1. Atmospheric Moisture Ch 6 Sec A I. Measuring Relative Humidity
A. A Psychrometer is an instrument for measuring relative humidity
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.

2. Atmospheric Moisture:Clouds
Ch 6 Sec B
II. A. Clouds form when the air is as cold as the dew
point and the water vapor in the air condenses onto
microscopic dust particles. Need 1. water vapor, 2. condensation surface, and 3. cooling.
B. Clouds are classified by their shape and altitude.
C. There are three main types of clouds:
cumulus, stratus, and cirrus
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
d. The suffix ...nimbus means rain.

3. Atmospheric Moisture:Clouds
Ch 6 Sec A
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.

4. Atmospheric Moisture:Clouds
Ch 6 Sec B
3. Cirrus Clouds (CI, CS, CC)
a. light feathery clouds that form at very
high altitudes between 6 and 12 Km.
b. made of ice crystals
c. usually indicate rain or snow is coming
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

5. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

6. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

7. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

8. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

9. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

10. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

11. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

12. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

13. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

14. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

15. Atmospheric Moisture:Clouds
Ch 6 Sec B 3.

16. Station Plot Cloud Types

17. Atmospheric Moisture:Clouds
The Bottom of clouds are called their bases
The Base of Cumulus Clouds( CU) can be
approximated by: H ~= Elev1000ft x (TF –TdpF)/4.5o F Fahrenheit o Where H is the height/altitude in feet
of the Base of the CU above the surface.
So Altitude of base of CU is:
Alt ~= Field Elevation ft x (TF –TdpF)/4.5o F Or Alt ~= Field Elevation ft x (TC –TdpC)/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:

18. Atmospheric Moisture:Visibility
Ch 6 Sec B
Tower Visibility: Horizontal Visibility from the tower.
Prevailing Visibility: Greatest horizontal visibility which bright lights or large objects can be seen. Statute Miles
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

19. Flight Rules Ch 6 Sec B Ceiling > 3000 feet AGL
VFR: Visual Flight Rules
Ceiling > 3000 feet AGL
Visibility > 5 Statute Miles
MVFR: Marginal Visual Flight Rules
Ceiling to 3000 feet AGL
Visibility 3 to 5 SM
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

20. Atmospheric Moisture:Precipitation
Ch 6 Sec B
Precipitation
A. Water vapor that has condensed and fallen to Earth
as rain, sleet, snow or hail.
B. Start as very small droplets and then hit and combine
with other droplets to become larger.
C. A rain drop contains about 1 million times the water as
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.
1. Hail is like an onion: it has several layers from
raindrops hitting it and freezing on it.
2. Can be up to 3 inches in size.

21. Atmospheric Moisture:Precipitation
Ch 6 Sec B
Intensity of Precipitation
Light: Indicated with a –’ve sign
Moderate: No intensity symbol
Heavy: Indicated with a + sign

22. From Fed. Met. Handbook 1
Ch 6 Sec B

23. Atmospheric Moisture:Precipitation
Ch 6 Sec B

24. Lab 11 Instead: do LAB 11 from the labs folder of our Web Page
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
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 and dewpoint equal each other?

25. Change of T-Tdp vs. Altitude Or
Is Replaced by:
Change of T-Tdp = 2.5oC/K feet
Cloud height = (T-Tdp)/2.5oC/K feet