Download Dew and Frost

10/20/16
Condensation: Dew, Fog, and Clouds
Dew and Frost
This chapter discusses:
0. How vertical motion effects parcel thermodynamics
1. How dew, frost, haze, fog, and clouds form from
atmospheric moisture
2. Classification of fog and cloud types from
observation
Formation of Dew & Frost
Figure 6.1
•  Recall that solid surfaces (blades of grass, glass
windshields, airframe surfaces) outside efficiently
radiatively cool at night, especially if the winds
are calm and there are no clouds (WHY?)
•  When this happens, they quickly become colder
than the overlying air.
•  Dew and frost occur when surfaces cool to a
temperature below the dewpoint(frostpoint)
temperature (DPT) of the surface air.
•  When this happens, water condenses out on the
cold surface, causing dew or frost to form
Haze & Water Seeking Nuclei
Figure 6.2
As air cools to its saturation, or dew point, vapor molecules slow
down and can adhere as dew on the ground surface or as frost
when air temperature drops below freezing.
Daily temperature lows often occur by radiational cooling,
forming dew at night or early morning. Dew is most likely when
winds are calm, and the nocturnal inversion is steep and shallow
Cloud Condensation Nuclei (CCN)
Above the ground surface, cooling and slowing water vapor instead condenses
upon cloud condensation nuclei (CCN, more on this later).
Hygroscopic particles such as salt or dust seek condensing vapor, and can form a
wet white haze when relative humidity is above 75%, or a dry blue haze when
drier.
Hygroscopic particles (e.g., salt, many dusts) seek condensing
vapor, and can form a wet white haze when relative humidity is
above 75%, or a dry blue haze when drier.
A few percent of the aerosol in the atmosphere are
referred to as Cloud Condensation Nuclei (CCN)
DEFINE: Cloud Condensation Nuclei (CCN) are
aerosols that are hygroscophic– they have an affinity
for (i.e. attract) water vapor molecules. CCN serve as
building sites for cloud (not rain) droplets.
CCN are essential for cloud droplet growth
Attracts water vapor
molecules
Repels water vapor
molecules
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How do clouds form?
At least 2 things are necessary:
•  A saturated parcel
•  Abundant CCN
Cooling by expansion
Moving a parcel upward to a region of lower
pressure causes the parcel to EXPAND and
COOL
If the parcel cools enough, it will become
saturated, and condensation– in the form of
cloud droplets– will occur.
Conversely, moving a parcel downward (higher
pressure) causes the parcel to CONTRACT and
WARM.
If initially saturated, as sinking parcels warm,
become subsaturated, and the cloud will
evaporate.
Cloud formation
•  For clouds to form, the atmospheric parcel must
saturate. the temperature needs to become the
same as the DPT (VP needs to equal SVP) This
can be done by (1) cooling the air parcel to DPT,
or (2) adding water vapor to raise the DPT.
•  (1) is much more common than (2). Cooling can
happen through
–  Longwave Radiation from the surface or cloud top, or,
more commonly
–  Cooling the parcel by expansion (raising to region of
lower pressure).
FOG
•  FOG— Simply put, fog is a stratus-type
cloud on the ground, not necessarily dense,
but often is (visiblity below 10 km for
aviation purposes, 6 km for NWS purposes
and 1 km for WMO purposes).
•  Unlike most other clouds, fog is typically
driven by some cooling mechanism other
than lifting (expansional cooling).
Ground Based Radiation Fog
Radiation fog, CA
Central Valley
radiation fog— occurs in clear-sky conditions when ground (and atmos
boundary layer) radiatively cools to below dewpoint.
Typically occurs in low-lying areas with light winds.
Dew formation is more likely if winds are dead calm.
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Advection Fog
Figure 6.5
Advection fog— occurs when warm, humid air mass advects over a cold
surface (snow, lake, ice), chilling that airmass to the dewpoint
temperature
This fog often forms above the ocean due to mixing currents, or when
warm ocean air rolls into the cooler waters at the Pacific Coastline.
Other fogs…
•  upslope fog— simply a surface-based stratus cloud that
forms as the result of orographic lifting (flow “up-slope”).
The kind of fog you experience in mountain tops.
•  ice fog— occurs not from cooling, but from injection of
moisture into the atmospheric boundary layer. It is
composed of ice crystals rather than cloud droplets
•  Usually occurs only in situations of strong temperature
inversions
•  only occurs at ~ -35° C or colder
Fog forest, Costa Rica
•  typically found near anthropogenic (man-made) sources of
water vapor (car exhaust, smokestacks, cooling towers at
power plants, etc)
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Fog & Human Safety
Foggy days in the US
have a predictable
distribution due to
ocean and mountain
influences.
Fog can help crop
growth in California,
but can also cause
severe automobile,
airplane, and boating
accidents.
Figure 6.8
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