Environment---Format of print

Environment

Evaporation

Oceans hold 97% of the water on the earth. The second largest amount of water, slightly over 2%, in frozen in huge, moving bodies of ice, called glaciers. The remaining water, less than 1%, is found flowing underneath and on top of the earth's surface. This 1% of water, a seemingly small amount, is the source of almost all of the water people every day.

As the sub beats down upon the earth, land surfaces absorb the heat and quickly release it back into the air again. Water, on the other hand, has a larger capacity than land for absorbing heat. Heat is not released into the air as quickly, which helps to keep the earth's atmosphere from becoming too hot. Many of the other planets in our solar system have wide ranges of temperature -- at times varying by hundreds of degrees. Water in the earth's atmosphere and on its surface helps prevent this from happening on our planet.

Surface water

Water in oceans, rivers, and lakes -- is an easily observed part of the hydrologic cycle. We can see surface water as it flows across the land. In fact, surface water often changes the land around us.

Flowing surface cuts into the land and erodes, or wears away, rock and soil. Pieces of eroded rock and soil, now called sediment, may be carried away by the water and deposited else and rolling it along ocean, lake, and river bottoms. Lighter sediment is carried suspended in the water. Sometimes so much sediment is suspended that the water appears cloudy. Water also moves sediment by dissolving it completely.

Each year oceans move billions of tons of sediment. Waves batter rocks and sand, changing cliff faces and beaches. Where there are significant changes in seasons, closely spaced, choppy winter waves crash onto beaches, narrowing them by pulling sand back into the ocean. The more widely spaced, lower waves of summer carry sand toward the shore, causing beaches to widen.

Powerful ocean currents carry sediment to new places, in turn building new land areas as the sediment is deposited. Many ocean side resorts and vacation homes are built on offshore ridges of sand known as barrier islands. Although these islands seem to be secure land, waves generated by powerful hurricanes can be strong enough to wash away buildings and sand.

Every time rivers overflow, they carry sediment. When flooding ends and rivers return to their normal channels, sediment is left behind. Over time and repeated flooding, some sediment deposits may form hills along the riverbanks. These hills, called levees, are natural barriers that can help prevent some future flooding.

Flat land alongside rivers is frequently covered by water when flooding occurs. These flat areas, called floodplains, are often covered with widespread layers of sediment that have been deposited by past floodwaters. Floodplain sediment can be many inches or feet thick. As a matter of act, much of the damage done inside homes that have been built on floodplains is caused by thick, muddy sludge left behind as rivers flow back into their channels.

Ground water

The hydrologic cycle continues as water seeps down into soil and rock beneath the earth's surface. Water flowing below the surface is called groundwater. Gravity, the force that prevents us from floating up into the air, pulls water downward through air spaces and cracks in rock and soil. This area is called the zone of aeration because of its many air spaces, or pores.

Water seeps into the ground at varying rates depending on the soil. Sandy soil has many little pores between its individual grains of sand. Gravel has larger pores between its rock pieces. The larger the pores and passageways, the easier it is for water to trickle through. If a bucket of water is poured onto a pebbly driveway, the water disappears faster than it would in sand because water flows more rapidly through the pebbles' larger pore spaces. Other soil and rocks have smaller air spaces between their grains. Clay, for example, has very tiny grains and pores; water does not flow as rapidly through the tiny air passageways. That's one of the reasons people make spots and bowls out of clay.

Rock that has many air spaces is called porous rock. Water often flows inside this kind of rock. When water moves easily through porous rock, the rock is said to be permeable.

Usually, groundwater that flows inside porous rock does not move very quickly, especially if the pores and pathways are tiny. Most groundwater flows an average of an inch or less per day, but water can flow as quickly as tens of yards per day in permeable rock. Groundwater follows the easiest, route, usually to a lake or stream, but may take thousands of years to reach the land's surface. To find out where groundwater travels and how quickly it is moving, scientists sometimes inject dye into groundwater. They measure how long the coloring takes to reach a nearby well.

