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

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

Evaporation, Condensation and Saturation Saturation: water molecules moving from liquid to vapor (evaporation) equal those moving from vapor to liquid (condensation) Saturation is more likely to occur in cool air Evaporation is increased by stronger wind; higher T Q: Water surface evaporation is stronger when air is moist; dry c) not too moist or dry

Evaporation, Condensation and Saturation condensation needs condensation nuclei (microscopic bits of dust, smoke, 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

Q: The favorable condition for ocean surface evaporation includes: a) lower ocean temperature, b) stronger wind, c) humid atmosphere Q: As the air temperature decreases, the maximum amount of water vapor that can exist in the air increases, b) decreases, c) does not change Q: What is condensation?

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 (%)

Vapor Pressure Saturated vapor pressure increases exponentially with T, and also depends on air pressure It is also provided in Table B.1 on p. 460. T= 7C, Pv = 10.2 mb T =10C, Pv = 12.3mb T =21C, Pv = 25.0 mb T = 24C, Pv = 29.6 mb (29.6-25.0) > (12.3-10.2)

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)

Relative Humidity and Dew Point dew point temperature (Td) dew point depression (T – Td) versus 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

Ta = -2C Ta = 35C Td = -2C Td = 10C (> -2C) Ta – Td = 0C Ta – Td = 25C RH = 100% RH = 21% (<100%)

Q: For Ta = 30C, Td = 10C, what is actual vapor pressure? a) 12 mb, b) 42 mb, c) 50 mb Td = 10C, what is RH? a) 12/42, b) 10/30, c) 50/86 Q: For the actual vapor pressure of 42 mb, Td is a) 10C, b) 20C, 30C

Q: Which would decrease with the increase of T? a) water vapor density, b) specific humidity, c) mixing ratio Q: Which would decrease significantly with the increase of T? a) vapor pressure, b) relative humidity, c) mixing ratio Q: 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 Q: The ratio of vapor pressure over dry air pressure near surface is around: a) 0.01, b) 0.1, c) 1.0 Q: The actual water vapor amount is NOT represented by a) vapor pressure, b) relatively humidity, c) mixing ratio

Average Surface dew-point temperature (oF) in July Monsoon onset in Tucson is defined as the first day when the daily Td is greater than 55oF for three consecutive days.

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 (without change of water vapor); a good measure of actual vapor content Heat index Q: Why do both temperature and relative humidity contribute to warm-weather discomfort? A: higher RH; less body moisture evaporation; less cooling; feel warmer

Relative Humidity and Human Discomfort Q: How to demonstrate Td < Tw < T Q: Under what conditions, Td = Tw = T? a) saturation, b) very dry, c) not too dry nor wet Q: Which is heavier? a) dry air, b) moist air, c) same (Note that the molecular mass of dry air is 29 g/mol)

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

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. 463-466) 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 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 Q: `Freeze’ condition is colder than frost, because a) frost would release latent heat; b) `freeze’ condition would release latent heat

Fog radiation fog: cooling from ground advection fog: warm, moist air over cold surface upslope fog: cooling evaporation (mixing) fog: haze: hygroscopic (`water seeking’) condensation nuclei allows water vapor to condense when RH < 100%; higher RH increases droplet size and concentration, leading to fog; haze has a better visibility than fog Q: The tiny cloud from your exhaled breach in winter is related to a) radiation fog, b) advection fog, c) mixing fog, d) haze

Foggy Weather coastal or water body: Annual number of fog days advection Interior: radiation and upslope hazard to aircraft: Some airports use fog-dispersal equipment Q: The London fog is caused by: a) radiation, b) advection, c) mixing

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 forecast many aspects of the weather using nothing but your eyes.

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

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)

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

Clouds with Vertical Development Cumulus (Cu): puffy, floating `cotton’ with flat base (difference from Sc: larger fraction of blue sky, tower-shaped top) cumulus congestus (Tcu): line of towering Cu, rain shower Cumulonimbus (Cb): with anvils, thunderstorm, lightning, and possibly large hail Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.

Some Unusual Clouds lenticular clouds: lens-like; UFO; caused by mountain waves Pileus: `cap’ Mammatus clouds: baglike Contrails: condensation trail from engine (“moist”) exhaust

Q: This is: a) Cc, b) Ac, c) Sc, d) Cu Q: This is: a) Cs, b) As, c) St, d) Ns Q: This is: a) Cu, b) Tcu, c) Cb