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
Q: do we have more water vapor in the hot, `dry’ air
the Sahara desert than in the cold, `damp’ polar air?
(A: yes and no, depending on the type of humidity
we mean)

3. Circulation of Water in the Atmosphere
evaporation
condensation
Precipitation
Runoff
hydrologic cycle
The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.

4. Figure 4.1: The hydrologic cycle.
Fig. 4-1, p. 80

5. Evaporation, Condensation and Saturation
saturation is more likely
to occur in cool air
Evaporation is increased by
stronger wind;
Higher T

6. Evaporation, Condensation and Saturation
condensation nuclei
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

7. Humidity Water vapor density (absolute humidity (kg/m3)
Specific humidity (kg/kg)
Mixing ratio (kg/kg)
Vapor pressure (mb)
Relative humidity

8. Vapor Pressure actual vapor pressure saturation vapor pressure
“Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.

9. The actual values are Also provided in Table B.1 on p. 440.
Figure 4.5: Saturation vapor pressure increases with increasing temperature. At a temperature of 10°C, the saturation vapor pressure is about 12 mb, whereas at 30°C it is about 42 mb. The insert illustrates that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice.
Watch this Active Figure on ThomsonNow website at
Fig. 4-5, p. 83

10. Relative Humidity definition of relative humidity:
vapor pressure divided by saturation vapor pressure
saturation and supersaturation
relative humidity and temperature

11. Figure 4.7: When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low. These conditions exist in clear weather when the air is calm or of constant wind speed.
Fig. 4-7, p. 85

12. Relative Humidity and Dew Point
dew point temperature
dew point depression (T – Td) and relative humidity
Higher (T – Td) indicates lower RH
The 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 water vapor content

13. Ta = -2C Ta = 35C Td = -2C Td = 10C Ta – Td = 0C Ta – Td = 25C
RH = 100% RH = 21%
Figure 4.9: The polar air has the higher relative humidity, whereas the desert air, with the higher dew point, contains more water vapor.
(a) POLAR AIR: Air temperature –2°C (28°F)
Dew point –2°C (28°F)
Relative humidity 100 percent
Fig. 4-9a, p. 87

14. Relative Humidity and Human Discomfort
wet bulb temperature
Tw: 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;
a good measure of actual vapor content
Heat index
“It’s not the heat, it’s the humidity” - both temperature and relative humidity contribute to warm-weather discomfort.

15. Figure 4.10: Air temperature (°F) and relative humidity are combined to determine an apparent temperature or heat index (HI). An air temperature of 95°F with a relative humidity of 55 percent produces an apparent temperature (HI) of 110°F.
Fig. 4-10, p. 89

16. Measuring Humidity Psychrometers Wet-bulb T (Tw); Hygrometers
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; or
Ta = 90F, Ta-Tw = 10F
Hygrometers
hair hygrometer and electrical hygrometer: RH
infrared hygrometer and dew cell: moisture content;
dew-point hygrometer (for ASOS)

17. Dew and Frost dew frost frost point and deposition
Frost is one of the few examples of deposition in nature.

18. Fog radiation fog: advection fog: upslope fog: cooling
cooling from ground
advection fog:
warm, moist air over cold surface
upslope fog: cooling
evaporation (mixing) fog
when moist air from your mouth or nose meets the cold air and mixes with it
haze: hygroscopic (`water seeking’) condensation nuclei
allows water vapor to condense when RH is below 100%

19. Foggy Weather coastal vs. interior areas hazard to aircraft
Some airports use fog-dispersal equipment.

20. Clouds
The cloud chart is provided at the end
of the textbook

21. Classification of Clouds
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.

22. Table 4-2, p. 98

23. High Clouds Cirrus: thin, wispy Cirrocumulus: small, white puffs
Cirrostratus: usually thin, often producing a halo
Cirrostratus clouds can sometimes be quite thick.

24. Middle Clouds Altocumulus: gray, puffy (larger, darker than Cc)
Altostratus: gray layer cloud with `watery sun’
(difference from Cs: darker, dimly visible, no ground
shallows)

25. Low Clouds Nimbostratus dark gray with light rain Stratocumulus:
larger cloud elements with lower cloud base than Ac
Stratus
uniform grayish cloud;
has a more uniform base than Ns;
has a lower base and dark gray than As;

26. Clouds with Vertical Development
Cumulus: puffy, floating `cotton’ with flat base
cumulus congestus: line of towering Cu
Cumulonimbus: thunderstorm with anvils
Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.

27. Some Unusual Clouds lenticular clouds: lens-like; UFO Pileus: `cap’
mammatus clouds: baglike
Contrails: condensation trail from engine exhaust