Transcript ATMO 1300-005 Class #2

ATMO 1300-006 Class #8
Wednesday, September 16, 2009
Chapter 4, Water in the
atmosphere
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Saturation vapor pressure
depends only on Temperature
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Table 4-1, p. 92
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
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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
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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
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Condensation in the
atmosphere (continued)
• Supersaturation (RH > 100%) does not
occur in the atmosphere
• 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
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Frozen dew
• When the dew point is above the freezing
point of water and cooling occurs, dew
forms when the temperature reaches the
dew point
• Sometimes further cooling occurs after the
dew forms
• If the temperature falls below 0°C, the dew
can freeze. When liquid dew freezes, it is
called frozen dew
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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
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Fog in a city
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Fig. 4-8, p. 95
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Fig. 4-9, p. 96
Radiation fog forms at night in
valleys due to cooling
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Fig. 4-10, p. 97
Advection fog: warm moist air
and a cool surface current
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Fig. 4-11, p. 97
Steam fog/evaporation fog:
cold air and warm water
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Fig. 4-12, p. 98
Ice in clouds
• Deposits (deposition) on small particles
called ice nuclei
• There is a scarcity of ice nuclei at high
temperatures (near but below 32ºF)
• Many water droplets do not freeze at
subfreezing temperatures. This is called
supercooling.
• Below -40ºC (or F), all water drops freeze
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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)
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Fig. 4-30, p. 112
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Fig. 4-35, p. 117
How clouds form above the
surface: lifting and cooling
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Fig. 4-13, p. 99
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Fig. 4-14, p. 100
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Fig. 4-15, p. 101