(WOE) Atmospheric Conditions and The Water Cycle Lesson

Atmospheric Conditions and The Water Cycle

The water cycle is critical not only to weather, but to life on earth.  Rain is necessary for the survival of plants and humans.  Condensation is necessary for cloud formation.  Evaporation is necessary to cooling and keeping a good balance of water vapor in the air.

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Evaporation

Water turns to water vapor and evaporates from the oceans. Water can also evaporate from the land, we call this evapotranspiration. This includes water vapor produced by plants during transpiration, water from lakes, streams, puddles and soil moisture, direct evaporation of snow and even water vapor from the breath of animals. 

Lower air pressure helps promote evaporation but temperature is the primary factor. For example, all of the water in a pot left on a table will eventually evaporate. It may take several weeks. But, if that same pot of water is put on a stove and brought to a boiling temperature, the water will evaporate more quickly. 

Temperature – As temperature increases, the rate of evapotranspiration increases.  Evaporation increases because there is a higher amount of energy available to convert the liquid water to water vapor.  Transpiration increases because at warmer temperatures plants open up their stomata and release more water vapor. Stomata are holes on the surfaces of leaves or stems that can open or close to allow the exchange of gases, including water vapor.

Humidity – If the air around the plant is too humid, the transpiration and evaporation rates drop.  It's the same reason sweat does not evaporate from our skin when it's too humid.

Wind speed – If the air is moving, the rate of evaporation will increase.  The wind will also clear the air of any humidity produced by the plant's transpiration, so the plant will increase its rate of transpiration.

Water availability – If the soil is dry and there is no standing water there will be no evaporation.  If plants can't get enough water they will conserve it instead of transpiring by closing their stoma.

Soil type – Soil type determines how much water soil can hold and how easy it is for the water to be drawn out of it, either by a plant or by evaporation.  For areas where the ground is covered by vegetation, the rate of transpiration is considerably higher than the rate of evaporation from the soil.

Plant type – Some plants, like cacti and other succulents, naturally hold onto their water and don't transpire as much.  Trees and crops are on the other end of the spectrum and can release copious amounts of water vapor in a day.  For example, an acre of corn can release 4,000 gallons of water vapor a day and a single large oak tree can transpire 40,000 gallons of water vapor in a year.

Condensation

Condensation is the process by which a gas, such as water vapor, changes to a liquid, such as water.

Once water has been evaporated and reaches the atmosphere, warm air will carry it upward. As the water vapor begins to rise, carried by that warm air it will begin to come into contact with cooler air. The higher up in the atmosphere we travel the cooler the air tends to become. Cold air holds less water vapor than warm air. When the water vapor begins to cool it will start to condense into liquid water. Condensed water molecules will clump together around tiny dust particles in the air,these participles could be dust from soil or dust from a fire, even car exhaust, forming clouds. The temperature at which condensation will begin is called the dew point. If the dew point is below freezing the water may condense directly into water crystals.

As more water vapor condenses, the water droplets in a cloud eventually become so heavy that they fall back to Earth. 

Precipitation

Water that falls to Earth as rain, snow, hail, or sleet is called precipitation. Much of the water cycle precipitation falls directly into the oceans. Water in the ocean may stay there for many years before evaporating. Once evaporated, water will usually stay in the atmosphere about ten days and then will finally fall as precipitation.

When precipitation falls on land, some of the water will be evaporated almost immediately, especially in climates where the temperature is hot, their is lots of wind and very little humidity. Some precipitation will land on the ground and run off the surface into a river or lake. Water in rivers or lakes will eventually either evaporate or find its way back to the ocean. Some water may trickle down into the ground, forming groundwater. Groundwater can move underground eventually reaching a river, lake, or ocean. Once groundwater reaches the surface, it will be available to begin the water cycle again by evaporating.

Precipitation is our source for freshwater, the water cycle helps to renew the usable supply of fresh water on Earth.

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