Chapter 4: Humidity, Condensation and CloudsCirculation of water in the atmosphereEvaporation, condensation and saturationHumidityDew and frostFogFoggy weatherClouds

Circulation of Water in the AtmosphereHydrologic 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 weeks supply of precipitation for the planet.

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

Fig. 4-1, p. 80

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 surfaceevaporation is strongerwhen air ismoist;dryc) not too moist or dry

Evaporation, Condensation and Saturationcondensation 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. Condensation occurs primarily when the air is cooled

Saturation is more likely to occur in cool air

HumidityHumidity 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 (%)

Fig. 4-5, p. 83Vapor Pressure

Saturated vapor pressureincreases exponentiallywith T

It is also provided in Table B.1 on p. 440.

T= 7C, Pv = 10.2 mbT =10C, Pv = 12.3mb

T =21C, Pv = 25.0 mbT = 24C, Pv = 29.6 mb

Relative Humiditydefinition 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)

Fig. 4-7, p. 85

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

Relative Humidity and Dew Pointdew point temperature (Td)

dew point depression (T Td) and relative humidity Higher (T Td) indicates lower RHIt is the T to which air would have to be cooled (with no change in air pressure and moisture content) for saturation to occurHigher Td indicates higher actual water vapor contentActual vapor pressure = saturated pressure at Td

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

Fig. 4-5, p. 83Q6: For Ta = 30C, Td = 10C, what is actualvapor 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

Relative Humidity and Human Discomfortwet 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 wetHeat 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

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

Measuring HumidityPsychrometers 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)

Dew and Frostdew: 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 TfQ11: `Freeze condition is colder than frost, because a) frost would release latent heat; b) `freeze condition would release latent heat

Fogradiation fog: cooling from groundadvection fog: warm, moist air over cold surfaceupslope fog: coolingevaporation (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

Foggy Weathercoastal or water body: radiationInterior: radiation and upslopehazard to aircraft: Some airports use fog-dispersal equipmentAnnualnumber of fog days

Classification of Clouds (chart at end of book)major cloud types low, middle, high, verticalcloud appearance sheetlike, puffy, wispy, rain cloudcloud base 0-2 Km, 2-6 km, 6-10kmIts 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.

Table 4-2, p. 98

High CloudsAll high clouds: thin, high Cirrus (Ci): wispyCirrocumulus (Cc): small, white puffs, ripplingCirrostratus (Cs): usually thin, often producing a halo

Middle CloudsAll 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)

Low CloudsAll low clouds: base < 2km, thicker than middle/high cloudsNimbostratus (Ns) dark gray with light rainStratocumulus (Sc): larger cloud elements with lower cloud base than AcStratus (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

Clouds with Vertical DevelopmentCumulus (Cu): puffy, floating `cotton with flat basecumulus congestus (Tcu): line of towering CuCumulonimbus (Cb): thunderstorm with anvilsNot all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.

Fig. 4-32, p. 104

Some Unusual Cloudslenticular clouds: lens-like; UFOPileus: `capMammatus clouds: baglikeContrails: condensation trail from engine exhaust

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

Figure 4.1: The hydrologic cycle.Figure 4.5: Saturation vapor pressure increases with increasing temperature. At a temperature of 10C, the saturation vapor pressure is about 12 mb, whereas at 30C 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 www.thomsonedu.com/login.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.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 2C (28F)Dew point 2C (28F)Relative humidity 100 percentFigure 4.5: Saturation vapor pressure increases with increasing temperature. At a temperature of 10C, the saturation vapor pressure is about 12 mb, whereas at 30C 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 www.thomsonedu.com/login.Figure 4.10: Air temperature (F) and relative humidity are combined to determine an apparent temperature or heat index (HI). An air temperature of 95F with a relative humidity of 55 percent produces an apparent temperature (HI) of 110F.Figure 4.32: A generalized illustration of basic cloud types based on height above the surface and vertical development.