# Science: An Elementary Teacher’s Guide/Water Cycle

Pour yourself a glass of water and take a sip. Did you know that the water you've just swallowed is the same water that wooly mammoths, King Tutankhamun and the first humans drank? That's because Earth has been recycling water for over 4 billion years!
The world's water moves between lakes, rivers, oceans, the atmosphere and the land in an ongoing cycle called - you guessed it! - the water cycle. As it goes through this continuous system, it can be a liquid (water), a gas (vapour) or a solid (ice).

## Water

The most abundant element on earth is Oxygen.
Most of this elements are formed in:
* Sand
* Water
71% of Earth's Surface
Another ingredient necessary for life on Earth.
Necessary for the economy such as Manufacturing and Agriculture.
Facts about Water: Human bones are 25% water. Water leads to increased energy levels. Less than 1% of the water supply on Earth can be used as drinking water. Drinking adequate amounts of water can decrease the risk of certain types of cancers, including colon cancer, bladder cancer, and breast cancer.

## The Water Cycle
### Evaporation

Energy from the sun heats up the surface of the Earth, causing the temperature of the water in our rivers, lakes, and oceans to rise. When this happens, the water slowly 'evaporates' into the air, turning into a gas called 'vapor'. Plants and trees also lose water to the atmosphere through their leaves. This process is known as 'transpiration'.

### Condensation

As water vapor rises up high into the sky, it cools and turns back into a liquid, forming clouds. This process is called 'condensation'. Condensation can also be seen at a lower level, typically in the mornings of most environments, in the form of dew. Another such example can be seen on glasses of liquids gaining water droplets on its surface, resulting in perspiration, a smaller form of condensation. Currents high up in the air move these clouds around the globe.

### Did you know?

The water cycle is also known as the hydrologic cycle.

### Precipitation

When too much water has condensed, the water droplets in the clouds become too big and heavy for the air to hold in the clouds. So the water droplets start to fall back down to Earth as rain, snow, hail or sleet, a process known as 'precipitation'.

### Collection

The fallen precipitation is then 'collected' in bodies of water - such as rivers, lakes, and oceans - from where it will eventually evaporate back into the air, beginning the cycle all over again. How it is collected, depends on where it lands...
* Some will fall directly into lakes, rivers or the sea, from where it will evaporate and begin the cycle all over again.
* If the water falls on vegetation, it may evaporate from leaves back into the air, or trickle down to the ground. Some of this water may then be taken up by the plant roots in the earth.
* In cold climates, the precipitation may build up on land as snow, ice or glaciers. If temperatures rise, the ice will melt to liquid water and then soak into the ground, or flow into rivers or the ocean.
* Water that reaches land directly may flow across the ground and collect in the oceans, rivers or lakes. This water is called 'surface run-off'. Some of the precipitation will instead soak (or 'infiltrate') into the soil, from where it will slowly move through the ground until eventually reaching a river or the ocean.
Click this link for a fun and catchy Water Cycle song that students are sure to enjoy and will help them learn the different stages.

## Dew Point

Dew point is the air temperature at which condensation takes place. The higher the specific humidity the higher the dew point will be. If more moisture is in the air dew point will be reached at a higher temperature.

#### Humidity

Given that all the other factors influencing humidity remain constant, at ground level the relative humidity rises as the temperature falls. This is because more water vapor condenses as the temperature falls further beneath the dew point. Dew point temperature is never greater than the air temperature because relative humidity cannot exceed 100%.
In technical terms, the dew point is the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, the rate of condensation will be greater than that of evaporation, forming more liquid water. The condensed water is called dew when it forms on a solid surface. The condensed water is called either fog or a cloud, depending on its altitude, when it forms in the air.
A high relative humidity implies that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the moisture content remains constant and temperature increases, relative humidity decreases.

## Dew and Frost

This forms as moisture from the air condenses and collects on objects and on the Earth's surface. If the air is cooled below its dew point water vapor in the air condenses on surfaces as droplets of water called dew. And if the dew point is below freezing temperature condensation occurs in the form of ice crystals leaving a layer of frost on the surface of the object.

## Clouds

When the air is cooled below its dew point the water vapor condenses around tiny droplets and a cloud is formed. When water vapor condenses on them they grow larger and heavier and eventually they fall out of the air as precipitation. The size depends largely on the amount of turbulence that holds them suspended in the air. And the shape of a cloud is determined by how its formed when the air movement is horizontal clouds form in layers are called cirrus and cumulus clouds tend to form in conditions of vertical air movement.

##### Cirrus

These clouds are feathery or wispy and are the highest clouds in the sky they are from 6.5 to 13 kilometers high and because of their altitude they always consist of ice crystals.
This cloud can form at any altitude between 16,500 ft (3.13 mi) and 45,000 ft (8.5 mi) above sea level.

##### Stratus

These type of clouds are form in layers and they are nearest to the Earth's surface are associated with stormy weather.
Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes over a region.

##### Cumulus

These type of clouds are flat on the bottom but they can extend very high are associated with fair weather but when they grow large and black they become thunderclouds and can bring heavy rains. We call these cumulonimbus clouds.

## Quiz

Take this quiz and test your knowledge!