Relative Humidity

Relative Humidity
Is how much moisture a given mass of air is
holding compared to how much water that air
is capable of holding expressed as a percent.
What % full?
What would
happen to that %
if the same
water were in a
smaller glass?
How to measure humidity • Remember - evaporation
cools things (sweat,
misters in outdoor cafes,
etc.)
• Evaporation goes faster
in dry air.
• The drier the air - the
greater the difference
between wet and drybulb temperatures.
Sling Psychrometer
Hair Hygrometer • Hair changes length depending on
humidity - “bad hair days”!
Cold is air’s kryptonite
• Warm air can hold more moisture.
• Cold air holds less moisture.
• So, cooling air reduces the air’s ability to
hold moisture, and if that air was holding a
lot of moisture, it may drop moisture
(condensation).
• What happens to RH if you warm the air?
Dew point -
• The temperature that you have to cool air to in
order to make the air drop its moisture is the dew
point.
• RH = 100% at DP.
• Is dew point high or low on a dry day?
If you cool air to its dew point,
condensation happens.
• If you breathe on cold glass, moisture from your
breath ____________ on the glass.
• If you cool air up in the sky to its dew point,
water vapor condenses into ___________.
• If you cool air near ground level to its dew point,
______ forms.
• On cool mornings, _____ forms on cool grass.
• If the dew point is below freezing (32f or 0c)
and the air gets that cold , _______ would form
on grass instead of dew.
Why do clouds have flat bottoms?