Chapter 4: Humidity, Condensation and Clouds

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. Fig. 4-1, p. 80

5. Evaporation, Condensation and Saturation • 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 nucleiare 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. 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. 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% 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 andrelative humidity contribute to warm-weather discomfort.

15. Fig. 4-10, p. 89

16. 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. 443-446) 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: 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 forecastmany aspects of the weather using nothing but youreyes.

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, butall 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