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Chapter 4 Atmospheric Moisture, Condensation, and Clouds

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Chapter 4 Atmospheric Moisture, Condensation, and Clouds 9/13/2012 Chapter 4 Atmospheric Moisture, Condensation, and Clouds. The sun’s electromagnetic spectrum and some of the descriptive names of each region. The numbers underneath the curve approximate the percent of energy the sun radiates in various regions. 0.4 μm = 400 nm 0.7 μm = 700 nm The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth’s surface. Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (gray The hotter sun not only radiates more energy than that of the cooler earth (the area shade), the air under the curve), but it also radiates the majority of its energy at much shorter temperature falls. wavelengths. (The area under the curves is equal to the total energy emitted, and the scales for the two curves differ by a factor of 100,000.) 1 9/13/2012 The average annual incoming solar radiation (yellow line) absorbed by the earth and the atmosphere along with the average annual infrared radiation (red line) emitted by the earth and the atmosphere. Water can exist in 3 phases, depending Evaporation, Condensation, upon pressure and temperature. & Saturation • Evaporation is the change of liquid into a gas and requires heat. • Condensation is the change of a gas into a liquid and releases heat. • Condensation nuclei • Sublimation: solid to gaseous state without becoming a liquid. • Saturation is an equilibrium condition http://www.sci.uidaho.edu/scripter/geog100/l http://chemwiki.ucdavis.edu/Physical_Che in which for each molecule that ect/05‐atmos‐water‐wx/ch5‐part‐2‐water‐ mistry/Physical_Properties_of_Matter/Phas phases.htm e_Transitions/Phase_Diagrams_1 evaporates, one condenses. 2 9/13/2012 http://clasfaculty.ucdenver.edu/callen/1202/Intro/EarthPlaceSpace/UniqueEarth.html Latent Heat of Vaporization = 600 calories / 1g Latent Heat of Condensation = 600 calories / 1g Latent Heat of Fusion= 80 calories / 1g Latent Heat of Vaporization = 600 calories / 1g Latent Heat of Condensation = 600 calories / 1g Latent Heat of Fusion= 80 calories / 1g How much energy to sublimate? http://www2.chemistry.msu.edu/courses/cem152/snl_cem152_SS12/pracprob/practiceexam1.html (a) Water molecules at the (b) When the number surface of the water are of water molecules evaporating (changing escaping from the liquid from liquid into vapor) (evaporating) balances and condensing (changing those returning from vapor into liquid). (condensing), the air Since more molecules are above the liquid is evaporating than saturated with water condensing, net vapor. (For clarity, only evaporation is occurring. water molecules are illustrated.) 3 9/13/2012 Condensation is (b) In the cool air, more likely to slow‐moving vapor occur as the air molecules are more cools. (a) In the likely to join warm air, fast‐ together on nuclei. moving H2O The condensing of vapor molecules many billions of tend to bounce water molecules away after produces tiny liquid colliding with water droplets. nuclei. 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 AH = mass of water vapor (g) / Volume of Air (m^3) http://www.srh.noaa.gov/jetstream/atmos/hydrocycle_max.htm 4 9/13/2012 Humidity • Vapor pressure: the pressure exerted by water vapor molecules in an air parcel – 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 Fig. 4‐4, p. 87 Saturation vapor pressure increases with Humidity increasing temperature. At a • Specific Humidity: mass of water vapor/mass temperature of 10°C, the saturation vapor of air pressure is about 12 • Mixing ratio: mass of water vapor/mass of dry mb, whereas at 30°C it air is about 42 mb. The • Neither measurement changes with volume, insert illustrates that the saturation vapor must add or subtract water vapor. pressure over water is greater than the saturation vapor Mixing Ratio = mass of water vapor (g) / Mass of pressure over ice. dry Air (kg) 5 9/13/2012 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 relation). (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. When the air is cool (morning), the relative humidity is high. (b) With the same water vapor content, an increase in air When the air is warm (afternoon), the relative humidity is temperature causes a decrease in relative humidity as the low. These conditions exist in clear weather when the air is air moves farther away from being saturated. calm or of constant wind speed. 6 9/13/2012 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 Average surface dew‐point temperatures (°F) across the United States and Canada for January. Inside the cloud the air temperature (T) and dew point (Td) are the same, the air is saturated, and the relative humidity (RH) is 100 percent. Average surface dew‐point temperature across the However, at the surface where the air temperature and dew point are not the same, the air is not saturated (even though it is raining), and the United States and Canada (°F) for July. relative humidity is considerably less than 100 percent. 7 9/13/2012 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 The polar air has the higher relative humidity, whereas the desert air, with the higher dew point, contains more water vapor. 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. 8 9/13/2012 Humidity • Measuring humidity – Sling psychrometer – Hygrometer The hair hygrometer measures relative humidity by amplifying and measuring changes in the length of human (or horse) hair. Dew and Frost Dew and Frost • Dew forms on objects near the ground surface • Particles suspended in the air around which when they cool below the dew point water condenses or freezes temperature. – Hydrophobic/hygroscopic – More likely on clear nights due to increased • Dry condensation nuclei (above dew point) radiative cooling reflect and scatter sunlight creating blueish • White frost forms when temperature cools haze. below the dew point and the dew point is • Wet condensation nuclei (75% relative below 0°C. humidity) reflect and scatter sunlight creating greyish or white haze. 9 9/13/2012 Fog • Saturation reached condensation forms a cloud near the ground • Radiation fog: forms when the ground cools through conduction and radiation; ground fog • Advection fog: forms when the wind moves moist air over a cold surface and the moist air cools to its dew point. • Upslope fog: forms as moist air slowly rises, cools, and condenses over elevated terrain. The high relative humidity of the cold air above the lake is causing a layer of haze to form on a still winter morning. 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. Radiation fog nestled in a valley in central Oregon. 10 9/13/2012 • Advection Fog: warm, moist fog moves Advection fog rolling in past the Golden Gate Bridge in San Francisco. As fog moves inland, the air warms and the fog lifts above the surface. horizontally (advects) over a cool surface. Eventually, the air becomes warm enough to totally evaporate the fog. – Summer fog on the Pacific coast Upslope fog forms as moist air slowly rises, cools, and condenses over elevated terrain. Even in summer, warm air rising above thermal pools in Fig. 4‐20, p. 100 Yellowstone National Park condenses into a type of steam fog. 11 9/13/2012 Average annual number of Clouds days with dense fog • Classification of clouds: use Latin words to (visibility less than 0.25 describe height and appearance miles) across North • Factors described America. – Height: low, mid, high, vertical (Dense fog observed in – Appearance: shape, density, color small mountain valleys and on mountain tops is not shown.) 12 9/13/2012 Cirrus clouds. Cirrocumulus clouds. Cirrostratus clouds with a faint halo encircling the sun. The sun is the bright white area in the center of the circle. Fig. 4‐26, p. 105 13 9/13/2012 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. The nimbostratus is the sheetlike cloud from which light rain is falling. The ragged‐appearing cloud beneath the nimbostratus is stratus fractus, or scud. Stratocumulus clouds forming along the south coast of Florida. Notice that the rounded masses are larger than A layer of low‐lying stratus clouds hides the mountains in those of the altocumulus. Iceland. 14 9/13/2012 Cumulus clouds. Small cumulus clouds such as these are Cumulus congestus. This line of cumulus congestus clouds is sometimes called fair weather cumulus, or cumulus humilis.
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