far away as a cloud to notice condensation, though. Condensation is responsible for ground-level fog, for your glasses fogging up when you go from a cold room to the outdoors on a hot, humid day, for the water that drips off the outside of your glass of iced tea, and for the water on the inside of your home windows on a cold day.

Condensation in the air

Even though clouds are absent in a crystal clear blue sky, water is still present in the form of water vapor and droplets which are too small to be seen. Depending on meteorological conditions, water molecules will combine with tiny particles of dust, salt, and smoke in the air to form cloud droplets, which grow and develop into clouds, a form of water we can see. Cloud droplets can vary greatly in size, from 10 microns (millionths of a meter) to 1 millimeter (mm), and even as large as 5 mm. This process occurs higher in the sky where the air is cooler and more condensation occurs relative to evaporation. As water droplets combine (also known as coalescence) with each other, and grow in size, clouds not only develop, but precipitation may also occur. Precipitation is essentially water cloud in its liquid or solid form falling form the base of a cloud. This seems to happen too often during picnics or when large groups of people gather at swimming pools.

You might ask ... why is it colder higher up?

As we said, clouds form in the atmosphere because air containing water vapor rises and cools. The key to this process is that air near the Earth's surface is warmed by solar radiation. But, do you know why the atmosphere cools above the Earth's surface? Generally, air pressure, is the reason. Air has mass (and, because of gravity on Earth, weight) and at sea level the weight of a column of air pressing down on your head is about 14 ½ pounds (6.6 kilograms) per square inch. The pressure (weight), called barometric pressure, that results is a consequence of the density of the air above. At higher altitudes, there is less air above, and, thus, less air pressure pressing down. The barometric pressure is lower, and lower barometric pressure is associated with fewer molecules per unit volume. Therefore, the air at higher altitudes is less dense. Since fewer air molecules exist in a certain volume of air, there are fewer molecules colliding with each other, and as a result, there will be less heat produced. This means cooler air. Do you find this confusing? Just think, clouds form all day long without having to understand any of this.

Percipitation

Precipitation: The discharge of water, in liquid or solid state, out of the atmosphere, generally upon a land or water surface

Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.

How do raindrops form?

The clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as precipitation, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud you can see some parts disappearing (evaporating) while other parts are growing (condensation). Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds. For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. Water droplets may grow as a result of additional condensation of water vapor when the particles collide. If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is not a trivial task since millions of cloud droplets are required to produce a single raindrop.

Precipitation rates vary geographically and over time

Precipitation does not fall in the same amounts throughout the world, in a country, or even in a city. Here in Georgia, USA, it rains fairly evenly all during the year, around 40-50 inches (102-127 centimeters (cm)) per year. Summer thunderstorms may deliver an inch or more of rain on one suburb while leaving another area dry a few miles away. But, the rain amount that Georgia gets in one month is often more than Las Vegas, Nevada observes all year. The world's record for average-annual rainfall belongs to Mt. Waialeale, Hawaii, where it averages about 450 inches (1,140 cm) per year. A remarkable 642 inches (1,630 cm) was reported there during one twelve-month period (that's almost 2 inches (5 cm) every day!). Is this the world record for the most rain in a year? No, that was recorded at Cherrapunji, India, where it rained 905 inches (2,300 cm) in 1861. Contrast those excessive precipitation amounts to Arica, Chile, where no rain fell for 14 years

The map below shows average annual precipitation, in millimeters and inches, for the world. The light green areas can be considered "deserts". You might expect the Sahara area in Africa to be a desert, but did you think that much of Greenland and Antarctica are deserts?

On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches (0.76 meters).

Water storage in ice and snow: Freshwater stored in frozen form, generally in glaciers, icefields, and snowfields

ice caps around the world

Although the water cycle sounds like it is describing the movement of water, in fact, much more water is in storage at any one time than is actually moving through the cycle. By storage, we mean water that is locked up in its present state for a relatively long period of time, such as in ice caps and glaciers.

The vast majority, almost 90 percent, of Earth's ice mass is in Antarctica, while the Greenland ice cap contains 10 percent of the total global ice-mass. The Greenland ice cap is an interesting part of the water cycle. The ice cap became so large over time (about 600,000 cubic miles (mi3) or 2.5 million cubic kilometers (km3)) because more snow fell than melted. Over the millenia, as the snow got deeper, it compressed and became ice. The ice cap averages about 5,000 feet (1,500 meters) in thickness, but can be as thick as 14,000 feet (4,300 meters). The ice is so heavy that the land below it has been pressed down into the shape of a bowl. In many places, glaciers on Greenland reach to the sea, and one estimate is that as much as 125 mi3 (517 km3) of ice "calves" into the ocean each year—one of Greenland's contributions to the global water cycle. Ocean-bound icebergs travel with the currents, melting along the way. Some icebergs have been seen, in much smaller form, as far south as the island of Bermuda.

Ice and glaciers come and go

The climate, on a global scale, is always changing, although usually not at a rate fast enough for people to notice. There have been many warm periods, such as when the dinosaurs lived (about 100 million years ago) and many cold periods, such as the last ice age of about 20,000 years ago. During the last ice age much of the northern hemisphere was covered in ice and glaciers, and, as this map from the University of Arizona shows, they covered nearly all of Canada, much of northern Asia and Europe, and extended well into the United States.

Some glacier and ice cap facts

Glacial ice covers 10 - 11 percent of all land.
According to the National Snow and Ice Data Center (NSIDC), if all glaciers melted today the seas would rise about 230 feet (70 meters).
During the last ice age (when glaciers covered more land area than today) the sea level was about 400 feet (122 meters) lower than it is today. At that time, glaciers covered almost one-third of the land.
During the last warm spell, 125,000 years ago, the seas were about 18 feet (5.5 meters) higher than they are today. About three million years ago the seas could have been up to 165 feet (50.3 meters) higher.

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