Below the zone of aeration, downward-moving water mixes with groundwater that has already seeped beneath the earth's surface. In this area, all the air spaces are already filled with water. Because the passageways between soil and rock grains are saturated, or completely filled with water, this area the zone of saturation. The top of the zone of the saturation is referred to as the water table. The water table can be far blow the ground or very near the surface. Streams and lakes mark the water table at the land's surface. A shallow hole dug fairly close to a lake's or ocean's shoreline quickly fills with water because the water table is near the surface. In other places, the water table may be many feet beneath the surface. During periods of drought, groundwater drains from the soil into streams, causing the water table to drop even farther below the surface. Sometimes the land surface ends abruptly, perhaps at a valley wall, cliff, or along cracks in rocks. If the water table crosses these places, groundwater flows out. This flowing groundwater is called a spring.

Groundwater is the main source of drinking water for many cities. When people drill wells looking for water, they try to find a aquifer. An aquifer is a layer of permeable underground rock saturated with groundwater that can flow easily into wells. Aquifers can be found under more than half the land areas of the United States.

Caves

Groundwater, like surface water, can change the shape of the land. As water trickles down through the soil can into the rock. Below, it may mix with a gas called carbon dioxide, which is found naturally in the air and the soil. When mixed with the carbon dioxide, water becomes mildly acidic, which makes it possible to dissolve some types of rock. The solution of water, dissolved rock, and carbon dioxide is carried away to streams and gradually flows to the sea, where it may combine with other solutions and form new rock material. If a hole with an opening to the surface is left where the rock dissolved, the hole is called a cave.Caves can be many shapes and sizes. Large caves with connecting chambers are known as caverns. They are cool, dark, and silent places, where the only sound may be the steady dripping of water as it trickles on rock surfaces.This water, which frequently carries dissolved rock material, can form marvelous deposits. The most common of these are stalactites. Like icicles made of stone, stalactites are formed when water rich in dissolved rock material drips from cave ceilings. Stalagmites are another deposit formed by water carrying dissolved rock material. They resemble icicles that grow upward from the cave floor. If a stalactite and a stalagmite join, they create a column.
Sinkholes, another land feature created by groundwater, are large pits formed in much the same way as caves are-acidic groundwater dissolves certain rock material. In fact, some sinkholes form when the roofs of caves collapse. And many sinkholes drain into caves.

Evaporation

Water found in the soil is not always pulled downward into the rock below. The hydrologic cycle continues as plants use their roots to pull water from the soil. Once inside a plant, the moisture, now called sap, travels through the plant's trunk or stem and out to its leaves. Tiny holes on the undersurface of leaves allow moisture to escape into the air. The process is known as transpiration. Water also evaporates from leaf surfaces. During the processes of evaporation and transpiration, a full-grown tree can release as much as 40,000 gallons of water a year-enough to fill a very large swimming pool!

Water also evaporates from oceans, lakes, rivers, and the land's surface, and becomes water vapor in the atmosphere. Heat energy from the sun is used to break the bonds between water molecules, causing evaporation. As molecules are heated, they begin moving rapidly. Scientists say that rapidly moving molecules are "excited." The rapid movement of excited water molecules is strong enough to break their bonds so they can change into vapor.
Water does not evaporate at the same rate everywhere. Warm air absorbs more moisture than cold air. For each 18° F (8° C) increase in air temperature, the air can hold two times more water. Also, warm water evaporates more quickly than cold water. The exact point at which air becomes saturated with water vapor varies according to the temperature.

Condensation

Molecules of water vapor in the air are so small you cannot see them. For a person to be able to see an object, light must strike the object, which then reflects, or bends, the light back toward a person's eyes. Water molecules are so small that they do not bend back enough light for us to see. As water molecules combine, they form water droplets. For a water droplet to be visible it must contain about 10 billion molecules of water.

When water vapor rises into the atmosphere high above the earth's surface and meets cooler air, it can change in two ways. One way the vapor may change is by turning into water droplets. But if the air temperature is very cold, deposition-the change of water vapor directly into solid ice crystals-may occur. Deposition also takes place on the ground. Frost is an example of water vapor that has gone through deposition, changing from a vapor into a solid without first becoming liquid water.

During warm months, the sun heats the air. But the high air temperatures often fall at night after the sun goes down. In the early morning, blades of grass may be coated with water droplets, or dew, the result of cooler night air temperatures. (Remember, cool air holds less water than warm air.) When water vapor condenses, dew forms on surfaces. As the air cools, it reaches a point, called the dew point, at which it is saturated with water vapor. Condensation begins at the dew point because the air is saturated and cannot hold any more water.

