Clouds Template

1
Humidity
Clouds, fogs, precipitation
2
The percentages dont add up! Water vapor is
considered an add-on and can be as much as 4.
Thats called absolute humidity.
3
Relative Humidity
The capacity of air to carry water vapor depends
on temperature. At 10C the air can only carry 9
grams in each cubic meter of air
4
Water vapor is invisible. If you are not at
capacity, skies are perfectly clear.
5
How close is the air to being at full capacity to
hold water as vapor?
That depends on (1) the actual amount of vapor
and (2) the capacity amount. This ratio is
called the Relative Humidity.
6
RH w/ws x 100 alternately, RH e/es x 100
w is called the mixing ratio ws is the saturation
mixing ratio. e is the vapor pressure, es is the
saturation vapor pressure.
When w ws or ees, then the RH is 100. We
call that
Saturation
When air is at saturation, you see clouds or fog.
7
Use the chart. If the temperature is 50F which
is 10C, what is the saturation mixing ratio (ws)?
Now suppose the actual water vapor content (w) is
only 4.5 g in the same volume. What is the RH?
8
Answer at 50F, ws 9 grams. If w 4.5
g, RH w/ws x 100 4.5/9 x 100
50
Since 1 cubic meter of air is around 1 kg in
mass, we express (saturation) mixing ratio in
g/kg.
Its hard to measure w and ws In a moment well
see an easy way to do it.
9
This is a fog in the Susquehanna River Valley
10
You cant see the valley here because of thick
fog. RH 100 in this fog.
11
Same view on a foggy day and a clear day.
12
In this fog the visibility is around ¼ mile (you
cant see farther than that)
13
Dew is another way you see 100 relative
humidity. Water vapor has condensed onto the
grass.
The dew point is the temperature at which dew
forms. RH 100 when the temperature reaches
the dew point temperature (call it Td)
14
Since ws depends only on Temperature, T, each one
has its own ws
15
Heres the fabulous part Since RH 100 when
T Td, and RH w/ws x 100, then it follows
that at saturation, RH 1 x 100 so w ws.
Since T Td at saturation and each T has its
own ws, then at saturation, the dew point also
shows you ws. Furthermore, since w ws, the
mixing ratio (w) at saturation has its own Td.
The dew point allows you to easily measure w,
which tells you exactly how many grams of water
vapor is actually in the air.
16
Remember, T ? ws and Td ? w, so NEVER calculate
Relative Humidity with the ratio T/Td. Always use
RH w/ws x 100
Find w and ws by using the Dew Point and
Temperature. We have tables for this, as youve
seen.
17
Heres the average distribution of fog in the
U.S. over the course of a year
Really foggy in NY and New England
Not very foggy in Arizona
Why is it often foggy in New England but rarely
foggy in Arizona?
1. Its humid in New England so w is high.
2. Its hot in Arizona, so ws is high
18
Whats the difference between a fog and a cloud?
Fog
Cloud
19
Next
Clouds