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 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)

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.

Figure 4.1: The hydrologic cycle. Fig. 4-1, p. 80

Evaporation, Condensation and Saturation saturation is more likely to occur in cool air Evaporation is increased by stronger wind; Higher T

Evaporation, Condensation and Saturation condensation nuclei 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

Humidity Water vapor density (absolute humidity (kg/m3) Specific humidity (kg/kg) Mixing ratio (kg/kg) Vapor pressure (mb) Relative humidity

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.

The actual values are Also provided in Table B.1 on p. 440. Figure 4.5: Saturation vapor pressure increases with increasing temperature. At a temperature of 10°C, the saturation vapor pressure is about 12 mb, whereas at 30°C 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. Fig. 4-5, p. 83

Relative Humidity definition of relative humidity: vapor pressure divided by saturation vapor pressure saturation and supersaturation relative humidity and temperature

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. Fig. 4-7, p. 85

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

Ta = -2C Ta = 35C Td = -2C Td = 10C Ta – Td = 0C Ta – Td = 25C RH = 100% RH = 21% 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 –2°C (28°F) Dew point –2°C (28°F) Relative humidity 100 percent Fig. 4-9a, p. 87

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 and relative humidity contribute to warm-weather discomfort.

Figure 4.10: Air temperature (°F) and relative humidity are combined to determine an apparent temperature or heat index (HI). An air temperature of 95°F with a relative humidity of 55 percent produces an apparent temperature (HI) of 110°F. Fig. 4-10, p. 89

Measuring Humidity Psychrometers Wet-bulb T (Tw); Hygrometers 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)

Dew and Frost dew frost frost point and deposition Frost is one of the few examples of deposition in nature.

Fog radiation fog: advection fog: upslope fog: cooling 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%

Foggy Weather coastal vs. interior areas hazard to aircraft Some airports use fog-dispersal equipment.

Clouds The cloud chart is provided at the end of the textbook

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

Table 4-2, p. 98

High Clouds Cirrus: thin, wispy Cirrocumulus: small, white puffs Cirrostratus: usually thin, often producing a halo Cirrostratus clouds can sometimes be quite thick.

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)

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;

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, but all thunderstorms start out as cumulus clouds.

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