As water vapor high in the atmosphere cools to the dew point, it condenses around dust particles, forming droplets. The droplets come together in larger and larger clumps until they are big enough to reflect light for us to see. These visible clumps of water droplets and if the air temperature is cold enough, ice crystals-are called clouds.
Millions of water droplets and ice crystals combine to form the large clouds we see in the sky. The cottony clouds that look so fluffy and light actually contain enough water droplets and ice crystals to weigh half a million tons. The enormous black clouds of a thunderstorm may weigh several million tons! When water droplets and ice crystals become too heavy for air currents to hold them suspended in the atmosphere, they fall toward the earth as precipitation.

Although clouds can contain enough water to weigh millions of tons, the total amount of water contained in all the clouds at any one time is actually less than .001 of I percent of the earth's water supply. If suddenly all the water droplets, ice crystals, and vapor contained in clouds and the atmosphere were to condense and fall evenly over the earth's surface, the total amount would only measure about 1 inch (2.54 cm) of rain.

Fog

A cloud that touches the earth's surface and is so thick that visibility is less than .62 mile (1 km) is called fog. If visibility is greater than .62 mile (1 km), the ground-based cloud is called mist.
Fog develops when air becomes chilled to the dew point. This usually occurs when warm, moist air flows over cold water. Fog often forms along the northeastern coast of the United States and Canada. The Gulf Stream-a warm ocean current that flows in a northeasterly pattern from the Gulf of Mexico toward Newfoundland, Canada-brings a steady stream of warm air over the colder North Atlantic Ocean, frequently causing heavy fog to develop. Sometimes the ground temperature helps to cause fog. When warm, moist air passes over colder or snow-covered ground, the cold temperature of the ground lowers the temperature of the air to the dew point, and fog forms.

In 1875 Paul jean Coulier, a Frenchman, experimented with air and fog. He sealed moist air in a glass container and applied pressure to the air. Fog appeared as the air was squeezed. After repeating the experiment a number of times, fog no longer appeared. Coulier wondered why. Finally, he decided to add new air to the container. He wanted to see if the new air would make a difference. It did. Fog reappeared when he applied pressure to the new air. Water vapor was condensing around something in the new air that was too small to see. Coulier believed that dust particles were attracting water vapor. Dust in the air, much finer than the dust you may find on the furniture in your home, most likely consists of salt particles from ocean spray, volcanic dust, and even molecules of certain gases that have condensed in the air. Coulier concluded that fog appears only when water vapor is able to condense around dust particles that are present in the air. Applying pressure forces vapor to attach itself to dust particles. Once all the dust particles have been used during condensation, no additional fog can appear. The discovery of the role of dust particles in the atmosphere was an important step toward understanding how the hydrologic cycle works.

Rainfall

Hail

Another potentially destructive form of precipitation is rounded, lumpy grains of ice, called hail. Balls of hail, usually called hailstones, form during thunderstorms when ice crystals are tossed high inside thunderclouds by strong, turbulent air currents. Supercooled water droplets inside the clouds freeze onto these ice crystals and make each crystal larger by adding new icy layers. This type of growth pattern, called concentric layering, has a small center with layers surrounding it, much like the layering you find when you cut an onion in half. New layers continue to be added until the hailstones become too heavy for the air currents to carry, and the hailstones fall to the ground.
Hailstones range greatly in size. Pea- to marble-sized hailstones are common. During severe storms, air currents can be strong enough to support hailstones the size of golf balls, which can do great damage to plant life and property.

Several things can happen to precipitation when it reaches the earth's surface. Most precipitation lands on the ground or in surface water. About 15 to 20 percent of the rain that falls on land surfaces soaks into the ground. Some precipitation lands on plants, where it either remains on the leaves and eventually evaporates, or slides off and falls to the ground. More than half of the precipitation is returned to the atmosphere through evaporation and transpiration.
Water can be on the earth's surface and not be an active part of the hydrologic cycle. In or near polar regions or on mountaintops, temperatures are so cold that snow and ice can accumulate in deep layers and eventually form glaciers. Glaciers may last hundreds or even thousands of years. During this time, the frozen water is temporarily removed from the hydrologic cycle. Eventually, however, when the air temperature warms and melting occurs, snow and ice occupy the same place in the hydrologic cycle as rainwater.

