# Moisture and Fog

```Hydrologic Cycle
1
Humidity

Refers to the amount of water vapor in the air

Specific



Relative


Ratio of amount of water vapor for a given parcel to amount of water
vapor if air was saturated
Absolute



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……

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
3
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

DEW POINT IS A GOOD INDICATOR OF
HOW MUCH MOISTURE IS IN THE AIR.
6
General Dewpoint Thresholds
Dew Point
Relative
Humidity
Human Perception
Temp. &deg;F (&deg;C)
Air Temp 90&deg;F
75&deg;F(24&deg;C)+
Extremely uncomfortable, oppressive
62%
70&deg;F(21&deg;C) 74&deg;F
Very Humid, quite uncomfortable
52% - 60%
65&deg;F(18&deg;C) 69&deg;F
60&deg;F(16&deg;C) 64&deg;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&deg;F(13&deg;C) 59&deg;F
Comfortable
31% - 41%
50&deg;F(10&deg;C) 54&deg;F
Very comfortable
31% - 37%
49&deg;F(10&deg;C)
Feels like the western US
or lower
a bit dry to some
7
8
Fun with the Heat Index Calculator

http://www.easysurf.cc/cnver16.htm
9
Fog



Fog is a cloud at ground level
Indicates saturated conditions at ground level
Fog types



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
10
11
WHAT KIND IS
THIS?
12
13
WHAT
KIND????
14
15
16
17
Heavy Fog Days
18
19
20
```