1. Chapter 4: Humidity, Condensation and Clouds
Circulation of water in the atmosphere
Evaporation, condensation and saturation
Humidity
Dew and frost
Fog
Foggy weather
Clouds

2. Circulation of Water in the Atmosphere
Hydrologic cycle
evaporation and transpiration (from vegetation)
condensation
precipitation (rain, snow, hail) – when cloud droplets
grow large enough to fall to surface
runoff
The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.
Q: The earth’s hydrological cycle is exceedingly efficient in circulating water in the atmosphere.
a) true; b) false

4. Evaporation, Condensation and Saturation
Saturation: water molecules moving from liquid to vapor
(evaporation) equal those moving from vapor to liquid
(condensation)
Saturation is more likely to occur in cool air
Evaporation is
increased by
stronger wind;
higher T
Q: Water surface
evaporation is stronger
when air is
moist;
dry
c) not too moist or dry

5. Evaporation, Condensation and Saturation
condensation needs condensation nuclei (microscopic bits of dust, smoke, and salt from ocean spray)
In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, a volume approximately the size of your fingertip.
Condensation occurs
primarily when the air
is cooled

6. Q: The favorable condition for ocean surface evaporation
includes: a) lower ocean temperature, b) stronger wind,
c) humid atmosphere
Q: As the air temperature decreases, the maximum amount
of water vapor that can exist in the air
increases, b) decreases, c) does not change
Q: What is condensation?

7. Humidity Humidity describes the amount of water vapor in the air.
Water vapor density (absolute
humidity): mv/V (kg/m3)
Specific humidity
mv/(mv+md) (kg/kg)
Mixing ratio: mv/md (kg/kg)
Vapor pressure: pv (mb)
p = pv + pd
Relative humidity (%)

8. Vapor Pressure Saturated vapor pressure increases exponentially
with T, and also
depends on air pressure
It is also provided in
Table B.1 on p. 460.
T= 7C, Pv = 10.2 mb
T =10C, Pv = 12.3mb
T =21C, Pv = mb
T = 24C, Pv = 29.6 mb
( ) > ( )

10. Relative Humidity
definition of relativ humidity:
actual vapor pressure divided by saturation vapor pressure
(RH = e/es * 100%)
Supersaturation (RH > 100%)
How to increase RH?
Increase e
decrease es (by
decreasing T)

12. Relative Humidity and Dew Point
dew point temperature (Td)
dew point depression (T – Td) versus
relative humidity
Higher (T – Td) indicates lower RH
It is the T to which air would have to be cooled (with
no change in air pressure and moisture content) for
saturation to occur
Higher Td indicates higher actual water
vapor content
Actual vapor pressure = saturated
pressure at Td

13. Ta = -2C Ta = 35C
Td = -2C Td = 10C (> -2C)
Ta – Td = 0C Ta – Td = 25C
RH = 100% RH = 21% (<100%)

14. Q: For Ta = 30C,
Td = 10C, what is actual
vapor pressure?
a) 12 mb, b) 42 mb,
c) 50 mb
Td = 10C, what is RH?
a) 12/42, b) 10/30,
c) 50/86
Q: For the actual vapor pressure of 42 mb, Td is
a) 10C, b) 20C, 30C

15. Q: Which would decrease with the increase of T?
a) water vapor density, b) specific humidity, c) mixing ratio
Q: Which would decrease significantly with the increase of T?
a) vapor pressure, b) relative humidity, c) mixing ratio
Q: For water vapor in the hot, `dry’ air in the Sahara desert
versus that in the cold, `damp’ polar air, which is true?
a) the former is higher in mixing ratio,
b) the former is higher in relative humidity
Q: The ratio of vapor pressure over dry air pressure near
surface is around: a) 0.01, b) 0.1, c) 1.0
Q: The actual water vapor amount is NOT represented by
a) vapor pressure, b) relatively humidity, c) mixing ratio

16. Average Surface dew-point temperature (oF) in July
Monsoon onset in Tucson is defined as the first day when the daily Td is greater than 55oF for three consecutive days.

