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 • 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. Q1: The earth’s hydrological cycle is exceedingly efficient in circulating water in the atmosphere. a) true; b) false

3. Fig. 4-1, p. 80

4. Evaporation, Condensation and Saturation • Saturation: water molecules moving from liquid to vapor • (evaporation) equal those moving from vapor to liquid • (condensation) • Evaporation is increased by • stronger wind; • higher T • Q2: 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 and salt from ocean spray) • 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 • Saturation is more likely • to occur in cool air

6. 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 (%)

7. Vapor Pressure Saturated vapor pressure increases exponentially with T It is also provided in Table B.1 on p. 440. T= 7C, Pv = 10.2 mb T =10C, Pv = 12.3mb T =21C, Pv = 25.0 mb T = 24C, Pv = 29.6 mb Fig. 4-5, p. 83

8. 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)

9. Fig. 4-7, p. 85

10. Q3: Which would decrease with the increase of T? a) water vapor density b) specific humidity c) mixing ratio Q4: Which would decrease significantly with the increase of T? a) vapor pressure b) relative humidity Q5: 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

11. Relative Humidity and Dew Point • dew point temperature (Td) • dew point depression (T – Td) and 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

12. Ta = -2C Ta = 35C Td = -2C Td = 10C Ta – Td = 0C Ta – Td = 25C RH = 100% RH = 21% Fig. 4-9a, p. 87

13. Q6: For Ta = 30C, • Td = 10C, what is actual • vapor pressure? • 12 mb, • 42 mb, • 50 mb • Q7: For Ta = 30C, • Td = 10C, what is RH? • a) 12/42, • b) 10/30, • c) 50/86 Fig. 4-5, p. 83

14. 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; a good measure of actual vapor content Graphically demonstrate Td < Tw < T Q8: Under what conditions, Td = Tw = T? a) saturation, b) very dry, c) not too dry nor wet • Heat index Q9: Why do both temperature and relative humidity contribute to warm-weather discomfort? A: higher RH; less body moisture evaporation; less cooling; feel warmer

15. Q10: Which has a higher heat index? a) Ta = 100F, RH = 20% (AZ), b) Ta = 95F, RH = 40% (FL)

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: 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)

17. 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 Q11: `Freeze’ condition is colder than frost, because a) frost would release latent heat; b) `freeze’ condition would release latent heat

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 < 100%; haze has a better visibility than fog Q12: The London fog is caused by a) radiation, b) advection, c) mixing

19. Foggy Weather • coastal or water body: radiation • Interior: radiation and upslope • hazard to aircraft: Some airports use fog-dispersal equipment Annual number of fog days

20. 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 forecastmany aspects of the weather using nothing but youreyes.

21. Table 4-2, p. 98

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

23. 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)

24. 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

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

26. Fig. 4-32, p. 104

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

28. Q13: This is: a) Cc, b) Ac, c) Sc, d) Cu Q14: This is: a) Cs, b) As, c) St, d) Ns Q15: This is: a) Cu, b) Tcu, c) Cb