Precipitation

Precipitation is another part of the hydrologic cycle. Precipitation begins when water vapor condenses and falls toward the earth as ice, snow, rain, or freezing rain. Ice crystals, tiny hexagonal, or six-sided, crystals found high in the atmosphere, are the first stage in the formation of snowflakes. If an ice crystal continues to grow larger, a snow crystal is formed. Snow crystals, like ice crystals, are six-sided, but they have a more complex shape. A snowflake forms when two or more snow crystals become joined. Some snowflakes may be made of several hundred snow crystals that have come together.
In 1880 Wilson Alwyn Bentley, a 15-year-old boy who lived in Jericho, Vermont, began examining snowflakes through a microscope. He noticed that snowflakes were crystals. Although he was not the first person to notice this, what he did five years later had never been done before and led to a lifelong study of snowflakes. In 1885 Bentley attached a special camera to his microscope and took the first successful photographs of snowflakes. "Snowflake' Bentley, as he came to be called, photographed many thousands of snowflakes during the 40 years that followed, and he never found two
that were identical.
Snowflakes come in many sizes. Snow crystals and flakes that form in especially cold air, where there is less water vapor available, tend to be small. Warmer air, with more available moisture, tends to favor the formation of large, wet flakes. These flakes frequently collide with other flakes and stick together while floating downward, sometimes forming snowflakes with diameters as large as 2 inches (5 cm).

Raindrops

Almost all raindrops begin as snowflakes. They are formed in clouds that are at least partly high enough for the air temperature to remain below freezing. When snowflakes drift into a lower, warmer part of the cloud, the flakes melt and become raindrops.
Like snowflakes, raindrops are not all the same size. They usually range from about .02 inches (.05 cm) to .2 inches (.5 cm) in diameter. The largest raindrops are the ones found during heavy rainstorms, when people are likely to say it's raining cats and dogs. A raindrop size can change as wind tosses it around or as it collides with other raindrops.
Throughout the years 1898 to 1904, Wilson Bentley not only photographed snowflakes, but studied raindrop size as well. He filled several pans with at least 1 inch (2.54 cm) of fine, sifted flour, placed them outside during a storm, and found that each raindrop that landed in the flour-filled pan formed a doughy pellet. After the pellets dried, Bentley measured them-some were almost .25 inch (.64 cm) in diameter. Bentley studied the raindrops from 70 different storms and made 344 raindrop measurements.

One of the least known facts about raindrops is that they are not shaped like teardrops. Special wind experiments done in laboratories have made it possible to suspend, or stop, raindrops in midair. Scientists have found that raindrops are actually shaped like hamburger buns!
A heavy rainfall can change the landscape. In moist places, rain falls fairly regularly. Floods in these areas are likely to be widespread and tend to rise and fall slowly, because the soil and plants absorb the rain. Desert areas, however, often have wild, rough floods. Storms in desert areas tend to be heavy rains that last for a short time. Because the soil has been baked dry by the sun, it is difficult for rain to soak in quickly. Since there is little or no vegetation to help slow water runoff and hold soil in place, rain rushes across the land in dirt-filled, swift-moving sheets or streams. This kind of rapidly flowing, dangerous water is called a flash flood. Flash floods usually occur very quickly, almost without warning-
A heavy rainfall in an agricultural area may wash away good topsoil. The faster the water flows, the more topsoil it sweeps away. Topsoil is important because healthy crops depend on this nutrient-rich layer of dirt to grow. Fortunately, farmers can use certain patterns of plowing to slow down flowing water and to reduce the amount of topsoil carried away.

Freezing rain forms when raindrops are supercooled, that is cooled below the freezing point without turning into ice. Upon contact with a cold surface, freezing rain immediately turns from a liquid to a solid, covering everything under a layer of ice. Although the icy layer becomes a sparkling winter spectacle when the sun shines on it, ice-coated tree branches and electrical wires can be dangerous if they become too heavy and snap. Streets and sidewalks covered with a slippery coating of freezing rain are treacherous for traffic and pedestrians.

