Transcript Water in the Atmosphere

Water in the Atmosphere

The movement of water between the
atmosphere and Earth’s surface is called the
WATER CYCLE.

Water enters the atmosphere by
EVAPORATION.

EVAPORATION is the process by which water
molecules in liquid water escape in to the air
as water vapor.

Water vapor also enters the air via plants.
Water Cycle

HUMIDITY is the amount of water vapor in the
air.
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The percentage of water vapor in the air
compared to the maximum amount the air can
hold is called the RELATIVE HUMIDITY.
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Warm air can hold more water vapor than cool
air.
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For instance at 10 degrees Celsius 1 cubic
meter can hold 8 grams of water vapor.
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If there were 8 grams of water vapor in that
cubic foot then the relative humidity would be
100 percent.

If the air held 4 grams of air the relative
humidity would be 50 percent.
IT’S NOT THE HEAT IT’S THE
HUMIDITY
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Even on a hot day you can feel comfortable if
the air is dry.

If the temperature and relative humidity are
high it is usually uncomfortable.

When the relative humidity is high, evaporation
slows down.

Evaporation has less of a cooling effect on the
body.
Measuring Relative Humidity
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Relative humidity can be measured with a
psychrometer.
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A psychometer has 2 thermometers, a wet bulb and a
dry bulb thermometer.
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The wet bulb is moistened with water and air is then
blown over both thermometers.

Because the wet bulb thermometer is cooled by
evaporation, its reading drops below that of the dry –
bulb thermometer.

If the relative humidity is high, the water on the
wet bulb will evaporate more slowly and the
wet bulb temperature will not change that
much.

If the relative humidity is low the water on the
wet bulb will evaporate rapidly and the wet
bulb temperature will drop.

The relative humidity can be found by
comparing the temperatures of the wet and dry
bulb thermometers using a chart like the one
on page 546.
How Clouds Form

Clouds of all kinds form when water
vapor in the air becomes liquid water or
ice crystals.
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The process by which molecules of
water vapor become liquid water is
called CONDENSATION.
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The temperature at which condensation begins is the dew point.
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If the dew point is below the freezing point the water vapor
changes directly into ice crystals.
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When you look at a cloud you are looking at millions of tiny ice
crystals or water droplets.

For water vapor to condense, tiny particles must be present i.e.
dust particles, smoke, salt crystals.
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Some time water vapor condenses on to blades of grass. This is
called dew.
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Frost is ice that has been deposited directly from the air onto a
cold surface.
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Clouds form whenever air is cooled to it’s dew point and particles
are present.