Transcript ATMO 1300-005 Class #2

ATMO 1300-006 Class #7
Monday, September 13, 2010
Chapter 4, Water
continued
Monday, 9/13/10
1
Saturation vapor pressure
depends only on Temperature
Monday, 9/13/10
2
Another way to measure
humidity: the relative humidity
• Relative humidity is expressed as a
percentage, where 100% is saturation
• Relative humidity can be defined in terms
of the vapor pressure
• Relative humidity = 100% x vapor
pressure ÷ saturation vapor pressure
• A low relative humidity allows a higher
evaporation rate
Monday, 9/13/10
3
Relative humidity has
disadvantages
• Relative humidity tells how the air is from
saturation
• 0% relative humidity: No water vapor
• 100% relative humidity is saturated
• But air at a high temperature with relative
humidity of 50% may have more water
vapor than air at a lower temperature with
a relative humidity of 90%
Monday, 9/13/10
4
The saturation water vapor
content varies greatly with T
Monday, 9/13/10
5
Relative humidity changes with
Temperature for the same air
Monday, 9/13/10
6
Another way to measure
humidity: dew point
• Dew point is also called the dewpoint
temperature, abbreviated as Td or TD
• Dew point is defined as the temperature to
which air must be cooled (without
changing the pressure) to become
saturated
• Dew point does not exceed the
temperature
Monday, 9/13/10
7
More on dewpoint
• When the dewpoint is below 0°C (32°F), it
is called the frost point, because
deposition (water vapor to ice) in the form
of frost will occur when the air becomes
saturated
• When air cools to the dewpoint,
condensation occurs
• On surfaces, this condensation is called
dew (or frost)
Monday, 9/13/10
8
Frozen Dew
• Occurs in two steps
• First, condensation occurs and the
temperature is above freezing (32ºF); that
is, the dew point is above freezing
• The condensation is dew
• Second, the temperature falls below
freezing. The dew freezes to frozen drops
• This ice is called frozen dew or black ice
Monday, 9/13/10
9
Frozen dew (continued)
• Frozen dew is also called “black ice”
• It is a major traffic hazard
• It also causes slips and falls for people on
foot
• Frozen dew is hard to see
• Frozen dew frequently forms on roads
where there is a significant slope, as well
as bridges and overpasses
Monday, 9/13/10
10
Monday, 9/13/10
11
Fig. 4-5, p. 91
Monday, 9/13/10
12
Fig. 4-6, p. 92
Monday, 9/13/10
13
Table 4-1, p. 92
Condensation in air (not on a
surface like frost or dew)
• In a lab with perfectly clean air, saturation
requires a relative humidity of more than
200%. RH > 100% is supersaturation
• Condensation is inhibited by the curvature
effect
• Small, very curved droplets have
molecules with few neighbors, and are
quick to evaporate
Monday, 9/13/10
14
Condensation
• In the lab with perfectly clean air (no
aerosol) takes a relative humidity of more
than 200%. RH > 100% is supersaturation
• Condensation is inhibited by the curvature
effect
• Small, very curved droplets have
molecules with few neighbors, and are
quick to evaporate
Monday, 9/13/10
15
Monday, 9/13/10
16
Fig. 4-7, p. 93
Condensation in the
atmosphere
• Is inhibited by the curvature effect
• Is enhanced by the solute effect
• Some aerosol, salt particles for example,
dissolve and have the ability to hold on to
water molecules and suppress
evaporation
• Other aerosol particles form nuclei, or
small surfaces for condensation
Monday, 9/13/10
17
Condensation in the
atmosphere (continued)
• A cloud nucleus gives water molecules
more neighbors, by acting like a small flat
surface
• There are always abundant cloud
condensation nuclei in the atmosphere—
dust, salt, pollen, pollutants
• The solute effect permits condensation at
RH < 100%. This is called haze
Monday, 9/13/10
18
Condensation in the
atmosphere (continued)
• Supersaturation (RH > 100%) does not
occur in the atmosphere. The solute effect
cancels the curvature effect.
• When the relative humidity reaches 100%,
cloud particles form.
• Cloud at the surface is called fog.
• Fog reduces visibility to less than 1 km or
0.6 miles
• Heavy fog is a travel
hazard
Monday, 9/13/10
19
Fog in a city
Monday, 9/13/10
20
Fig. 4-8, p. 95
Monday, 9/13/10
21
Fig. 4-9, p. 96
Radiation fog forms at night in
valleys due to cooling
Monday, 9/13/10
22
Fig. 4-10, p. 97
Advection fog: warm moist air
and a cool surface current
Monday, 9/13/10
23
Fig. 4-11, p. 97
Steam fog/evaporation fog:
cold air and warm water
Monday, 9/13/10
24
Fig. 4-12, p. 98
Ice in clouds
• Deposits (deposition) on small particles
called ice nuclei (clay minerals, tiny ice
crystals)
• There is a scarcity of ice nuclei at high
subfreezing temperatures near but < 32ºF
• Many water droplets do not freeze at
subfreezing temperatures, called
supercooling.
• Below -40ºC (or F), all water drops freeze
Monday, 9/13/10
25
Ice in clouds (continued)
• Ice takes on different crystal shapes in
clouds, depending on temperature and
supersaturation
• Clouds are saturated with respect to
water, supersaturated with respect to ice
• The saturation vapor pressure over ice is
less than that over water
• There can be ice fog (inland Alaska)
Monday, 9/13/10
26
Monday, 9/13/10
27
Fig. 4-30, p. 112
Monday, 9/13/10
28
Fig. 4-35, p. 117
How clouds form above the
surface: lifting and cooling
Monday, 9/13/10
29
Fig. 4-13, p. 99