Advanced knowledge

The water cycle

The most noteworthy characteristic of any small body of fresh water-be it a pond, a stream, an icicle, or a rain cloud-is its impermanence. Ponds evaporate, streams flow to the sea, icicles melt and dribble away, rain falls; water is forever on the move, repeatedly changing its state-liquid, ice, or vapor-in the process. In the word, it is dynamic. In much the same way that every living organism has a life cycle, water has a water cycle; it circulates. Indeed, all the water on earth is constantly circulating.
In one form or another it is familiar to almost everybody. Water falls to earth as rain or snow, some on land and some at sea; much of what reaches the land drains into the sea, by surface routes below the ground. The rest evaporates back into the atmosphere, either directly or through the vegetation: all plants on the land absorb water from the ground and exhale most of it into the air, as vapor, in a process called transpiration. The vapor from the land mixes with vapor evaporating from the sea, and, together, they condense into rain-giving clouds again. The cycle keeps on indefinitely, with the world ocean as the reservoir. The bulk of all this water is therefore salty sea water. Only the water temporarily withdrawn from the ocean is fresh.
Now for some statistics. The total amount of water on earth at the present time is approximately 1.4 billion cubic kilometers, an impossibly large quantity to visualize. If it were solidified into a cube, each edge of the cube would be about 1,120 kilometers long, approximately twice the length of Lake Superior.
The amount of fresh water in the world today is approximately 36 million cubic kilometers, a mere 2.6 percent of the total; of this fresh water, only 11 million cubic kilometers (0.77 percent of the total, or 30 percent of the fresh water) counts as part of the water cycle, in the sense that it circulates comparatively fast. Of the water not in circulation, most is immobilized in long-lived polar ice sheets, and some is trapped, stagnant, under the ground.
No single answer can be given to the question, how fast does the water circulate? This is because the time taken by a given drop of water to complete the cycle, from ocean back to ocean, varies tremendously. It ranges from minutes or hours, as when a rainstorm blows inland from the sea, to thousands of years, the time during which a water drop may be frozen into a glacier. Indeed, there isn't a sharp distinction between circulating and noncirculating water: given enough time-hundreds of thousands, or millions, of years-all water circulates.
These statistics apply to what is happening at the present day. Until the 1980s, it was believed that the total amount of water on the earth had remained the same, with only negligible changes, throughout the lifetime of the planet; in other words, that the earth's water cycle was virtually closed-no new water ever entered it and no existing water ever left it. New discoveries in the late 1980s have inspired new speculation. Many scientist now believe that the earth's water supply is growing all the time, though not fast enough to solve humanity's water-shortage problems. According to the new theory, loosely packed "snowballs" of nearly pure snow, the size of small house, are entering the earth's gravity field from the outer parts of the solar system every few seconds; they have been dubbed small comets, and most of them appear to weigh between 20and40tons.They melt and vaporize when they get near the earth. If this "rain of snowballs" has been going on at the current rate for billions of years (and there is no reason to suppose that it hasn't), the amount of new water we are continually gaining is equivalent to about 6 millimeters of rainfall over the whole earth, or 3 trillion tons of water, every 10,000years.
The amount of fresh water on earth, as a fraction of the sum total of all water, fresh and salt, varies from one geological epoch to another. So does speed of the water cycle. At times when the whole world was warm-no ice sheets anywhere-and shallow, inland seas were more extensive than they are now, the amount of liquid water in circulation would have been much greater than it is at present, and the ratio of salt water to fresh would undoubtedly have been different. Conversely, at the height of an ice age, liquid fresh water was comparatively scarce, because much was immobilized in ice sheets. At the same time, the low temperatures would have reduced evaporation rates and precipitation rates, slowing the whole cycle.
To return to the present, to the fresh water the world holds now. People is general are at last becoming aware that human demands for fresh water are beginning to outstrip the supply. Quite probably, fresh water will turn out to be the factor that limits population growth, as limited it obviously must be.
Fresh water has a "natural history" of its own: the water cycle can be thought of as a "life cycle". Water comes from the sea as a vapor, travels for a time as a liquid, sometimes lingers as ice, and then returns to the sea again. Fresh water" behaves" in a number of different ways: it moves over and through the ground, sometimes fast and sometimes slowly; it pauses in lakes and ponds; it freezes; it vaporizes; it creates habitats for a wide range of ecosystems; it shapes the land. In brief, it is active and powerful, indeed more active and more powerful than the living things whose lives depend on it.

