What would decrease the stability of an air mass? 





   ANSWER: Warming from below. 

   When air is warmed from below, even 
   though cooling adiabatically, it remains warmer than the 
   surrounding air. The colder, more dense surrounding air 
   forces the warmer air upward and an unstable condition 
   develops. 

What feature is associated with a temperature inversion? 





   ANSWER: A stable layer of air. 

   A temperature inversion is associated 
   with an increase in temperature with height, a reversal of 
   normal decrease in temperature with height. Thus, any warm 
   air rises to where it is the same temperature and forms a 
   stable layer of air. 

An almond or lens-shaped cloud which appears stationary, but 
which may contain winds of 50 knots or more, is referred to as 





   ANSWER: a lenticular cloud. 

   Lenticular clouds are lens-shaped 
   clouds which indicate the crests of standing mountain 
   waves. They form in the updraft and dissipate in the 
   downdraft, so they do not move as the wind blows through 
   them. Lenticular clouds may contain winds of 50 kt. or more 
   and are extremely dangerous. 

Crests of standing mountain waves may be marked by stationary, 
lens-shaped clouds known as 





   ANSWER: standing lenticular clouds. 

   Lens-shaped clouds, which indicate 
   crests of standing mountain waves, are called standing 
   lenticular clouds. They form in the updraft and dissipate in 
   the downdraft so that they do not move as the wind blows 
   through them. 

Clouds are divided into four families according to their 





   ANSWER: height range. 

   The four families of clouds are high 
   clouds, middle clouds, low clouds, and clouds with 
   extensive vertical development. Thus, they are based upon 
   their height range. 

The suffix "nimbus," used in naming clouds, means 





   ANSWER: a rain cloud. 

   The suffix nimbus or the prefix nimbo 
   means a rain cloud. 

What clouds have the greatest turbulence? 





   ANSWER: Cumulonimbus. 

   The greatest turbulence occurs in 
   cumulonimbus clouds, which are thunderstorm clouds. 

What cloud types would indicate convective turbulence? 





   ANSWER: Towering cumulus clouds. 

   Towering cumulus clouds are an early 
   stage of cumulonimbus clouds, or thunderstorms, which are 
   based on convective turbulence, i.e., an unstable lapse rate. 

The boundary between two different air masses is referred to as a 





   ANSWER: front. 

   A front is a surface, interface, or 
   transition zone of discontinuity between two adjacent air 
   masses of different densities. It is the boundary between 
   two different air masses. 

One weather phenomenon which will always occur when flying 
across a front is a change in the 





   ANSWER: wind direction. 

   The definition of a front is the zone of 
   transition between two air masses of different air pressure or 
   density, e.g., the area separating high and low pressure 
   systems. Due to the difference in changes in pressure 
   systems, there will be a change in wind. 

One of the most easily recognized discontinuities across a front is 





   ANSWER: a change in temperature. 

   Of the many changes which take 
   place across a front the most easily recognized is the change 
   in temperature. When flying through a front you will notice a 
   significant change in temperature, especially at low altitudes. 

Steady precipitation preceding a front is an indication of 





   ANSWER: stratiform clouds with little or no turbulence. 

   Steady precipitation preceding a front 
   is usually an indication of a warm front, which results from 
   warm air being cooled from the bottom by colder air. This 
   results in stratiform clouds with little or no turbulence. 

One in-flight condition necessary for structural icing to form is 





   ANSWER: visible moisture. 

   Two conditions are necessary for 
   structural icing while in flight. First, the airplane must be 
   flying through visible moisture, such as rain or cloud 
   droplets. Second, the temperature at the point where the 
   moisture strikes the airplane must be freezing or below. 

Possible mountain wave turbulence could be anticipated when 
winds of 40 knots or greater blow 





   ANSWER: across a mountain ridge, and the air is stable. 

   Always anticipate possible mountain 
   wave turbulence when the air is stable and winds of 40 kt. or 
   greater blow across a mountain or ridge. 

Where does wind shear occur? 





   ANSWER: At all altitudes, in all directions. 

   Wind shear is the eddies in between 
   two wind currents of differing velocities, direction, or both. 
   Wind shear may be associated with either a wind shift or a 
   wind speed gradient at any level in the atmosphere. 

A pilot can expect a wind-shear zone in a temperature inversion 
whenever the windspeed at 2,000 to 4,000 feet above the surface is 
at least 





   ANSWER: 25 knots. 

