Transcript Water Vapor and Humidity

Water Vapor and
Humidity
D14-20
Where does water vapor come
from?
 When
warm air touched cold glass, the air
cools and droplets form
 Water Vapor the water in the air


Water in the form of a gas
It’s invisible, colorless, odorless, and tasteless
 Humidity
the amount of water vapor in
the air

This is NOT rain, fog, or clouds

Evaporation the changing of a liquid into a
gas

This takes a large amount of energy
• The source of the energy is the Sun; each day the Sun turns
trillions of tons of ocean water into water vapor

When air is cooled, molecules in the air slow
down; molecules of water vapor slow down too;
if they slow down enough they collide and stick
together and change to water droplets on cool
surfaces

Condensation the changing of a gas into a
liquid

This is what you see on shower doors, on a cold drink
glass, and on the grass in the morning
What is Relative Humidity?
 Humidity
and Relative Humidity both
describe the amount of water vapor in the
air
 2 factors determine the amount of humidity
in the air


There has to be water available to evaporate
The warmer the temperature, the faster the
water evaporates
• This means if water is available, warm air will take
on more water vapor than cold air
 Relative
Humidity a comparison
between how much water vapor is in the
air and how much air could hold (at a
given temp) if it were fully saturated

It is written as a %; if the relative humidity is
50%, then the air is holding half of what it
could if it were full
 Relative
day
humidity can change from day to

Can be used to predict how the air will feel


The higher the relative humidity the less water can
evaporate, the less sweat can evaporate, and the
“stickier” the air feels
Relative humidity can also be used to predict
when condensation will occur
This is a hygrometer;
they use it to find the
relative humidity.
What happens when warm moist
air cools?
1.
Air can cool by being pushed upward over
mountains by wind
Heating the air also causes it to rise.
2.

When the Sun heats the ground, air rises above the
ground warms and rises, as it rises, its expands and
cools
Air can also pushed upward when cooler air
and warmer air meet
3.


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The don’t mix when they meet
The lighter air is pushed over the heavier, cold air
The result is the warm air gets pushed higher into
the atmosphere and it cools