Every physical process of weather is accompanied by, or is the 
result of, a 





   ANSWER: heat exchange. 

   Every physical process of weather is 
   accompanied by, or is the result of, a heat exchange. A heat 
   differential (difference between the temperatures of two air 
   masses) causes a differential in pressure, which in turn 
   causes movement of air. Heat exchanges occur constantly, 
   e.g., melting, cooling, updrafts, downdrafts, wind, etc. 

What causes variations in altimeter settings between weather 
reporting points? 





   ANSWER: Unequal heating of the Earth's surface. 

   Unequal heating of the Earth's 
   surface causes differences in air pressure, which is reflected 
   in differences in altimeter settings between weather 
   reporting points. 

The most frequent type of ground or surface-based temperature 
inversion is that which is produced by 





   ANSWER: terrestrial radiation on a clear, relatively still night. 

   An inversion often develops near the 
   ground on clear, cool nights when wind is light. The ground 
   loses heat and cools the air near the ground while the 
   temperature a few hundred feet above changes very little. 
   Thus, temperature increases in height, which is an inversion. 

A temperature inversion would most likely result in which weather 
condition? 





   ANSWER: An increase in temperature as altitude is increased. 

   By definition, a temperature inversion 
   is a situation in which the temperature increases as altitude 
   increases. The normal situation is that the temperature 
   decreases as altitude increases. 

Which weather conditions should be expected beneath a low-level 
temperature inversion layer when the relative humidity is high? 





   ANSWER: Smooth air, poor visibility, fog, haze, or low clouds. 

   Beneath temperature inversions, there 
   is usually smooth air because there is little vertical 
   movement due to the inversion. There is also poor visibility 
   due to fog, haze, and low clouds (when there is high relative 
   humidity). 

Under what condition is pressure altitude and density altitude the 
same value? 





   ANSWER: At standard temperature. 

   Pressure altitude and density altitude 
   are the same when temperature is standard. 

Under which condition will pressure altitude be equal to true 
altitude? 





   ANSWER: When standard atmospheric conditions exist. 

   Pressure altitude equals true altitude 
   when standard atmospheric conditions (29.92" Hg and 15°C 
   at sea level) exist. 

If a pilot changes the altimeter setting from 30.11 to 29.96, what is 
the approximate change in indication? 





   ANSWER: Altimeter will indicate 150 feet lower. 

   Atmospheric pressure decreases 
   approximately 1" of mercury for every 1,000 ft. of altitude 
   gained. As an altimeter setting is changed, the change in 
   altitude indication changes the same way (i.e., approximately 
   1,000 ft. for every 1" change in altimeter setting) and in the 
   same direction (i.e., lowering the altimeter setting lowers the 
   altitude reading). Thus, changing from 30.11 to 29.96 is a 
   decrease of .15 in., or 150 ft. (.15 x 1,000 ft.) lower. 

If a flight is made from an area of low pressure into an area of high 
pressure without the altimeter setting being adjusted, the altimeter 
will indicate 





   ANSWER: lower than the actual altitude above sea level. 

   When an altimeter setting is at a lower 
   value than the correct setting, the altimeter is indicating less 
   than it should and thus would be showing lower than the 
   actual altitude above sea level. 

If a flight is made from an area of high pressure into an area of 
lower pressure without the altimeter setting being adjusted, the 
altimeter will indicate 





   ANSWER: higher than the actual altitude above sea level. 

   When flying from higher pressure to 
   lower pressure without adjusting your altimeter, the altimeter 
   will indicate a higher than actual altitude. As you adjust an 
   altimeter barometric setting lower, the altimeter indicates 
   lower. 

Which condition would cause the altimeter to indicate a lower 
altitude than true altitude? 





   ANSWER: Air temperature warmer than standard. 

   In air that is warmer than standard 
   temperature, the airplane will be higher than the altimeter 
   indicates. Said another way, the altimeter will indicate a 
   lower altitude than actually flown. 

Under what condition will true altitude be lower than indicated 
altitude? 





   ANSWER: In colder than standard air temperature. 

   The airplane will be lower than the 
   altimeter indicates when flying in air that is colder than 
   standard temperature. Remember that altimeter readings are 
   adjusted for changes in barometric pressure but not for 
   changes in temperature. When one flies from warmer to cold 
   air and keeps a constant indicated altitude at a constant 
   altimeter setting, the plane has actually descended. 

Which factor would tend to increase the density altitude at a given 
airport? 





   ANSWER: An increase in ambient temperature. 

   When air temperature increases, 
   density altitude increases because, at a higher temperature, 
   the air is less dense. 

The wind at 5,000 feet AGL is southwesterly while the surface wind 
is southerly. This difference in direction is primarily due to 





   ANSWER: friction between the wind and the surface. 

   Winds aloft at 5,000 ft. are largely 
   affected by Coriolis force, which deflects wind to the right, 
   in the Northern Hemisphere. But at the surface, the winds 
   will be more southerly (they were southwesterly aloft) 
   because Coriolis force has less effect at the surface where 
   the wind speed is slower. The wind speed is slower at the 
   surface due to the friction between the wind and the surface. 

