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Answer to #1
ANSWER: Atmospheric stability is a condition of equilibrium. Air is in stable equilibrium when, after being lifted or lowered, it tends to return to its original positionit resists upward and downward air motions. Air that is in unstable equilibrium will, when given a little push, move farther away from its original positionit favors vertical air currents. In addition, the atmosphere is stable when the environmental lapse rate is small; that is, when the difference in temperature between the surface air and the air aloft is relatively small. Stable atmospheres form when surface air cools or higher-altitude air warms, or both.
Answer to #2
ANSWER: If a parcel of air expands and cools, or compresses and warms, with no interchange of heat with its surroundings, this situation is called an adiabatic process. As long as the air in the parcel is unsaturated (the relative humidity is less than 100 percent), the rate of adiabatic cooling or warming remains constant. This rate of heating or cooling is about 10 degrees Celsius for every 1000 m of change in elevation (5.5 degrees Fahrenheit per 1000 ft). Because this rate applies only to unsaturated air, it is called the dry adiabatic rate. As the rising air cools, its relative humidity increases as the air temperature approaches the dew-point temperature. If the rising air cools to its dew-point temperature, the relative humidity becomes 100 percent. Further lifting results in condensation; a cloud forms, and latent heat is released inside the rising air parcel. Because the heat added during condensation offsets some of the cooling due to expansion, the air no longer cools at the dry adiabatic rate but at a lesser rate called the moist adiabatic rate. If a saturated parcel containing water droplets were to sink, it would compress and warm at the moist adiabatic rate because evaporation of the liquid droplets would offset the rate of compressional warming. Hence, the rate at which rising or sinking saturated air changes temperaturethe moist adiabatic rateis less than the dry adiabatic rate.
Answer to #3
ANSWER: Surface heating and free convection: Air in contact with the warm surface becomes warmer than its surroundings and rises. Example: cumulus cloud. Topography: Horizontally moving air forced to rise over a mountain range. Example: orographic clouds, which form on the upwind side of the range. Widespread ascent: Rising air due to convergence in low pressure systems. Example: stratocumulus and nimbostratus. Uplift along weather fronts: fronts lift the air, causing adiabatic cooling, condensation and clouds. Example: cumulonimbus clouds in the vicinity of a cold front.
Answer to #4
ANSWER: The atmosphere becomes more unstable as the environmental lapse rate steepens; that is, as the air temperature drops rapidly with increasing height. This circumstance may be brought on by either air aloft becoming colder or the surface air becoming warmer. The cooling of the air aloft may be due to winds bringing in colder air (cold advection), and clouds (or the air) emitting infrared radiation to space (radiational cooling). The warming of the surface air may be due to daytime solar heating of the surface, an influx of warm air brought in by the wind (warm advection), or air moving over a warm surface.
Answer to #5
ANSWER: The level at which clouds form is called the condensation level. Above the condensation level the rising air is saturated and cools at the moist adiabatic rate. Condensation continues to occur, and since water vapor is transforming into liquid cloud droplets, the dew point within the cloud now drops more rapidly with increasing height than before. The air remains saturated as both the air temperature and dew point decrease at the moist adiabatic rate. Inside the cloud the rising air remains warmer than the air surrounding it and continues its spontaneous rise upward. The top of the bulging cloud at 2000 m (about 6600 ft) represents the top of the rising air, which has now cooled to a temperature equal to its surroundings. The air would have a difficult time rising much above this level because of the stable subsidence inversion directly above it. The subsidence inversion, associated with the downward air motions of a high-pressure system, prevents the clouds from building very high above their bases. Hence, an afternoon sky full of flat- base cumuli with little vertical growth indicates fair weather. As we can see, the stability of the atmosphere above the condensation level plays a major role in determining the vertical growth of a cumulus cloud. Seldom do cumulonimbus clouds extend very far above the tropopause. The stratosphere is quite stable, so once a cloud penetrates the tropopause, it usually stops growing vertically and spreads horizontally. The low temperature at this altitude produces ice crystals in the upper section of the cloud. In the middle latitudes, high winds near the tropopause blow the ice crystals laterally, producing the flat anvil-shaped top so characteristic of cumulonimbus clouds.
Answer to #6
ANSWER: Stable air flowing over a mountain often moves in a series of waves that may extend for several hundred kilometers on the leeward side. These waves resemble the waves that form in a river downstream from a large boulder. As moist air rises on the upwind side of the wave, it cools and condenses, producing a cloud. On the downwind side of the wave, the air sinks and warms; the cloud evaporates. These clouds are called lenticular clouds, and because they appear motionless viewed from the ground, they are often referred to as standing wave clouds even though the air in their vicinity is not motionless.
Answer to #7
ANSWER: Air descending a mountain warms by compressional heating and, upon reaching the surface, can be much warmer than the air at the same level on the windward side, especially when condensation occurs and latent heat is released on the windward side. In addition, air on the leeward side of a mountain is normally drier (has a lower dew point) than the air on the windward side because water in the form of clouds and precipitation often remains on the windward side. The lower dew point and higher air temperature on the leeward side produce a lower relative humidity, a greater potential for evaporation of water, and a rain shadow desert. This scenario can be found in the Rocky Mountain Region (e.g. Colorado) of the United States. Colorado's Front Range tends to be both warmer and drier than areas of equivalent altitude on the Western Slope.
Answer to #8
ANSWER: On a typical day, the atmosphere is usually most stable in the early morning around sunrise, when the lowest surface air temperature is recorded. Hence, daytime heating makes the atmosphere more unstable.
Answer to #9
ANSWER: An inversion represents an atmospheric condition where the air becomes warmer with height. Inversions that form as air slowly sinks over a large area are called subsidence inversions. They sometimes occur at the surface, but more frequently, they are observed aloft and are often associated with large high-pressure areas because of the sinking air motions associated with these systems.
An inversion represents an atmosphere that is absolutely stable. The reason for this is that within the inversion, warm air overlies cold air, and, if air rises into the inversion, it is becoming colder, while the air around it is getting warmer. Obviously, the colder air would tend to sink. Inversions, therefore, act as lids on vertical air motion. When an inversion exists near the ground, stratus clouds, fog, haze, and pollutants are all kept close to the surface.
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