Hydrologic Cycle
1
Humidity
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Refers to the amount of water vapor in the air
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Specific
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Relative

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Ratio of amount of water vapor for a given parcel to amount of water
vapor if air was saturated
Absolute

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Ratio of water vapor to air (dry air + water vapor) for a given parcel
Does not change with expansion or compression
Density of water in a given parcel of air
Subjected to volume changes
Warm air has greater capacity to hold moisture than
cool air
2
Relative Humidity

A ratio of the amount of water vapor in the air, relative to how
much water vapor that air can hold……
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WHAT IS THE PROBLEM WITH USING RELATIVE
HUMIDITY TO SEE IF THE AIR IS HUMID?
Relative humidity =
Actual water vapor content of the air
Maximum water vapor capacity of
the air at that temperature
X 100
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4
Specific Humidity
5
Air Saturation and Dew Point

= 100% relative humidity; that is the air is
holding as much moisture as it can at its
temperature = saturation and dew point

Dew Point is temperature to which air would
have to be cooled to achieve saturation assuming
there is no change in pressure or moisture
content
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DEW POINT IS A GOOD INDICATOR OF
HOW MUCH MOISTURE IS IN THE AIR.
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General Dewpoint Thresholds
Dew Point
Relative
Humidity
Human Perception
Temp. °F (°C)
Air Temp 90°F
75°F(24°C)+
Extremely uncomfortable, oppressive
62%
70°F(21°C) 74°F
Very Humid, quite uncomfortable
52% - 60%
65°F(18°C) 69°F
60°F(16°C) 64°F
Somewhat uncomfortable for most people
44% - 52%
at upper limit
OK for most, but everyone perceives the
humidity
37% - 46%
at upper limit
55°F(13°C) 59°F
Comfortable
31% - 41%
50°F(10°C) 54°F
Very comfortable
31% - 37%
49°F(10°C)
Feels like the western US
or lower
a bit dry to some
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Fun with the Heat Index Calculator
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http://www.easysurf.cc/cnver16.htm
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Fog
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Fog is a cloud at ground level
Indicates saturated conditions at ground level
Fog types
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Advection fog- warm moist air moves over a cold surface
Evaporation or steam fog- cold air moves over warm water
Radiation fog- air layer in contact with the ground cools to
dew point…..creates shallow fog on calm clear nights
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WHAT KIND IS
THIS?
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13
WHAT
KIND????
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Heavy Fog Days
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