Transcript Humidity Notes

Water In The Atmosphere
•
The presence of water in the
atmosphere is a major influence on the
formation of clouds, rain, snow,
lightning, fog, and most other weather
events. This chapter will provide
students with an understanding of the
importance of atmospheric moisture
and its effects on weather and climate.
Heat Energy and Water
• How does water in the
atmosphere mainly exist?
water vapor
• It can also exist as a
liquid or a solid.
• Water can change from a
solid to a liquid to a gas
by adding heat.
• However, if I have a
special kind of ice
(like CO2), it can change
directly from a solid to a
vapor without becoming a
liquid. This is called:
Sublimation and an
example is with dry ice. It
can also happen with
frozen water or snow if
the air is dry, and its
temperature is below
freezing.
• Although some water
enters the atmosphere
through sublimation, most
water gets into the
atmosphere through
evaporation.
When water evaporates and enters the
atmosphere, the molecules move much
faster than they did when they were in a
liquid form. This is due to “hidden” heat
within the molecules called: latent heat.
Example of Latent Heat:
• On a cold night when temperature is below
freezing, a farmer can spray plants with
water and they won’t freeze due to latent
heat.
• http://www.youtube.com/watch?v=1PcnC
WZP7I0
Humidity
• When we discuss how
much of this water vapor
is in the atmosphere, we
are talking about:
Humidity.
• As more water vapor is
added to the air, the
humidity increases. When
the air has all the water
vapor it can possibly hold
at a given temperature, it
is said to be saturated.
• Warm air can hold more
water than cold air.
Therefore, as
temperature increases,
the amount of water it can
hold also increases.
• When we discuss how
close to saturation the air
actually is, we call it:
relative humidity.
• Relative humidity
compares how much
water is in the air to how
much water the air can
hold.
When they say on the news that the
relative humidity is 90%
-That Does NOT mean…
that 90% of our atmosphere is
water.
-It Does mean…
that the atmosphere can hold a
certain amount of water and 90%
of that amount is in the air.
Example
Think of the atmosphere as a cup of water
• This cup can hold 10
ml of water. It is
holding 5 ml of water.
• This cup can hold 8
ml of water. It
contains 6 ml.
• What % is it filled?
• What % is it filled?
What is the Relative Humidity
If a temperature can hold 10 grams of water and contains 5
grams?
50%
If a temperature can hold 25 grams and contains 10 grams
of water?
40%
If a temperature can hold 30 grams and contain 30 grams?
100% saturated
If a temperature can hold 20 grams and contains 30
grams?
150% supersaturated-probably raining
(need enough nuclei for rain to condense on)
What would happen to the relative
humidity if the amount of moisture
stays the same but….
1. The temperature drops?
it would increase
2. The temperature increases?
it would decrease
See cup drawings on the board
Measuring Relative Humidity
• Relative Humidity is measured with a psychrometer.
• A psychrometer has 2 parts:
Wet bulb- thermometer covered with a damp wick.
Dry bulb- a normal thermometer.
The person measuring relative humidity holds the
psychrometer and whirls it around. Water evaporates
from the wet wick.
When it evaporates what does it pull with it? Latent heat.
Therefore, what will happen to the temperature in the wet
bulb thermometer? It will drop.
• By looking at the difference is temperature
readings between the dry bulb and the wet bulb,
and by using that information and applying it to
the chart, one can determine relative humidity.
• Another way to measure is with a hair
hygrometer. It contains a bundle of hair that
increases in length as humidity increases.
• To express the actual amount of moisture in the
air, scientists use specific humidity.
• Specific Humidity= # of grams of H2O kg. of air.
• Below are 3 regions of the earth and 3
different specific humidity readings. Match
the specific humidity to its region.
• California
18 g/kg
• South Pole
1 g/kg
• Hawaii
10 g/kg
Dew Point
• We already said that warm air can hold
more water than cold air. Therefore, if air
is cooled until it reached the point of
saturation, it is called its dew point.
• When temperature is lowered below dew
point, water vapor from the atmosphere
condenses into a liquid which we call dew.