Dew, Frost and Fog
RECAP
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• mixing ratio, relative humidity.
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• to be cooled in order to get saturated.
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Hydrological cycle: transport of water and energy.
Humidity: absolute humidity, specific humidity, water
Saturation pressure: the maximum vapor pressure at
a given T.
Dew point: the temperature to which air would have
The effect of large water bodies on the humidity of a
climate (Florida/California).
The effect of cooling/heating systems on the
humidity in the house.
Heat index: what the air temperature “feels like” to
the average person for different combinations of
temperature and relative humidity
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Condensation
As air cools, it becomes first
saturated and then supersaturated
-> condensation/deposition
Condensation requires some
initiator
♦ surface (dew, frost)
♦ nuclei (haze, fog, clouds)
Classification of condensation
nuclei according to
♦ size (not important)
♦ properties
hygroscopic: “waterseeking”, condensation at
RH<100%
hydrophobic: “waterrepelling”, condensation at
RH>100%.
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saturated
Supersaturated
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unsaturated
Dew
• Condensation of water vapor on cool surfaces.
• Usually forms at ground level (air is coldest there).
• Frozen
dew: air temperature drops below zero after dew has
formed
• Atmospheric conditions: clear, calm nights.
♦ Clear air: the ground emits IR radiation to space. There
are no clouds to absorb and reradiate the IR energy.
♦ Calm air (no winds): The wind enhances evaporation and
inhibits condensation. It also enhances mixing in the air,
so in the absence of wind, the ground gets colder (recall
radiation inversion).
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Frost
Deposition of water vapor on cool surfaces.
Atmospheric conditions: cold, clear, calm nights.
♦ Clear air: the ground emits IR radiation to
space. There are no clouds to reabsorb IR
energy.
♦ Cold: the temperature of the ground is below
freezing.
♦ Calm air: no winds- wind enhances evaporation
and inhibits condensation. It also enhances
mixing in the air, so if there is no wind, the
ground gets colder (recall radiation inversion).
Frost on Windows
• water vapor in the room is cooled down to below
Cold, outside temperatures cool the window. The
freezing temperatures and is deposited on the inside
of the window, forming delicate ice crystals.
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Haze
A layer of small nuclei (dust, aerosols, salt particles).
♦ Dry haze: at low relative humidity
♦ Wet haze: at relative humidity 75%-100%. The
presence of hygroscopic (water-seeking) nuclei
facilitates condensation at RH<100%.
Haze is more visible in the morning and in the evening
(higher relative humidity) because the wet particles
are bigger.
Book sez: Dry haze appears bluish on dark background
and yellowish on light background (why? Wait for the
answer until Chapter 19).
Haze on Titan
Cassini spacecraft imaging system
Fog
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• Fog is a cloud resting near the ground.
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Wet haze near RH=100% is called fog.
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Wet haze or fog? International definition: fog, if
visibility < 1km. The individual water particles are
bigger than the haze particles and they become
visible to the eye.
Fog scatters light at all wavelengths (geometric
scattering) and as a result it appears white/gray.
Fog is most likely to form in the morning or evening
when the RH is highest.
Making Fog
Radiation (Ground) Fog:
• Preconditions – local radiative processes
dominate.
• Formation.
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♦ The ground cools down due to
thermal (IR) radiation;
♦ The layer next to the ground cools down;
♦ The RH is increasing and the air saturates forming fog close
to the ground.
When does it occur?
♦ Late night hours when the ground temperature is minimum.
♦ Late fall and winter (long nights)
When does it dissipate?
♦ As the sun comes up it heats up the ground. The ground air is
heated by conduction and the fog droplets evaporate.
♦ The fog dissipates from the ground up.
Advection Fog:
• Preconditions.
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♦ Air mass movements (advection,
weak wind) over a moisture source.
♦ Temperature difference between the
water surface and the land.
Formation.
♦ Warm land and cold water: warm air mass moves over to the
water. It cools down, the RH rises, the air saturates;
♦ Cold land and warm water: cold air mass moves over the
water. It mixes up with warmer moist air from above the
water surface. The resulting mixed air is saturated.
♦ Cold water and warm water: two ocean currents with
different temperatures flow next to each other. Warm air
moving over the cold current produces fog.
Upslope Fog
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Preconditions.
♦ Weak winds
♦ Presence of topography
Formation.
♦ Moist air moved up the
slope of a hill;
♦ The air mass cools down
and the RH increases;
♦ The saturated air forms
fog
Evaporation Fog
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Preconditions:
♦ Cool air over a body of warm
water (no need for advection).
Formation:
♦ Water evaporates from the warm lake (pool).
♦ The cool air moistens and saturates.
♦ It is a local effect.
When does it occur?
♦ Often during the fall and early winter (water is
warmer than the land)
♦ After a rainshower as the sun heats up the ground
Foggy weather in the US
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The nuisance of foggy weather
The limited visibility leads to
♦ Poor driving conditions
Use fog lights
Do not use high beams
♦ Poor sailing conditions
Lighthouses
♦ Poor flying conditions
Airport closure
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Railways also are affected