17. Relative Humidity and Human Discomfort
wet bulb temperature
Tw: lowest T attained by evaporating water into the air;
a good measure of how cool the skin can become
Td: reached by cooling the air to saturation (without change
of water vapor);
a good measure of actual
vapor content
Heat index
Q: Why do both temperature and
relative humidity contribute to
warm-weather discomfort?
A: higher RH; less body
moisture evaporation;
less cooling; feel warmer

18. Relative Humidity and Human Discomfort
Q: How to demonstrate
Td < Tw < T
Q: Under what conditions,
Td = Tw = T?
a) saturation, b) very dry,
c) not too dry nor wet
Q: Which is heavier?
a) dry air, b) moist air, c) same
(Note that the molecular mass of
dry air is 29 g/mol)

19. Q: Which has a higher heat index?
a) Ta = 100F, RH = 20% (AZ), b) Ta = 95F, RH = 40% (FL)

20. Measuring Humidity Psychrometers Wet-bulb T (Tw); dry-bulb T (Ta);
wet-bulb depression (Ta-Tw);
Find Td and RH (based on
Table D on p ) for
Ta = 20C, Ta-Tw = 5C:
Td = 12C, RH = 58%
Ta = 90F, Ta-Tw = 10F: Td = 76F, RH = 65%
Hygrometers
hair hygrometer and electrical hygrometer: RH
infrared hygrometer: moisture content;
dew cell: vapor pressure
dew-point hygrometer (for ASOS)

23. Dew and Frost
dew: condensation at Td > 0C (spherical beads of water)
frozen dew: dew forms and freezes (spherical beads)
frost: deposition (vapor to solid) at Tf < 0C (tree-like branch)
freeze (black frost): Ta drops below 0C without reaching Tf
Q: `Freeze’ condition is colder than frost, because
a) frost would release latent heat;
b) `freeze’ condition would release latent heat

24. Fog radiation fog: cooling from ground advection fog:
warm, moist air over cold surface
upslope fog: cooling
evaporation (mixing) fog:
haze: hygroscopic (`water seeking’) condensation nuclei allows water vapor to condense when RH < 100%;
higher RH increases droplet size and concentration, leading to fog;
haze has a better visibility than fog
Q: The tiny cloud from your exhaled breach in winter is related to
a) radiation fog, b) advection fog,
c) mixing fog, d) haze

25. Foggy Weather coastal or water body: Annual number of fog days
advection
Interior:
radiation and upslope
hazard to aircraft:
Some airports use
fog-dispersal equipment
Q: The London fog is caused by: a) radiation, b) advection, c) mixing

26. Classification of Clouds (chart at end of book)
major cloud types
low, middle, high, vertical
cloud appearance
sheetlike, puffy, wispy, rain cloud
cloud base
0-2 Km, 2-6 km, 6-10km
It’s easy to identify clouds, but it takes practice. The ability to identify clouds allows you to forecast many aspects of the weather using nothing but your eyes.

28. High Clouds All high clouds: thin, high Cirrus (Ci): wispy
Cirrocumulus (Cc): small, white puffs, rippling
Cirrostratus (Cs): usually thin, often producing
a halo

29. Middle Clouds
All middle clouds: thicker than high clouds, cloud base > 2 km
Altocumulus (Ac): gray, puffy (larger, darker than Cc)
Altostratus (As): gray layer cloud with `watery sun’
(difference from Cs: darker, dimly visible, no ground shallows)

30. Low Clouds
All low clouds: base < 2km, thicker than middle/high clouds
Nimbostratus (Ns)
dark gray with light rain
Stratocumulus (Sc):
larger cloud elements with lower cloud base than Ac
Stratus (St)
uniform grayish cloud;
has a more uniform base than Ns;
has a lower base and dark gray than As;
resembles a fog that does not reach the ground

31. Clouds with Vertical Development
Cumulus (Cu): puffy, floating `cotton’ with flat base (difference from Sc: larger fraction of blue sky, tower-shaped top)
cumulus congestus (Tcu): line of towering Cu, rain shower
Cumulonimbus (Cb): with anvils, thunderstorm, lightning, and possibly large hail
Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.

33. Some Unusual Clouds lenticular clouds:
lens-like; UFO; caused by mountain waves
Pileus: `cap’
Mammatus clouds: baglike
Contrails: condensation trail from engine (“moist”) exhaust

34. Q: This is: a) Cc, b) Ac,
c) Sc, d) Cu
Q: This is: a) Cs, b) As,
c) St, d) Ns
Q: This is: a) Cu,
b) Tcu, c) Cb