Chapter 6
When a parcel of air cools to near the dew point, water vapor will start to condense out of
the air.
Condensation is greatly aided by cool surfaces or condensation nuclei.
Without a surface to condense on, air can become supersaturated.
Frost occurs by deposition (vapor → ice) if the dew point is below freezing.
Dew generally occurs on clear, calm nights when radiation emitted by the earth cools the
surface and an inversion occurs. Air 1 meter above the surface may remain above the dew
point.
If the air above the ground drops below the dew point, water can condense on
hygroscopic nuclei, forming haze.
When visibility drops to 1km (0.62mi) the wet haze is called fog.
Fog is basically a cloud on the ground!
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•
•
•
Radiation fog: air cools by radiation
Advection Fog: warm moist air moves over cool surface
Up slope fog: moist air flows up hillside and cools
Fog due to mixing: 2 air masses, each just below saturation, mix. The resulting
mixture is above saturation. An example is cold air moving over warm water.
Example: Find RH of point X
Actual VP = 25mb
Saturation VP = 57mb
RH =
25mb
× 100 = 44%
57 mb
Dew point temperature = 22°C
Clouds
Clouds are identified by form and height above the ground :
(Latin names are used)
Low (water)
Stratus (sheetlike) – think of stratified
Cumulus (puffy) – think of acumulate
High (ice)
Cirrus (wispy) – think of circular
Nimbus (violent rain)
Review pictures of cloud types.
Measuring cloud ceilings
1)
2)
h2
h1
θ1
θ2
d
detector
beam
d
detector
beam
Place the laser beam source and the detector a known distance, d, apart. Point the laser
beam straight up and scan the detector until scattered light from the laser is detected.
Measure the angle, θ.
In figure 1, d = 1000 ft, θ1 = 45°
tan θ =
Opposite side h
=
Adjacent side d
Thus h1 = 1000 ft × tan 45° = 1000 ft
In fig 2, d =1000 ft, θ2 = 60°
So, h2 = 1000 ft × tan 60° = 1730 ft
Geostationary satellites:
Always are directly above equator!
This is because all satellites orbit around the center of the earth, because that is where the
net gravitational force originates. The only locations on earth that orbit about the center
of the earth are places on the equator. All other locations orbit about a point on a line
between the N and S poles.
Interesting fact: Geostationary satellites are approximately 6 times the radius of the earth
above the surface of the earth (One tenth of distance to the moon!)
R
v
satellite
earth
How fast is satellite moving?
v=
2Π R 2Π (26,300mi )
=
≅ 6900 mi
hr
24hrs
24hrs
Note: 26,300mi is distance of satellite from center of earth (not surface).
Applying Newton’s second law to the orbiting satellite gives the following relation
between R and v:
R=
constant
v2
But, from the first equation,
R = constant × v
The only solution that satisfies both of these equations at the same time is a unique,
constant value for R.
There is no such restriction on polar orbiting satellites.
They can be stable at any height and their orbit is determined by convenience.
Satellites detect scattered light and emitted radiation (mostly infrared).
Warm, low clouds are dark in IR, meaning more intense.