Geology 1347 Meteorology Exam 2 Review 1. The scattering of light by small particles in the atmosphere causes many familiar effects such as white clouds, blue skies, hazy skies, crepuscular rays, and colorful sunrises and sunsets. In the absence of any scattering, one would observe a white sun against a black sky. 2. In order to produce a rainbow sunlight must be refracted (bent) as it enters the raindrop, reflected off the back interior surface of the raindrop and finally dispersed as the light exits the raindrop. 3. If the sun is near the horizon in such a configuration that the sun, ice crystals, and observer are all in the same horizontal plane, the observer will see a pair of brightly colored spots, one on either side of sun. These colored spots are called sundogs. Sundogs are caused by the refraction of sunlight through the ice crystals. As light passes through these ice crystals, red light is bent the least and is, therefore observed on the inside of sundogs. 4. Scattering is the process by which small particles in the atmosphere deflect radiation (light) from its path into different directions. This scattering can limit visibility while driving at night in thick fog with the high-beam light on. The light is scattered back to the driver s eyes from the fog droplets making it difficult to see.
5. Definitions. a. corona a series of colored rings concentrically surrounding he disk of the sun or moon. Smaller than the halo, the corona is often caused by the diffraction of light around small water droplets of uniform size. b. halo ring or arc that encircles the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. The halo is produced by refraction of light. c. sundog a colored luminous spot produced by refraction of light through ice crystals that appears on either side of the sun. d. rainbow an arc of concentric colored bands that spans a section of the sky when rain is present and the sun is positioned at the observer s back. e. sun pillar a vertical streak of light extending above (or below) the sun. It is produced by the reflection of sunlight off ice crystals.
Looking toward the sun one would be able to observe a corona, halo, sundogs, and a sun pillar. 6. Because of the selective scattering of incoming solar radiation by a thick section of the atmosphere, the sun at sunrise and sunset appears yellow, orange, or red in color. The more particles in the atmosphere, the more scattering of sunlight, and the redder the sun appears. Hence, the shorter wavelengths of light are scattered by the atmosphere s molecules and particles and the longer wavelengths of light are being seen by an observer. 7. Rainbows occur when rain is falling in one part of the sky, and the sun is shining in another. To view a rainbow, one must face the falling rain with the sun to the back of the observer. To see a rainbow in the evening one must face east toward the rain. Since weather patterns normally move from west to east in the middle latitudes, clear skies in the west suggest that the showers will end and clear skies will prevail. This observation is noted in the following weather rhyme: Rainbow in morning, sailors take warning, Rainbow at night, sailor s delight. 8. The blue haze over the Blue Ridge Mountains in Virginia is callused by the scattering of blue light by extremely small particles smaller than the wavelengths of visible light.
9. Saturation vapor pressure increases with increasing temperature. 10. Definitions a. condensation process by which water vapor becomes a liquid b. evaporation process by which a liquid changes to a gas c. deposition a process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first d. sublimation the process whereby ice changes directly into water vapor without melting 11. The dew-point temperature or dew point represents the temperature to which air would have to be cooled (no change in air pressure or moisture content) for saturation to occur. Since atmospheric pressure varies only slightly at the earth s surface, the dew point is a good indicator of the air s actual water vapor content. 12. High dew points indicate high water vapor content; low dew points indicated low water vapor content. Addition of water vapor to the air increases the dew point, whereas removing water vapor from the air lowers the dew point. 13. In the summertime air over southern California is less humid than air over the Gulf coast because the Pacific water is much cooler than the Gulf water. 14. Polar air is often described as being dry when the relative humidity is high (often close to 100%). In cold, polar air, the dew point and air temperature are normally close together. The low dew-point temperature means that there is little water vapor in the air. Hence, the air is said to be dry even though the relative humidity is quite high.
15. Condensation is more likely to occur as air is cooled. As the air temperature increases, condensation is less likely. As air rises an air parcel expands and cools, increases the chance for condensation. 16. Definitions: a. nacreous clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km (middle of the stratosphere) above the earth s surface. Also called mother-of-pearl clouds. b. pileus a smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud. Also referred to as a cap cloud. c. castillanus little castles refers to altocumulus clouds. These are middle clouds that are composed mostly of water droplets and are rarely more than 1 km thick. d. scud ragged-appearing clouds that form beneath nimbostratus. Also called stratus fractus. e. mammatus clouds that look like pouches hanging from t he underside of a cloud.
