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

2. Circulation of Water in the Atmosphere Hydrologic cycle Hydrologic cycle evaporation and transpiration (from vegetation) evaporation and transpiration (from vegetation) condensation condensation precipitation (rain, snow, hail) – when cloud droplets precipitation (rain, snow, hail) – when cloud droplets grow large enough to fall to surface grow large enough to fall to surface runoff runoff The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.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 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 a)moist; b)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) condensation needs condensation nuclei (microscopic bits of dust 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.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 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): m v /V (kg/m 3 ) Specific humidity m v /(m v +m d ) (kg/kg) Mixing ratio: m v /m d (kg/kg) Vapor pressure: p v (mb) p = p v + p d Relative humidity (%)

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

8. Relative Humidity definition of relativ humidity: definition of relativ humidity: actual vapor pressure divided by saturation vapor pressure actual vapor pressure divided by saturation vapor pressure (RH = e/e s * 100%) (RH = e/e s * 100%) Supersaturation (RH > 100%) Supersaturation (RH > 100%) How to increase RH? How to increase RH? Increase e Increase e decrease e s (by decrease e s (by decreasing T) 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 (T d ) dew point temperature (T d ) dew point depression (T – T d ) and relative humidity dew point depression (T – T d ) and relative humidity Higher (T – T d ) indicates lower RH Higher (T – T d ) indicates lower RH It is the T to which air would have to be cooled (withIt is the T to which air would have to be cooled (with no change in air pressure and moisture content) for no change in air pressure and moisture content) for saturation to occur saturation to occur Higher T d indicates higher actual water vapor contentHigher T d indicates higher actual water vapor content Actual vapor pressure = saturated pressure at T dActual vapor pressure = saturated pressure at T d

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

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

14. Relative Humidity and Human Discomfort wet bulb temperature wet bulb temperature T w : lowest T attained by evaporating water into the air; T w : lowest T attained by evaporating water into the air; a good measure of how cool the skin can become a good measure of how cool the skin can become T d : reached by cooling the air to saturation; T d : reached by cooling the air to saturation; a good measure of actual vapor content a good measure of actual vapor content Graphically demonstrate T d < T w < T Graphically demonstrate T d < T w < T Q8: Under what conditions, T d = T w = T? a) saturation, b) very dry, c) not too dry nor wet a) saturation, b) very dry, c) not too dry nor wet Heat index Heat index Q9: Why do both temperature and relative humidity contribute to warm-weather discomfort? A: higher RH; warm-weather discomfort? A: higher RH; less body moisture evaporation; less cooling; feel warmer less body moisture evaporation; less cooling; feel warmer

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

16. Measuring Humidity Psychrometers Psychrometers Wet-bulb T (T w ); dry-bulb T (T a ); Wet-bulb T (T w ); dry-bulb T (T a ); wet-bulb depression (T a -T w ); wet-bulb depression (T a -T w ); Find T d and RH (based on Find T d and RH (based on Table D on p. 443-446) for Table D on p. 443-446) for Ta = 20C, Ta-Tw = 5C: Ta = 20C, Ta-Tw = 5C: Td = 12C, RH = 58% Td = 12C, RH = 58% Ta = 90F, Ta-Tw = 10F: Td = 76F, RH = 65% Ta = 90F, Ta-Tw = 10F: Td = 76F, RH = 65% Hygrometers Hygrometers hair hygrometer and electrical hygrometer: RH hair hygrometer and electrical hygrometer: RH infrared hygrometer: moisture content; infrared hygrometer: moisture content; dew cell: vapor pressure dew cell: vapor pressure dew-point hygrometer (for ASOS) dew-point hygrometer (for ASOS)

17. Dew and Frost dew: condensation at T d > 0C (spherical beads of water) dew: condensation at T d > 0C (spherical beads of water) frozen dew: dew forms and freezes (spherical beads) frozen dew: dew forms and freezes (spherical beads) frost: deposition (vapor to solid) at T f < 0C (tree-like branch) frost: deposition (vapor to solid) at T f < 0C (tree-like branch) freeze (black frost): T a drops below 0C without reaching T f freeze (black frost): T a drops below 0C without reaching T f Q11: `Freeze’ condition is colder than frost, because a) frost would release latent heat; a) frost would release latent heat; b) `freeze’ condition would release latent heat b) `freeze’ condition would release latent heat

18. Fog radiation fog: radiation fog: cooling from ground cooling from ground advection fog: advection fog: warm, moist air over cold surface warm, moist air over cold surface upslope fog: cooling upslope fog: cooling evaporation (mixing) fog: evaporation (mixing) fog: when moist air from your mouth or nose meets the cold air and mixes with it 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: hygroscopic (`water seeking’) condensation nuclei allows water vapor to condense when RH < 100%; haze has a better visibility than fog haze has a better visibility than fog Q12: The London fog is caused by a) radiation, b) advection, c) mixing a) radiation, b) advection, c) mixing

19. Foggy Weather coastal or water body: radiation coastal or water body: radiation Interior: radiation and upslope Interior: radiation and upslope hazard to aircraft: Some airports use fog-dispersal equipment 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 major cloud types low, middle, high, vertical low, middle, high, vertical cloud appearance cloud appearance sheetlike, puffy, wispy, rain cloud sheetlike, puffy, wispy, rain cloud cloud base cloud base 0-2 Km, 2-6 km, 6-10km 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.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.

21. Table 4-2, p. 98

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

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

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

25. Clouds with Vertical Development Cumulus (Cu): puffy, floating `cotton’ with flat base Cumulus (Cu): puffy, floating `cotton’ with flat base cumulus congestus (Tcu): line of towering Cu cumulus congestus (Tcu): line of towering Cu Cumulonimbus (Cb): thunderstorm with anvils Cumulonimbus (Cb): thunderstorm with anvils Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.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: lenticular clouds: lens-like; UFO lens-like; UFO Pileus: `cap’ Pileus: `cap’ Mammatus clouds: baglike Mammatus clouds: baglike Contrails: condensation trail from engine exhaust 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