Class #3: Humidity, condensation, and clouds Chapters 4 and 5 Class #3 July 9, 2010

Chapter 4 Atmospheric Humidity Class #3 July 9, 2010

Circulation of Water in the Atmosphere A general definition of humidity is the amount of water vapor in the air. Remember, humidity is not constant through time or space, there is constant circulation of water through the hydrologic cycle. Class #3 July 9, 2010

FIGURE 4.1 The hydrologic cycle. Class #3 July 9, 2010

Stepped Art Figure 4.1: The hydrologic cycle. Class #3 July 9, 2010 Fig. 4-1, p. 90

Figure 4.2 The water molecule. Class #3 July 9, 2010 Fig. 4-2, p. 91

The Many Phases of Water Phase is related to molecular motion, an increase or decrease in motion creates a phase change. Ice is the coolest/slowest phase Water vapor is the warmest/fastest phase Class #3 July 9, 2010

FIGURE 4.3 The three states of matter. Water as a gas, as a liquid, and as a solid. Class #3 July 9, 2010

Evaporation, Condensation, & Saturation Evaporation is the change of liquid into a gas a requires heat. Condensation is the change of a gas into a liquid and releases heat. Condensation nuclei Saturation is an equilibrium condition in which for each molecule that evaporates, one condenses. Class #3 July 9, 2010

FIGURE 4.4 (a) Water molecules at the surface of the water are evaporating (changing from liquid into vapor) and condensing (changing from vapor into liquid). Since more molecules are evaporating than condensing, net evaporation is occurring. (b) When the number of water molecules escaping from the liquid (evaporating) balances those returning (condensing), the air above the liquid is saturated with water vapor. (For clarity, only water molecules are illustrated.) Class #3 July 9, 2010

FIGURE 4.5 Condensation is more likely to occur as the air cools. (a) In the warm air, fast-moving H2O vapor molecules tend to bounce away after colliding with nuclei. (b) In the cool air, slow-moving vapor molecules are more likely to join together on nuclei. The condensing of many billions of water molecules produces tiny liquid water droplets. Class #3 July 9, 2010

Humidity Any of a number of ways of specifying the amount of water vapor in the air. Absolute humidity: mass of water vapor/volume of air Water vapor density Not commonly used due to frequent change of volume Class #3 July 9, 2010

FIGURE 4.6 The water vapor content (humidity) inside this air parcel can be expressed in a number of ways. Class #3 July 9, 2010

FIGURE 4.7 With the same amount of water vapor in a parcel of air, an increase in volume decreases absolute humidity, whereas a decrease in volume increases absolute humidity. Class #3 July 9, 2010

Humidity Specific Humidity: mass of water vapor/mass of air Mixing ratio: mass of water vapor/mass of dry air Neither measurement changes with volume, must add or subtract water vapor. Class #3 July 9, 2010

FIGURE 4.8 The specific humidity does not change as air rises and descends. Class #3 July 9, 2010

FIGURE 4.9 The average specific humidity for each latitude. The highest average values are observed in the tropics and the lowest values in polar regions. Class #3 July 9, 2010

Humidity Vapor pressure: the pressure exerted by water vapor molecules in an air parcel (Dalton’s Law of Partial Pressure) Fraction of total vapor pressure (1% or so) More water molecules = high vapor pressure Saturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature Class #3 July 9, 2010

ACTIVE FIGURE 4.10 Saturation vapor pressure increases with increasing temperature. At a temperature of 10oC, the saturation vapor pressure is about 12 mb, whereas at 30oC it is about 42 mb. The insert illustrates that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice. Visit the Meteorology Resource Center to view this and other active figures at academic.cengage.com/login Class #3 July 9, 2010

Humidity Special Topic: Vapor Pressure & Boiling Once water boils it requires more energy to increase temperature. Water boils at a low temperature in the mountains and thus needs more energy and time to cook items as compared to sea level. Class #3 July 9, 2010

Humidity Relative Humidity: (actual water vapor/saturation water vapor)*100 RH can be changed two ways: Change vapor content Change saturation Decrease temperature causes an increase in relative humidity (inverse relationship). Class #3 July 9, 2010

FIGURE 4.11 (a) At the same air temperature, an increase in the water vapor content of the air increases the relative humidity as the air approaches saturation. (b) With the same water vapor content, an increase in air temperature causes a decrease in relative humidity as the air moves farther away from being saturated. Class #3 July 9, 2010