17. In order for condensation to occur naturally, water vapor needs a surface on which to form. In air these surfaces are called condensation nuclei. Condensation nuclei are small, but allow the water vapor to condense. Without condensation nuclei relative humidities of several hundred percent would be required. 18. Definitions a. radiation fog fog produced over land when radiational cooling reduces the air temperature to or below its dew point. Also known as ground fog or valley fog b. evaporating (mixing) fog fog produced when sufficient water vapor is added to the air by evaporation, and the most air mixes with relatively drier air. The two common types are steam fog, which forms when cold air moves over warm water, and frontal fog, which forms as warm raindrops evaporate in a cool air mass. c. upslope fog fog formed as most, stable air flows upward over a topographic barrier d. advection fog fog produced when warm, moist air moves over a cold surface and the air cools to below its dew point
19. The terms little castles in the sky refer to castellanus or altocumulus clouds. 20. Cold fog can usually be dispersed by seeding the fog with dry ice. 21. Advection fog is formed by cooling surface air to its saturation point. Radiation fog is formed by cooling nighttime air near the ground. Upslope fog forms as most air slowly rises, cools, and condenses over elevated terrain. Evaporation of mixing fog forms when cold air moves over warm water. 22. Definitions a. altostratus a middle cloud composed of gray or bluish sheets or layers of uniform appearance. In the thinner regions, the sum or moon usually appears dimly visible. b. cirrostratus high, thin, sheetlike clouds, composed of ice crystals. Cirrostratus clouds frequently cover the entire sky of often produce a halo. These clouds are so thin that the sun and moon can clearly seen through them. c. stratus a low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle. d. nimbostratus a dark, gray, cloud usually characterized by continuously falling precipitation. e. stratocumulus a low cloud, predominantly stratiform, with low, lumpy, rounded masses, often with blue sky between them. 23. The highest clouds in the earth s atmosphere are called noctilucent clouds. These clouds are composed of tiny ice crystals and may be seen in the upper mesosphere, at altitudes above 75 km (46 miles).
24. 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. 25. Fog produced by the earth s radiational cooling is called radiation fog, or ground fog. Radiation fog can form in low-lying areas. Hence, radiation fog is also called valley fog. Radiation fog is the type of fog that often occurs in the central valley of California. 26. There are two primary types of weather satellites in use for viewing clouds. a. geostationary satellites orbit the equator at the same rate the earth spins, hence, remain above a fixed spot on the earth s surface. These satellites are at an altitude of nearly 36,00 km (22,300 miles) above the earth s surface. b. polar-orbiting satellites pass over the north and south polar regions on each revolution. These satellites scan from north to south, and on each successive orbit the satellite scans an area farther to the west.
27. Definitions a. nimbostratus a dark, gray cloud characterized by more or less continuously falling precipitation. Rarely accompanied by lightning, thunder, or hail b. cumulus congestus a cumulus type cloud that exhibit extensive vertical growth. Also called towering cumulus. These clouds may form into cumulonimbus given suitable atmosphere conditions. c. cumulonimbus a dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail. It is also known as a thunderstorm cloud. d. stratus a low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle. e. cirrocumulus a high cloud that appears as a white patch of clouds without shadows. It consists of small elements in the form of grains or ripples. The small ripples in the cirrocumulus resemble the scales of a fish. Hence, a sky covered with cirrocumulus clouds is called a mackerel sky.
28. Definitions a. pileus a smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud. Also called a cap cloud. b. mammatus clouds that look like pouches hanging from the underside of a cloud. Form in descending air. c. castellanus little castles. Altocumulus clouds that indicate the presence of rising air at the cloud level. d. stratocumulus a low cloud, predominantly stratiform, with low, lumpy, rounded masses, often with blue sky between them. Form in rising air. e. cumulus humilis cumulus clouds that show only slight vertical growth. Also called fair weather cumulus f. cumulus fractus ragged-edge cumulus clouds that are smaller that cumulus humilis and scattered across the sky.
29. Polar-orbiting satellites scan from north to south along earth s longitude lines. On each successive orbit the satellite scans are area farther to the west because of the earth s rotation. 30. Dew and frost need a surface on which to form. Likewise, water vapor must have airborne particles on which condensation can occur in order to produce cloud droplets. The surfaces on which water vapor can condense are called condensation nuclei. Without these condensation nuclei, relative humidities of several hundred percent would be required before condensation could begin. 31. The rate at which rise air cools depends if the rising air parcel is saturated or unsaturated. The rate of change of temperature in a rising or descending unsaturated air parcel is called the dry adiabatic rate. The rate of adiabatic cooling or warming is about 10 C per 1000 m (5.5 F per 1000 ft). The rate of change of temperature in a rising or descending saturated air parcel is called the moist adiabatic rate. The rate of cooling or warming is about 6 C per 1000 m (3.3 F per 1000 ft). The difference in the two rates is due to the latent heat of condensation being released in a rising parcel of saturated air. 32. The atmosphere is always absolutely stable when the environmental lapse rate is less than the moist adiabatic rate. 33. Absolute instability results when the environmental lapse rate is greater than the dry adiabatic rate. 34. A layer of air may become unstable by either mixing or lifting. Occasionally, the mixing of a moist layer of stable air will produce a deck of stratocumulus clouds.