   When taking off or landing in calm 
   wind under clear skies within a few hours before or after 
   sunset, prepare for a temperature inversion near the ground. 
   You can be relatively certain of a shear zone in the inversion 
   if you know the wind is 25 kt. or more at 2,000 to 4,000 ft. 
   Allow a margin of airspeed above normal climb or approach 
   speed to alleviate the danger of stall in the event of 
   turbulence or sudden change in wind velocity. 

When may hazardous wind shear be expected? 





   ANSWER: In areas of low-level temperature inversion, frontal zones, 
   and clear air turbulence. 

   Wind shear is the abrupt rate of 
   change of wind velocity (direction and/or speed) per unit of 
   distance and is normally expressed as vertical or horizontal 
   wind shear. Hazardous wind shear may be expected in areas 
   of low-level temperature inversion, frontal zones, and clear 
   air turbulence. 

Why is frost considered hazardous to flight? 





   ANSWER: Frost spoils the smooth flow of air over the wings, 
   thereby decreasing lifting capability. 

   Frost does not change the basic 
   aerodynamic shape of the wing, but the roughness of its 
   surface spoils the smooth flow of air, thus causing an 
   increase in drag and an early airflow separation over the 
   wing, resulting in a loss of lift. 

How does frost affect the lifting surfaces of an airplane on takeoff? 





   ANSWER: Frost may prevent the airplane from becoming airborne at 
   normal takeoff speed. 

   Frost that is not removed from the 
   surface of an airplane prior to takeoff may make it difficult to 
   get the airplane airborne at normal takeoff speed. The frost 
   disrupts the airflow over the wing, which increases drag. 

In which environment is aircraft structural ice most likely to have 
the highest accumulation rate? 





   ANSWER: Freezing rain. 

   Freezing rain usually causes the 
   highest accumulation rate of structural icing because of the 
   nature of the supercooled water striking the airplane. 

If there is thunderstorm activity in the vicinity of an airport at 
which you plan to land, which hazardous atmospheric 
phenomenon might be expected on the landing approach? 





   ANSWER: Wind-shear turbulence. 

   The most hazardous atmospheric 
   phenomenon near thunderstorms is wind shear turbulence. 

A nonfrontal, narrow band of active thunderstorms that often 
develop ahead of a cold front is known as a 





   ANSWER: squall line. 

   A nonfrontal, narrow band of active 
   thunderstorms that often develop ahead of a cold front is 
   known as a squall line. 

What conditions are necessary for the formation of 
thunderstorms? 





   ANSWER: High humidity, lifting force, and unstable conditions. 

   Thunderstorms form when there is 
   sufficient water vapor, an unstable lapse rate, and an initial 
   upward boost (lifting) to start the storm process. 

During the life cycle of a thunderstorm, which stage is 
characterized predominately by downdrafts? 





   ANSWER: Dissipating. 

   Thunderstorms have three life cycles: 
   cumulus, mature, and dissipating. It is in the dissipating 
   stage that the storm is characterized by downdrafts as the 
   storm rains itself out. 

Thunderstorms reach their greatest intensity during the 





   ANSWER: mature stage. 

   Thunderstorms reach their greatest 
   intensity during the mature stage, where updrafts and 
   downdrafts cause a high level of wind shear. 

What feature is normally associated with the cumulus stage of a 
thunderstorm? 





   ANSWER: Continuous updraft. 

   The cumulus stage of a thunderstorm 
   has continuous updrafts which build the storm. The water 
   droplets are carried up until they become too heavy. Once 
   they begin falling and creating downdrafts, the storm 
   changes from the cumulus to the mature stage. 

Which weather phenomenon signals the beginning of the mature 
stage of a thunderstorm? 





   ANSWER: Precipitation beginning to fall. 

   The mature stage of a thunderstorm 
   begins when rain begins falling. This means that the 
   downdrafts are occurring sufficiently to carry water all the 
   way through the thunderstorm. 

Thunderstorms which generally produce the most intense hazard 
to aircraft are 





   ANSWER: squall line thunderstorms. 

   A squall line is a nonfrontal narrow 
   band of active thunderstorms. It often contains severe, 
   steady-state thunderstorms and presents the single most 
   intense weather hazard to airplanes. 

The conditions necessary for the formation of cumulonimbus 
clouds are a lifting action and 





   ANSWER: unstable, moist air. 

   Unstable moist air in addition to a 
   lifting action, i.e., convective activity, are needed to form 
   cumulonimbus clouds. 

Upon encountering severe turbulence, which flight condition 
should the pilot attempt to maintain? 





   ANSWER: Level flight attitude. 

   Attempting to hold altitude and 
   airspeed in severe turbulence can lead to overstressing the 
   airplane. Rather, you should set power to what normally will 
   maintain VA, and simply attempt to maintain a level flight 
   attitude. 

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