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Condensation: Dew, Fog, &
Clouds
Chapter 5
The Formation of Dew & Frost
• Dew forms on objects near the ground
surface when they cool below the dew
point temperature
temperature.
– More likely on clear nights due to increased
radiative cooling
• White frost forms when temperature cools
below the dew point and the dew point is
below 0°C
Condensation Nuclei
• Particles suspended in the air that around
which water condenses or freezes.
– Hygroscopic: water attracted
– Hydrophobic: water repelling
Haze
• Dry condensation nuclei (above dew point)
reflect and scatter sunlight creating blueish
haze
• Wet condensation nuclei (75% relative
humidity) reflect and scatter sunlight
creating grayish or white haze
Fog
• Saturation reached condensation forms a
cloud near the ground
• Radiation fog: ground cools through
conduction and radiation; ground fog
– Valley
V ll ffog created
t db
by cold
ld air
i d
drainage
i
– High inversion fog
Fog
• Advection Fog: warm moist fog moves
horizontally (advects) over a cool surface.
– Summer fog on the Pacific coast
• Observation: Headlands
– Air
Ai converges and
d rises
i
over h
headlands
dl d
forming fog as compared to lower elevation
beaches.
beaches
Advection Fog in San Francisco
Fog
• Upslope Fog: moist air flows up an
orographic barrier
– East side of the Rockies
• Evaporation Fog:
– Warm
W
moist
i t surface
f
provides
id enough
h
moisture to saturate a dry air parcel; short
lived
• Steam fog
• Breath in winter
Upslope Fog
Example of Evaporation Fog
Foggy Weather
• In general fog not common for most
location in the US. However several areas
do exist with a high frequency of fog
fog. Two
causes:
– Elevation
– Ocean currents
Average Annual Days of Dense Fog
Clouds
• Classification of clouds: use Latin words to
describe height and appearance
• Factors described
– Height: low, mid, high, vertical
– Appearance:
A
shape,
h
d
density,
it color
l
Major Cloud Types
Cloud Base Heights
Basic Cloud Types and Heights
Cirrus
above 20,000 ft, thin, ice crystals
Cirrocumulus
Cirrostratus
sometimes get haloshalos often in advance of a storm
Altocumulus
thunderstorms often develop
Altostratus
Nimbostratus
usually have light to moderate precipitation- darker than altostratus
Stratocumulus
Stratus
Cumulus
Also called air weather cumulus or cumulus humilis
Cumulus Congestus
(well defined top- not fuzzy)-usually summertime
Cumulonimbus
This has an “anvil” top from wind shear
Usually fuzzy top
top- thunderstorm clouds
From continued growth of cumulus congestus
Some Unusual Clouds
• Not all clouds can be placed into the ten
basic cloud forms
• Unique atmospheric processes and
environmental conditions create dramatic
and exotic clouds
• Unusual clouds and weather balloons
often
ft cause off UFO reports
t
Atmospheric Rotors
Rotors and Mountain Waves in the lee of the Sierra Nevada
•
•
Improved understanding and prediction important for aviation
safety in complex terrain
Forming a strongly coupled system with overlying mountain
waves and underlying boundary layer
August 7, 2007
Grubisic, AMS 12th Mesoscale
Conference, Waterville Valley, NH
32
Lenticular
More Lenticular Clouds
Mammatus
Associated with sinking air – often with severe weather
Cloud Observations
• Sky conditions: cloud coverage divided
into eighths and each amount associated
with term such as scattered clouds
• Observations: cloud ceilings
•
Ceilometer used at airports to
determine height from clouds by
l
laser
striking
t iki clouds
l d and
d th
then
amount of reflected light recorded
• Lidar = Light Detection And Ranging
Cloud Observations
• Satellite Observations
– Geostationary, polar orbiting
– Visible light provides a black and white picture
of clouds
– Infrared approximates cloud temperature
which infers height
– Satellites measure many other variables: sea
surface temperatures, ozone, upper level
features, snow cover, land cover
Geostationary Satellite
Polar Orbiting Satellites
Lower clouds
“warmer”
High clouds
“cold”
Just cold
ground