35. Billow clouds can form is a region of rapidly changing wind speed (wind shear). These clouds look like waves breaking onshore. 36. Maximum atmospheric instability occurs during the hottest part of the day, usually in the afternoon. 37. The atmosphere becomes more unstable when the environmental lapse rate steepens. Atmospheric instability can be caused by the air aloft becoming colder or the surface air becoming warmer. 38. The atmosphere becomes more stable when the environmental lapse rate is small. This is a result of the air aloft becoming warmer or the surface air becoming cooler. 39. Most clouds form as air rises, cools, and condenses. The majority of clouds are formed by: a. surface heating and free convection b. uplift along topography c. widespread ascent due to convergence of surface air d. uplift along weather fronts. 40. A temperature inversion or inversion represents an increase in air temperature with height. An inversion represents an atmosphere that is absolutely stable. Within an inversion, warm air overlies cold air. As air rises into an inversion, the air parcel becomes colder, while the air around the parcel gets warmer. 41. Inversions that form as air slowly sinks over a large area are called subsidence inversions. These inversions usually occur aloft and are associated with large high-pressure areas because of the sinking air motions associated with these systems.
42. The moist adiabatic rate is different from the dry adiabatic rate because latent heat is released inside a parcel of rising saturated air. 43. Definitions a. sleet a type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Same as ice pellets. b. freezing rain and freezing drizzle rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Both can produce a coating of ice on objects which is called glaze. c. snow grains precipitation in the form of small, opaque grains of ice. The solid equivalent of drizzle. d. snow pellets white, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion. Unlike snow grains, snow pellets are brittle, crunch, and bounce (or break apart) upon hitting a hard surface. Usually fall as showers. 44. Definitions a. solute effect the dissolving of hygroscopic particles, such as salt, in pure water, thus reducing the relative humidity required for the onset of condensation. As a result of the solute effect, a droplet containing salt can be in equilibrium with its environment when the atmospheric relative humidity is lower than 100%. Should the relative humidity of the air increase, the droplet would grow larger in size. b. curvature effect as cloud droplets decrease in size, the droplets exhibit a greater surface curvature that causes an increased rate of evaporation. c. hydrophobic the ability to resist the condensation of water vapor. Usually used to describe water-repelling condensation nuclei. d. adiabatic process a process that takes place without a transfer of heat between the system (such as an air parcel) and its surroundings. In an adiabatic process, compressions always results in warming, and expansion results in cooling.
45. The ice-crystal (Bergeron) process. 1. The greater number of water vapor molecules around the liquid droplet causes water molecules to diffuse from the liquid droplet toward the ice crystal. 2. The ice crystal absorbs the water vapor and grows larger. 3. As a result the water droplet grows smaller. 46. A warm stratus cloud is typically less than 500 m thick and has slow upward air movement. If the air beneath the cloud is moist, the droplets may reach the ground as drizzle, the lightest form of rain. 47. An excessively heavy shower is termed a cloudburst. Beneath a cumulonimbus cloud, which normally contains large convection currents of rising and descending air, it is possible that one side of a street may be dry (updraft side), while a heavy shower is occurring across the street (downdraft side). 48. In the collision-coalescence process the most important factor in the production of raindrops is the cloud s liquid water content. In a cloud with sufficient water, other significant factors are: a. the range of droplet sizes b. the cloud thickness c. the updrafts of the cloud d. the electric charge of the droplets and the electric field in the cloud. 49. As rain falls, the drops take on a characteristic shape. Large raindrops, with diameters exceeding 2 mm, become slightly elongated, flattened on the bottom, and rounded on the top.
50. Natural cloud seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain. The cirrus clouds are above the altostratus. 51. In some clouds, especially those with relatively warm tops, ice crystals might collide with supercooled droplets. Upon contact, the liquid droplets freeze into ice and stick together. This process of ice crystals growing larger as they collide with suppercooled cloud droplets is called accretion. The icy matter that forms is called graupel or snow pellets. 52. Hail is produced in cumulonimbus clouds when graupel, or large frozen raindrops act as emryos that grow by accumulating supercooled liquid droplets accretion. 53. Definitions a. coalescence the merging of cloud droplets into a single larger droplet. b. riming the growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact accretion. c. homogeneous (spontaneous) freezing the freezing of pure water. For tiny cloud droplets, homogeneous freezing does not occur until the air temperature reaches about -40 C. d. contact freezing the process by which contact with a nucleus such as an ice crystal causes supercooled liquid droplets to change into ice.