• evaporation
    Boiling water evaporates into thin air.

    Photograph by Dean McCartney, My Shot

    Rate of Evaporation
    The National Weather Service in the United States measures the rate of evaporation at different locations every year. Scientists there found that the rate of evaporation can be below 76 centimeters (30 inches) per year at the low end, to 305 centimeters (120 inches) per year on the high end.

    Evaporation happens when a liquid substance becomes a gas. When water is heated, it evaporates. The molecules move and vibrate so quickly that they escape into the atmosphere as molecules of water vapor.

    Evaporation is a very important part of the water cycle. Heat from the sun, or solar energy, powers the evaporation process. It soaks up moisture from soil in a garden, as well as the biggest oceans and lakes. The water level will decrease as it is exposed to the heat of the sun.

    Although the level of a lake, pool, or glass of water will decrease due to evaporation, the escaped water molecules dont disappear. They stay in the atmosphere, affecting humidity, or the amount of moisture in the air. Areas with high temperatures and large bodies of water, such as tropical islands and swamps, are usually very humid for this reason. Water is evaporating, but staying in the air as a vapor.

    Once water evaporates, it also helps form clouds. The clouds then release the moisture as rain or snow. The liquid water falls to Earth, waiting to be evaporated. The cycle starts all over again.

    Many factors affect how evaporation happens. If the air is already clogged, or saturated, with other substances, there wont be enough room in the air for liquid to evaporate quickly. When the humidity is 100 percent, the air is saturated with water. No more water can evaporate.

    Air pressure also affects evaporation. If air pressure is high on the surface of a body of water, then the water will not evaporate easily. The pressure pushing down on the water makes it difficult for water to escape into the atmosphere as vapor. Storms are often high-pressure systems that prevent evaporation.

    Temperature, of course, affects how quickly evaporation happens. Boiling-hot water will evaporate quickly as steam.

    Evaporation is the opposite of condensation, the process of water vapor turning into liquid water.

  • Term Part of Speech Definition Encyclopedic Entry
    air pressure Noun

    force pressed on an object by air or atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    cloud Noun

    visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

    Encyclopedic Entry: cloud
    condensation Noun

    process by which water vapor becomes liquid.

    Encyclopedic Entry: condensation
    decrease Verb

    to lower.

    disappear Verb

    to vanish or leave without a trace.

    earth Noun

    soil or dirt.

    evaporation Noun

    process by which liquid water becomes water vapor.

    Encyclopedic Entry: evaporation
    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    high-pressure system Noun

    weather pattern characterized by high air pressure, usually as a result of cooling. High-pressure systems are usually associated with clear weather.

    humidity Noun

    amount of water vapor in the air.

    Encyclopedic Entry: humidity
    liquid Noun

    state of matter with no fixed shape and molecules that remain loosely bound with each other.

    molecule Noun

    smallest physical unit of a substance, consisting of two or more atoms linked together.

    saturate Verb

    to fill one substance with as much of another substance as it can take.

    snow Noun

    precipitation made of ice crystals.

    solar energy Noun

    radiation from the sun.

    Encyclopedic Entry: solar energy
    storm Noun

    severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

    stream Noun

    body of flowing water.

    Encyclopedic Entry: stream
    swamp Noun

    land permanently saturated with water and sometimes covered with it.

    Encyclopedic Entry: swamp
    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    tropical Adjective

    existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

    vapor Noun

    visible liquid suspended in the air, such as fog.

    water Noun

    chemical compound that is necessary for all forms of life.

    water cycle Noun

    movement of water between atmosphere, land, and ocean.

    Encyclopedic Entry: water cycle
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