Humidity Relative Humidity and Dew Point Dew point is the temperature at which saturation occurs Cool air parcel to dew point and liquid water condenses A good measure of actual water vapor content Relative humidity indicates how close to saturation, dew point indicates the amount of water vapor Class #3 July 9, 2010

FIGURE 4.12 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. Class #3 July 9, 2010

Figure 4.13 On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.) Class #3 July 9, 2010 Fig. 4-13, p. 98

Figure 4.13 On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.) Class #3 July 9, 2010 Fig. 4-13, p. 98

Figure 4.13 On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.) Class #3 July 9, 2010 Fig. 4-13, p. 98

Figure 4.13 On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.) Class #3 July 9, 2010 Fig. 4-13, p. 98

FIGURE 4.14 Average surface dew-point temperatures (oF) for (a) January and for (b) July. Class #3 July 9, 2010

FIGURE 4.14 Average surface dew-point temperatures (oF) for (a) January and for (b) July. Class #3 July 9, 2010

FIGURE 4.16 Relative humidity averaged for latitudes north and south of the equator. Class #3 July 9, 2010

FIGURE 4.17 Air from the Pacific Ocean is hot and dry over land, whereas air from the Gulf of Mexico is hot and muggy over land. For each city, T represents the air temperature, Td the dew point, and RH the relative humidity. (All data represent conditions during a July afternoon at 3 p.m. local time.) Class #3 July 9, 2010

Figure 4.15 The polar air has the higher relative humidity, whereas the desert air, with the higher dew point, contains more water vapor. Class #3 July 9, 2010 Fig. 4-15a, p. 100

Figure 4.15 The polar air has the higher relative humidity, whereas the desert air, with the higher dew point, contains more water vapor. Class #3 July 9, 2010 Fig. 4-15b, p. 100

Humidity Relative Humidity in the Home Due to an increase in temperature in a heated home there is a decrease in relative humidity, causing more evaporation from body, plants, etc Humidifier, chapped lips Swamp cooler Class #3 July 9, 2010

FIGURE 4.18 When outside air with an air temperature and a dew point of o15oC (5oF) is brought indoors and heated to a temperature of 20oC (68oF) (without adding water vapor to the air), the relative humidity drops to 8 percent, placing adverse stress on plants, animals, and humans living inside. (T represents temperature; Td, dew point; and RH, relative humidity.) Class #3 July 9, 2010

Humidity Relative humidity & human comfort “It’s not the heat, it’s the humidity.” High relative humidity equates to less evaporative cooling. Sweat cannot evaporate and cool the body Wet bulb temperature Heat Index Class #3 July 9, 2010

FIGURE 4.19 Air temperature (oF) and relative humidity are combined to determine an apparent temperature or heat index (HI). An air temperature of 95oF with a relative humidity of 55 percent produces an apparent temperature (HI) of 110oF. Class #3 July 9, 2010

Humidity Special Topic: Heavier humid air Due to the molecular weight of water as compared to nitrogen, humid air is lighter than dry air. Baseball announcers are incorrect. Class #3 July 9, 2010

Humidity Measuring humidity Sling psychrometer Hygrometer Class #3 July 9, 2010

FIGURE 4.20 The sling psychrometer. Class #3 July 9, 2010

Condensation: DEW, Fog, & clouds Chapter 5 Condensation: DEW, Fog, & clouds Class #3 July 9, 2010

The Formation of Dew & Frost Dew forms on objects near the ground surface when they cool below the dew point temperature. More likely on clear nights due to increased radiative cooling White frost forms when temperature cools below the dew point and the dew point is below 0°C Class #3 July 9, 2010

FIGURE 5.1 Dew forms on clear nights when objects on the surface cool to a temperature below the dew point. If these beads of water should freeze, they would become frozen dew. Class #3 July 9, 2010

FIGURE 5.2 These are the delicate ice-crystal patterns that frost exhibits on a window during a cold winter morning. Class #3 July 9, 2010

Condensation Nuclei Particles suspended in the air that around which water condenses or freezes. Hydrophobic/hygroscopic Class #3 July 9, 2010

Class #3 July 9, 2010 Table 5-1, p. 113

FIGURE 5.1 Dew forms on clear nights when objects on the surface cool to a temperature below the dew point. If these beads of water should freeze, they would become frozen dew. Class #3 July 9, 2010