Humidity

When a lot of water vapor is in the air, we say the air is humid. Scientists frequently use the term relative humidity, which describes how much water vapor is in the air at a particular temperature compared with how much water the air at that same temperature is able to hold. An easy way to understand this is to imagine that the air is a towel. If you spill a glass of water, you can wipe up the water with a towel. But the towel probably could absorb more than just a glassful of water. Perhaps it could hold water spilled from 5 or 10 glasses before becoming completely soaked. The amount of water vapor actually present in the air is often only a fraction of the total amount that the air can hold, so relative humidity is expressed as a percentage. When the relative humidity is 100 percent, the air is saturated. Like a towel totally soaked with water, the air can hold no more moisture. When the relative humidity is 100 percent and the air is saturated, evaporation and precipitation are in a state of balance. As moisture precipitates, the amount of evaporation increases to reach a balanced state again.

Water vapor in the air is called humidity. Because molecules of water vapor are so small they can't be seen, people who study humidity have developed creative ways to measure the amount of vapor in the air.
Probably the first person to think of an instrument to measure the vapor content of the air was Leonardo da Vinci, a man who was born in Italy in the 15th century. He placed a small wad of dry cotton on one side of a balance scale. Then he placed an object of exactly the same weight as the wad of cotton on the other side of the scale. As the dry cotton absorbed water vapor from the air, it became heavier and the balance pan lowered. The difference between the two weights was the measure of the humidity.
Now scientists use an instrument called a psychrometer to measure relative humidity. A psychrometer is made of two thermometers that are fastened next to one another. The bulb of one thermometer is wrapped in material that is soaked with purified water. To begin measuring relative humidity, a person whirls the psychrometer around until the thermometer with the wet material reaches a steady temperature, which is always lower than the temperature on the dry bulb. The actual air temperature is measured by the thermometer with the dry bulb. The difference between the two temperatures is called the wet-bulb depression and is the result of the evaporation of water from the material. Scientists mark the dry-bulb temperature and the wet-bulb depression on two charts, called Psychrometric Tables, to calculate relative humidity and dew point temperature.

Scientists also measure humidity with an instrument called a hair hygrometer. Materials such as wood, cotton, skin, and hair absorb moisture from the air. Human hair gets longer as it absorbs water, increasing its length by about 21/2 percent over a relative humidity range of 0 to 100 percent.
On a hair hygrometer, strands of hair are attached to a pointer on a mechanical dial that has been specially marked to indicate relative humidity. The pointer moves as the hair lengthens or shortens. If a written record of relative humidity is needed, a hair hygrometer can be connected to a hygrograph, which has a clock-driven pen that marks a continuous line on graph paper.

The amount of water vapor in the air determines how comfortable we feel on a given day. One way the human body releases heat is by sweating. Evaporation of sweat from our skin helps us cool down on hot days. We feel cooler because during the evaporation process, water molecules require energy to change into vapor. The kind of energy they use is heat energy taken from the water molecules' environment, which in this case is our skin. If the air already contains a lot of water vapor, however, sweat does not evaporate quickly, and we continue to feel hot and uncomfortable.

The water budget

Hydrologists, scientists who study water, have compared the amount of water in oceans with the amount of water on land. The result of their findings is referred to as the water budget. Hydrologists add and subtract the amount of water in much the same way as families add and subtract money in household budgets. After calculating how much precipitation falls on the oceans and how much water evaporates from the oceans, then subtracting evaporation (loss) from precipitation (gain), hydrologists have found that the oceans lose about .36 x 10" cubic meters of water per year. More water evaporates from oceans than precipitates into oceans.
Then hydrologists made the same calculations for land and found that a surplus of about .36 x 1014 cubic meters of water falls onto the land per year. The excess amount of water that precipitates onto land is equal to the excess amount of water that evaporates from oceans!
The reason that the land doesn't become soggy and the oceans don't dry up is that the extra precipitation on land ends up in rivers and groundwater. This excess water flows, drips, and seeps its way back to the oceans, where evaporation occurs on a large scale, continuing the hydrologic cycle.