The presence of ice pellets at the surface is evidence that there 





   ANSWER: is a temperature inversion with freezing rain at a higher 
   altitude. 

   Rain falling through colder air may 
   freeze during its descent, falling as ice pellets. Ice pellets 
   always indicate freezing rain at a higher altitude. 

What is meant by the term "dewpoint"? 





   ANSWER: The temperature to which air must be cooled to become 
   saturated. 

   Dew point is the temperature to which 
   air must be cooled to become saturated, or have 100% 
   humidity. 

The amount of water vapor which air can hold depends on the 





   ANSWER: air temperature. 

   Air temperature largely determines 
   how much water vapor can be held by the air. Warm air can 
   hold more water vapor than cool air. 

What are the processes by which moisture is added to unsaturated air? 





   ANSWER: Evaporation and sublimation. 

   Evaporation is the process of 
   converting a liquid to water vapor, and sublimation is the 
   process of converting ice to water vapor. 

Which conditions result in the formation of frost? 





   ANSWER: The temperature of the collecting surface is at or below 
   the dewpoint of the adjacent air and the dewpoint is below 
   freezing. 

   Frost forms when both the collecting 
   surface is below the dew point of the adjacent air AND the 
   dew point is below freezing. Frost is the direct sublimation of 
   water vapor to ice crystals. 

Clouds, fog, or dew will always form when 





   ANSWER: water vapor condenses. 

   As water vapor condenses, it 
   becomes visible as clouds, fog, or dew. 

At approximately what altitude above the surface would the pilot 
expect the base of cumuliform clouds if the surface air temperature 
is 82°F and the dewpoint is 38°F? 





   ANSWER: 10,000 feet AGL. 

   The height of cumuliform cloud bases 
   can be estimated using surface temperature/dew point 
   spread. Unsaturated air in a convective current cools at 
   about 5.4°F/1,000 ft., and dew point decreases about 
   1°F/1,000 ft. In a convective current, temperature and dew 
   point converge at about 4.4°F/1,000 ft. Thus, if the 
   temperature/dew point spread is 44° (82° - 38°), divide 44 by 
   4.4 to obtain 10,000 ft. AGL. 

What is the approximate base of the cumulus clouds if the surface 
air temperature at 1,000 feet MSL is 70°F and the dewpoint is 48°F? 





   ANSWER: 6,000 feet MSL. 

   The height of cumuliform cloud bases 
   can be estimated using surface temperature/dew point 
   spread. Unsaturated air in a convective current cools at 
   about 5.4°F/1,000 ft., and dew point decreases about 
   1°F/1,000 ft. In a convective current, temperature and dew 
   point converge at about 4.4°F/1,000 ft. Thus, if the 
   temperature and dew point are 70°F and 48°F, respectively, 
   at 1,000 ft. MSL, there would be a 22° spread which, divided 
   by the lapse rate of 4.4, is approximately 5,000 ft. AGL, or 
   6,000 ft. MSL (5,000 + 1,000). 

What is a characteristic of stable air? 





   ANSWER: Stratiform clouds. 

   Characteristics of a stable air mass 
   include stratiform clouds, continuous precipitation, smooth 
   air, and fair to poor visibility in haze and smoke. 

Moist, stable air flowing upslope can be expected to 





   ANSWER: produce stratus type clouds. 

   Moist, stable air flowing upslope can 
   be expected to produce stratus type clouds as the air cools 
   adiabatically as it moves up sloping terrain. 

If an unstable air mass is forced upward, what type clouds can be 
expected? 





   ANSWER: Clouds with considerable vertical development and 
   associated turbulence. 

   When unstable air is lifted, it usually 
   results in considerable vertical development and associated 
   turbulence, i.e., convective activity. 

What are characteristics of unstable air? 





   ANSWER: Turbulence and good surface visibility. 

   Characteristics of an unstable air 
   mass include cumuliform clouds, showery precipitation, 
   turbulence, and good visibility, except in blowing 
   obstructions. 

A stable air mass is most likely to have which characteristic? 





   ANSWER: Smooth air. 

   Characteristics of a stable air mass 
   include stratiform clouds and fog, continuous precipitation, 
   smooth air, and fair to poor visibility in haze and smoke. 

What are characteristics of a moist, unstable air mass? 





   ANSWER: Cumuliform clouds and showery precipitation. 

   Characteristics of an unstable air 
   mass include cumuliform clouds, showery precipitation, 
   turbulence, and good visibility, except in blowing 
   obstructions. 

What measurement can be used to determine the stability of the 
atmosphere? 





   ANSWER: Actual lapse rate. 

   The stability of the atmosphere is 
   determined by vertical movements of air. Warm air rises 
   when the air above is cooler. The actual lapse rate, which is 
   the decrease of temperature with altitude, is therefore a 
   measure of stability. 

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