FIGURE 5.3 Hygroscopic nuclei are “water-seeking,” and water vapor rapidly condenses on their surfaces. Hydrophobic nuclei are “water- repelling” and resist condensation. Class #3 July 9, 2010

Haze Dry condensation nuclei (above dew point) reflect and scatter sunlight creating blueish haze. Wet condensation nuclei (75% relative humidity) reflect and scatter sunlight creating grayish or white haze. Class #3 July 9, 2010

Figure 5.4 The high relative humidity of the cold air above the lake is causing a layer of haze to form on a still winter morning. Class #3 July 9, 2010 Fig. 5-4, p. 114

Fog Saturation reached condensation forms a cloud near the ground Radiation fog: ground cools through conduction and radiation; ground fog Valley fog created by cold air drainage High inversion fog Class #3 July 9, 2010

FIGURE 5.5 Radiation fog nestled in a valley. Class #3 July 9, 2010

FIGURE 5.6 Visible satellite image of dense radiation fog in the southern half of California’s Central Valley on the morning of November 20, 2002. The white region to the east (right) of the fog is the snowcapped Sierra Nevada range. During the late fall and winter, the fog, nestled between two mountain ranges, can last for many days without dissipating. The fog on this day was responsible for several auto accidents, including a 14-car pileup near Fresno. Class #3 July 9, 2010

Fog Advection Fog: warm moist fog moves horizontally (advects) over a cool surface. Summer fog on the Pacific coast Observation: Headlands Air converges and rises over headlands forming fog as compared to lower elevation beaches. Class #3 July 9, 2010

FIGURE 5.7 Advection fog forms as the wind moves moist air over a cooler surface. Here advection fog, having formed over the cold, coastal water of the Pacific Ocean, is rolling inland past the Golden Gate Bridge in San Francisco. As fog moves inland, the air warms and the fog lifts above the surface. Eventually, the air becomes warm enough to totally evaporate the fog. Class #3 July 9, 2010

FIGURE 5.9 (a) Radiation fog tends to form on clear, relatively calm nights when cool, moist surface air is overlain by drier air and rapid radiational cooling occurs. (b) Advection fog forms when the wind moves moist air over a cold surface and the moist air cools to its dew point. Class #3 July 9, 2010

Fog Upslope Fog: moist air flows up an orographic barrier East side of the Rockies Evaporation Fog: Warm moist surface provides enough moisture to saturate a dry air parcel; short lived Steam fog Breath in winter Class #3 July 9, 2010

FIGURE 5.10 Upslope fog forms as moist air slowly rises, cools, and condenses over elevated terrain. Class #3 July 9, 2010

FIGURE 5.11 Even in summer, warm air rising above thermal pools in Yellowstone National Park condenses into a type of steam fog. Class #3 July 9, 2010

Foggy Weather In general fog not common for most location in the US. However several areas do exist with a high frequency of fog. Two causes: Elevation Ocean currents Class #3 July 9, 2010

FIGURE 5.12 Average annual number of days with dense fog (visibility less than 0.25 miles) through the United States. (NOAA) Class #3 July 9, 2010

Foggy Weather Environmental Issue: Fog dispersal Mix air with air craft or fans Introduce large particle into air to reduce total number of cloud droplets. Use dry ice to lower temperature below freezing. Class #3 July 9, 2010

Clouds Classification of clouds: use Latin words to describe height and appearance. Factors described Height: low, mid, high, vertical Appearance: shape, density, color Class #3 July 9, 2010

TABLE 5.2 The Four Major Cloud Groups and Their Types Class #3 July 9, 2010

TABLE 5.3 Approximate Height of Cloud Bases Above the Surface for Various Locations Class #3 July 9, 2010

FIGURE 5.24 A generalized illustration of basic cloud types based on height above the surface and vertical development. Class #3 July 9, 2010

FIGURE 5.13 Cirrus clouds. Class #3 July 9, 2010

FIGURE 5.14 Cirrocumulus clouds. Class #3 July 9, 2010

FIGURE 5.15 Cirrostratus clouds with a faint halo encircling the sun. The sun is the bright white area in the center of the circle. Class #3 July 9, 2010

