Ch 6
Ch 6
Atmospheric Moisture
Sec A
• I. Measuring Relative Humidity
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A. A Psychrometer is an instrument for measuring relative humidity
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B. A common psychrometer uses two thermometers with a wet
gauze wrapped over the bulb of one. Air is blown
over both thermometers and the difference in temperature
between the two thermometers can be used to calculate
the Relative Humidity.
C. For Example the dry-bulb thermometer reads 25 degrees C
and the wet-bulb reads 20 degrees C,
the relative humidity is 63%. This is commonly found in a table
that comes with the psychrometer.
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Ch 6
Sec A
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Atmospheric Moisture:Clouds
2. Stratus clouds (STratus, AS:AltoStratus, CS:CirroStratus)
a. Large flat clouds often covering whole sky
b. Form at about 2.5 kilometers
c. Can bring light rain or drizzle
d. Nimbostratus are stratus clouds that are raining
e. Stratus clouds close to the ground are called fog.
f. Our fog in SJ and Santa Cruz is called Pacific Stratus
g. Ground fog forms when the ground cools quickly
at night causing the temperature of the air
in contact with ground to go below the dew point.
Fog is usually as stratus cloud.
1. Cumulus Clouds (SC,AC,CC,CU,CB)
a. Look like piles of white cotton balls.
b. Usually have flat bottoms
c. Form at 2.4 to 13.5 kilometers.
c. Cumulonimbus clouds are large cumulus
clouds that are grey and usually bring rain
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d. The suffix ...nimbus means rain.
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Ch 6
Atmospheric Moisture:Clouds
Sec B
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3. Cirrus Clouds (CI, CS, CC)
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a. light feathery clouds that form at very
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high altitudes between 6 and 12 Km.
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b. made of ice crystals
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c. usually indicate rain or snow is coming
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Atmospheric Moisture:Clouds
Sec B
• II. A. Clouds form when the air is as cold as the dew
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point and the water vapor in the air condenses onto
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microscopic dust particles. Need 1. water vapor,
2. condensation surface, and 3. cooling.
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B. Clouds are classified by their shape and altitude.
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C. There are three main types of clouds:
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cumulus, stratus, and cirrus
4. Naming of clouds
a. The three types cumulus, stratus, and cirrus
b. add nimbus after the word or put nimbo before
the cloud type to indicate it is raining or
threatening rain.
c. add alto before the word to indicate high
d. add cirro before the cloud type for very high
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
Ch 6
Sec B
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3.
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
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Atmospheric Moisture:Clouds
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
Ch 6
Sec B
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3.
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
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Atmospheric Moisture:Clouds
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Ch 6
Sec B
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Atmospheric Moisture:Clouds
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
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Ch 6
Sec B
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3.
Atmospheric Moisture:Clouds
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Station Plot Cloud Types
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4
Atmospheric Moisture:Clouds
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The Bottom of clouds are called their bases
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The Base of Cumulus Clouds( CU) can be
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approximated by: H ~= 1000ft x (T –Tdp)/4.5 F Fahrenheit Degs
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• of the Base of the CU above the surface.
• So Altitude of base of CU is:
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• Alt ~= Field Elevation + 1000ft x (T –Tdp)/4.5 F
Alt ~= Field Elevation + 1000ft x (T –Tdp)/2.5 o C Celsius Degrees
• T is the OAT(Outside Air Temperature ) at the field
• Tdp is the dew point temperature at the air field
• See:
• http://cimss.ssec.wisc.edu/satmet/modules/4_clouds/clouds-4.html#tag
• http://www.csgnetwork.com/estcloudbasecalc.html
Ch 6
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Prevailing Visibility: Greatest horizontal visibility which
bright lights or large objects can be seen. Statute Miles
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Runway Visibility: Horizontal Visibility of a bright objects
down a given runway. Given in Hundreds of feet.
• Ceiling: the height above the Earth’s surface of the lowest
cloud layer that has 5/8 or greater sky coverage.
• Vertical Visibility: The vertical distance you can see into a
ground based obsuration: fog, smoke, ash, or haze.
Reported in Hundreds of feet eg. VV005
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Ch 6
Sec B
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VFR: Visual Flight Rules
– Ceiling > 3000 feet AGL
– Visibility > 5 Statute Miles
• MVFR: Marginal Visual Flight Rules
– Ceiling 1000 to 3000 feet AGL
– Visibility 3 to 5 SM
Atmospheric Moisture:Precipitation
Sec B
Precipitation
• A. Water vapor that has condensed and fallen to Earth
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as rain, sleet, snow or hail.
• B. Start as very small droplets and then hit and combine
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with other droplets to become larger.
• C. A rain drop contains about 1 million times the water as
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a droplet in a cloud
• D. Frozen rain is called sleet
• E. Snow is a six pointed crystal
• F. Snow forms when water vapor changes directly to a crystal.
• G. Hail is like large sleet ranging in size from .5 to 7.5 cm.
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1. Hail is like an onion: it has several layers from
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raindrops hitting it and freezing on it.
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2. Can be up to 3 inches in size.
• IFR: Instrument Flight Rules
– Ceiling: 500 to 1000 Feet AGL
– Visibility: 1 to 3 SM
• LIFR: Low Instrument Flight Rules
– Ceiling < 500 Feet AGL
and/or
– Visibility < 1 SM
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Ch 6
Flight Rules
Atmospheric Moisture:Visibility
Sec B
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Tower Visibility: Horizontal Visibility from the tower.
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Ch 6
Atmospheric Moisture:Precipitation
Sec B
Ch 6
From Fed. Met. Handbook 1
Sec B
• Intensity of Precipitation
Light: Indicated with a –’ve sign
Moderate: No intensity symbol
Heavy: Indicated with a + sign
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Ch 6
Atmospheric Moisture:Precipitation
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Lab 11
• Answer the all of the questions questions.
• Make a table for questions 1,2,&3 using data the cumulus
calculator(2nd web site) and the equation given in these notes.
A row for each question and a col for each method of
calculating the cloud base. Field Elevation=1ft for 1st 3 rows
• Table of 4 rows and 6 columns Q#, FE, T, Tdp, Web, Equa
• Find a metar today w Cb cloulds and do above for it.
• Print metar at bottom of SpreadSheet
Sec B
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1. What is the height of the base of a cumulus cloud with surface temperature 70
degrees Fahrenheit and dewpoint 48 degrees F?
2. What is the height of the base of a cumulus cloud with surface temperature 95
degrees Fahrenheit and dewpoint 75 degrees F?
3. What is the height of the base of a cumulus cloud with surface temperature 1
degree Fahrenheit and dewpoint -4 degrees Fahrenheit?
4. What happens to the base of a cumulus cloud when the surface temperature is
held constant and the dewpoint increases?
5. What happens to the base of a cumulus cloud when the surface temperature
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and dewpoint equal each other?
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Change of T-Tdp vs. Altitude
• http://cimss.ssec.wisc.edu/satmet/modules
/4_clouds/clouds-4.html#tag
• Or http://cimss.ssec.wisc.edu/satmet.2012-Feb14/modules/clouds/makeCU.html
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Replaced by:
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• Change of T-Tdp = 2.5 C/K feet
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• Cloud height = (T-Tdp)/2.5 C/K feet
• http://www.csgnetwork.com/estcloudbasecalc.html
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