The hydrologic or water cycle is a major driving force on our planet. Water is in constant motion, evaporating into the atmosphere to from oceans, lakes, rivers, and streams. When the atmosphere can no longer support the moisture within the clouds we experience rain, snow, hail, or sleet. Some water is locked in the form of ice at the polar caps and in alpine glaciers. Water is returned to the system through drainage, which results from the melting of snow that has accumulated during the winter months. This water flows on the surface of the Earth and percolates through the Earth as groundwater. Water is not actually consumed but is continuously recycled. Can we survive without water? No, life as we are familiar with, could not function. From the primordial Earth and the evolution of life, water has played a nurturing role in sustaining the cells of organisms in fluids that created the key to duplication and replication. What makes water so important? Water is a peculiar substance with properties that make it an ideal fluid. If you theoretically calculated the boiling and freezing temperatures of water you would find that water has an unusually low freezing point and high boiling point compared to other molecules that have similar structures.

   How can you explain such a big difference? The molecular structure of water resembles that of Mickey Mouse's head or a teddy bear (figure below). The hydrogen and the oxygen have a very tight covalent bond, where the hydrogen and the oxygen share electrons as they dance and twirl around in the molecule. The individual molecules of water are also held together very tightly by what is called a hydrogen bond. A hydrogen bond is much stronger than other bonds that molecules have. Ionic bonds are weaker bonds, and many substances like salt can be easily be "broken up" by a stronger covalent bond. Water is a package of power that is hard to break, and it is this strength that allows other substances to dissolve or break up in it, hence the name, the "universal solvent."

   Water is a transparent, odorless, and tasteless liquid. It illustrates the three states of matter: solid (ice), gas (steam), and liquid (water). The form it takes depends upon the temperature. At low temperatures, the molecules do not move around as much and form a crystalline structure that is rigid (ice). In the liquid state, water molecules move more freely. Water molecules in the form of steam are moving very fast with large spaces between the molecules. Although ice is crystalline, it tends to have the molecules in a rigid structure that is spaced farther than the molecules of liquid water and this is quite important, for if ice were denser, it would sink in water. Imagine what would happen if icebergs grew from the bottom of the ocean instead of floating on the surface.

   Another chemical quality of water is that water has a very large heat capacity, meaning that it can absorb a great deal of heat without itself becoming extremely hot. This fact makes the oceans large reservoirs of heat that greatly affect the overall weather and climate patterns of the world.

   Water's surface tension (the ability of a substance to stick to itself) makes it an excellent substance to float heavy objects upon. Water not only sticks to itself, but also to other surfaces, and this allows it to move against gravity, which is very important to plants when transporting water form the soil to their leaves. This upward motion is known as capillarity or capillary movement.

   Water is an important commodity to all organisms that live on Earth. Although it appears as if we will never run out of water, only 3% (2/3 is locked up as ice, 1/3 as groundwater, lakes, and atmosphere) of all of the earth's water is fresh, 97% is salt water which is unusable by most land organisms' metabolic systems. The ocean water was formed as fresh water that eroded the land and "dissolved" different salts that remained in the water. The oceans play an important role in maintaining the water cycle, because only pure water will evaporate, leaving the "salts" behind. The water from the oceans, the movement of the oceans due to the spinning of the Earth, and the currents of the oceans that retain heat or cold all help to fuel the weather. Understanding weather is actually a complex subject.

   Every day we look outside to see if it is raining or sunny. We check to see if it is warm or cold outside. If it is raining out, we can't go on a picnic and if it is cold outside, we don't want to go to the beach. The weather affects just about everything we do, but many of us don't know what causes the weather.

   The Earth has a blanket of air called the atmosphere around it. Plants, animals, and people need this air to breathe. The atmosphere is also the place where weather begins. Weather means the current state of certain weather elements of the atmosphere at a particular place and time. The elements that make up the weather include the type of clouds in the sky, the temperature of the air, whether or not it is raining, the amount of moisture or humidity in the air, and the speed and direction or the wind. What causes these weather elements? What causes the weather that we see every day?

   In order to understand what makes clouds and temperature, we should look again at the Earth. The Earth is a planet that revolves around the Sun. The Sun heats up our atmosphere in much the same way that you heat a pan of water on the stove. The part of the atmosphere that is directly under the Sun's rays heats quickly. This is the air at the equator. Because the Earth is tilted, the polar regions receive only a little bit of sunlight, and for part of the year they receive none at all. The different amounts of sunlight received by the atmosphere cause differential heating of the air. The areas that receive lots of sun will heat up and the areas that do not receive very much sun will be cooler. As air warms, it rises, and moves toward cooler air. Rising warm air will cool as it rises, and will gradually sink back to Earth. When the warm air rises, cooler air moves in to replace it. The air is warmed, and it too begins to rise. Thus the atmosphere is constantly moving both horizontally (along the ground) and vertically (higher into the atmosphere). The general movement of the air is called circulation.

   The Earth is made up of both land and water. When water is heated it changes from the liquid to the gas water vapor. This process is called evaporation. When water vapor is cooled, as it would be if it were taken higher in the atmosphere, this gas will condense, or change back to liquid form.

   We can see water vapor condensing when we watch clouds. A cloud is nothing more than water vapor that has condensed back to a liquid form. A cloud is made of extremely tiny drops of water which can remain suspended in the air. As a cloud grows, and more and more water condenses in the same place, the cloud droplets get larger. Eventually, these cloud droplets will be too large to remain in the air. The cloud is then said to be saturated. A saturated cloud will usually precipitate its excess water, or cause it to fall. This is how it rains or snows. Moisture falling from clouds is called precipitation.

   The amount of the gaseous water vapor that is in the air tells us the relative humidity of the air. Humidity is a measure of how moist or dry feeling the air is. When the air is warm, it can hold more water vapor than when it is cold. When the humidity is high, it is very difficult to dry off after swimming, since there is so much moisture already in the air, it is not easy for more to evaporate. When the humidity is low, dry air easily evaporates water.

   The cycle of evaporation, condensation, and precipitation of water is called the water or hydrologic cycle. Since clouds move across the sky, the precipitation does not usually fall in the same place that the water came from. This is how the earth spreads water across land areas and allow us to live in so many different areas.

   The Sun is the driving force behind our weather. The temperature of the air, winds, clouds, humidity, and rain all depend on heat from the Sun. Our atmosphere, Sun, and oceans, combine to cause our weather.

   Combining scientific information in a coherent manner and teaching methods is stressed in the water cycle of the Integrating, Science, Math, and Technology Program. Within the 34 week program the Water Cycle constitutes 4 weeks and presents content from the themes of Water, Oceans, Atmosphere and Weather. These four units can develop an understanding to your students of how the oceans and atmosphere work together to form weather.

   During the water units students begin to see that the chemical combination of hydrogen and oxygen creates a compound that is universally important to humans. Students discover and explore the properties of water, so they understand its importance. In the Oceans units, students discover and evaluate the importance of mixing water with salts in the development of this planet. The distinguish salt water and fresh water to find the geographic locations of these bodies. In the Atmosphere unit, students learn the different types of gases which make up the air and how the movement of air helps control world weather patterns. In the Weather units, students will learn how to differentiate weather systems and also how to be able to recognize these patterns or systems from satellite photos.

   The units unify the major components of the water cycle, so students appreciate and understand the conditions not only in their local areas, but also in far away places.