FIGURE 5.16 Altocumulus clouds. Class #3 July 9, 2010

FIGURE 5.17 Altostratus clouds. The appearance of a dimly visible “watery sun” through a deck of gray clouds is usually a good indication that the clouds are altostratus. Class #3 July 9, 2010

FIGURE 5.18 The nimbostratus is the sheetlike cloud from which light rain is falling. The ragged-appearing clouds beneath the nimbostratus is stratus fractus, or scud. Class #3 July 9, 2010

FIGURE 5.19 Stratocumulus clouds forming along the south coast of Florida. Notice that the rounded masses are larger than those of the altocumulus. Class #3 July 9, 2010

FIGURE 5.20 A layer of low-lying stratus clouds hides these mountains in Iceland. Class #3 July 9, 2010

FIGURE 5.21 Cumulus clouds. Small cumulus clouds such as these are sometimes called fair weather cumulus, or cumulus humilis. Class #3 July 9, 2010

FIGURE 5.22 Cumulus congestus. This line of cumulus congestus clouds is building along Maryland’s eastern shore. Class #3 July 9, 2010

FIGURE 5.23A cumulonimbus cloud (thunderstorm). Strong upper-level winds blowing from right to left produce a welldefi ned anvil. Sunlight scattered by falling ice crystals produces the white (bright) area beneath the anvil. Notice the heavy rain shower falling from the base of the cloud. Class #3 July 9, 2010

Some Unusual Clouds Not all clouds can be placed into the ten basic cloud forms. Unique atmospheric processes and environmental conditions create dramatic and exotic clouds. Unusual clouds and weather balloons often cause of UFO reports. Class #3 July 9, 2010

TABLE 5.4 Common Terms Used in Identifying Clouds Class #3 July 9, 2010

FIGURE 5.25 Lenticular clouds forming on the leeward side of the Sierra Nevada near Verdi, Nevada. Class #3 July 9, 2010

Figure 5.26 The cloud forming over and downwind of Mt. Rainier is called a banner cloud. Class #3 July 9, 2010 Fig. 5-26, p. 130

Figure 5.27 A pileus cloud forming above a developing cumulus cloud. Class #3 July 9, 2010 Fig. 5-27, p. 130

Figure 5.28 Mammatus clouds forming beneath a thunderstorm. Class #3 July 9, 2010 Fig. 5-28, p. 130

Figure 5.29 A contrail forming behind a jet aircraft. Class #3 July 9, 2010 Fig. 5-29, p. 130

Figure 5.30 The clouds in this photograph are nacreous clouds. They form in the stratosphere and are most easily seen at high latitudes. Class #3 July 9, 2010 Fig. 5-30, p. 131

Figure 5.31 The wavy clouds in this photograph are noctilucent clouds. They are usually observed at high latitudes, at altitudes between 75 and 90 km above the earth’s surface. Class #3 July 9, 2010 Fig. 5-31, p. 131

Cloud Observations Sky conditions: cloud coverage divided into eighths and each amount associated with term such as scattered clouds. Observations: cloud ceilings Ceilometer used at airports to determine height from clouds by light or laser striking clouds and then amount and speed of reflected light recorded. Class #3 July 9, 2010

Figure 5 The laser-beam ceilometer sends pulses of infrared radiation up to the cloud. Part of this beam is reflected back to the ceilometer. The interval of time between pulse transmission and return is a measure of cloud height, as displayed on the indicator screen. Class #3 July 9, 2010 Fig. 5, p. 132

TABLE 5.5 Description of Sky Conditions Class #3 July 9, 2010

Cloud Observations Satellite Observations Geostationary, polar orbiting Visible light provides a black and white picture of clouds Infrared approximates cloud temperature which infers height Satellites measure many other variables: sea surface temperatures, ozone, upper level features, snow cover, land cover Class #3 July 9, 2010

FIGURE 5.32 The geostationary satellite moves through space at the same rate that the earth rotates, so it remains above a fixed spot on the equator and monitors one area constantly. Class #3 July 9, 2010

FIGURE 5.33 Polar-orbiting satellites scan from north to south, and on each successive orbit the satellite scans an area farther to the west. Class #3 July 9, 2010

FIGURE 5.34 Generally, the lower the cloud, the warmer its top. Warm objects emit more infrared energy than do cold objects. Thus, an infrared satellite picture can distinguish warm, low (gray) clouds from cold, high (white) clouds. Class #3 July 9, 2010