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Humidity
Water Vapour in the
Air
The more water vapour in the atmosphere
• A) the air becomes heavier.
• B) the air becomes lighter.
• C) the air stays the same
What are the components of air?
Air components
• Dry air = 99% Nitrogen & Oxygen gas
• These gases are heavier than water vapour
• SO if water vapour replaces dry air the atmospheric
pressure will go down.
• Therefore, humid or ‘wet’ air exerts less atmospheric
pressure than dry air.
What does a decrease in atmospheric
pressure suggest about the weather?
Warm, humid air is approaching and the
temperature will increase.
So an increase in atmospheric pressure suggests that cool, dry weather is on its way.
Humidity is:
• A measure of the amount of water
vapour in the air (page 20)
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What factor primarily determines how
much water vapour the air can hold?
Temperature Which temperature can hold more water vapor?
The factor that primarily determines
how much water vapour the air can
hold is:
• Temperature
• Warmer air has a greater capacity for
holding water vapour than cooler air
• The amount of water vapour actually
present in the air is called specific humidity
• The air becomes saturated when the
specific humidity equals the capacity of air
to hold water at a specific temperature
• What happens when the air cools below
this temperature?
Condensation
• DEW POINT is the
temperature at which
condensation will
occur. The Dew point is
the temperature the air
must be in order to be
saturated.
Remember, cooling the The ice makes the air
air makes it unable to
near the glass cooler
and it reaches its’ dew
hold as much water
point, cool air can’t hold
vapor, so water will
as much moisture and
condense out at a
condensation occurs
certain temperature.
Most often we here another
term when referring to humidity
Relative Humidity
• RELATIVE HUMIDITY is the amount of water
in the air compared to the amount of water
the air could possibly hold.
• Humidity values are usually given as Relative Humidity.
– Examples:
– if the air is holding half the water it could hold,
it’s Relative Humidity is 50%.
– If the air is holding ALL the water it can hold it is
saturated and the relative humidity is 100%
– If the air is holding no water, relative humidity is 0%
– If it holding a quarter of the water it could hold, 25%
Saturation
• If the air is SATURATED, it is holding ALL of the
water that it can hold.
– WARM air expands and can hold more water vapor
than COLD air, so it takes more water to saturate
warm air.
If the air is saturated, its Relative Humidity is 100%,
and if the temperature drops
PRECIPITATION will occur.
Water capacity of air at different
temperatures
Measuring Humidity
• Relative Humidity is measured using a
PSYCHROMETER.
– A psychrometer is made of two thermometers.
One is covered with a wet cloth. When air
moves over the wet cloth, evaporation occurs
and lowers the temperature on that thermometer.
If you compare the temperature on the two
thermometers you can get the relative humidity.
Ex. 1
Dry Bulb = 14 degrees C
Wet Bulb = 10 degrees C
Difference is 14-10 = 4
1st – look at dry bulb
reading (14)
2nd – find difference (4)
3rd – RH is where they
meet = 60%
Dry Bulb= 4 degrees
Wet Bulb= 3 degrees
Difference is 4-3 = 1
RH = 85 %
Ex. 3
Dry Bulb = 6 degrees C
Wet Bulb = 6 degrees C
Difference is 6-6 = 0
RH =
100%
Activity 2-1B
Using the Humidex Scale
• Complete this activity on page 51 pass in
when completed.