Weather Ch. 2

Question 1

DEFINE relative humidity

Answer 1

• the percent of saturation of the air
• Abrev. RH
• The air can become saturated (RH = 100%) by one of two ways. If the air is cooled, the falling air temperature decreases the dew point spread closer to zero, while the RH rises closer to 100%. If evaporation occurs, this adds moisture to the atmosphere, increasing the dew point, which again lowers the dew point spread and increases the RH. Once the dew point spread reaches 4° F, the RH will be 90%, and the water vapor will begin to condense into fog or clouds. Any further cooling or evaporation will produce precipitation, as there will be more water present in the air than it can hold.

Question 2

Two principal cloud forms

Answer 2

1. Cumuliform – A lumpy, billowy cloud with a base showing a definite pattern or structure.
2. Stratiform – A cloud with a uniform base, formed in horizontal, sheet-like layers.

Question 3

DESCRIBE the types of atmospheric stability

Answer 3

• stable: object displaced but returns to original postion
• neutral: object displaced and tends to remain in new postion
• unstable: object displaced and never returns to original position

Question 4

EXPLAIN and identify gradient winds and Buys Ballot’s Law with respect to the isobars around pressure systems in the Northern Hemisphere

Answer 4

• THE RESULT OF THESE TWO FORCES FORM GRADIENT WINDS:
  
  • pressure gradient force causes air to flow from high pressure to low pressure across the isobar pattern
  • The Coriolis force, created by the Earth’s rotation, diverts the air to the right–with respect to its initial direction of motion–regardless of whether the air is near a high or a low pressure system
• Gradient winds flow perpindicular to the pressure gradient force, parallel to isobars
• Gradient winds found above 2,000 ft AGL
• Highs gradient winds flow clockwise, lows are counter-clockwise
• Buy Ballot’s Law: if the wind is at your back, the area of lower pressure will be to your left. When standing on the Earth’s surface, the low will be slightly forward or directly left because the winds flow across the isobars.

Question 5

DESCRIBE the weather conditions associated with Middle clouds

Answer 5

• composed of ice crystals, water droplets, or a mixture of the two
• Visibility varies depending on cloud density from ½ mile to a few feet
• Turbulence may be encountered
• clouds are dark and turbulent enough to make formation flying difficult. Icing is common due to the presence of super-cooled water droplets.
• Rain, rain and snow mixed, or snow can be encountered in thick middle clouds
• Virga, which is rain or snow that evaporates before reaching the ground, may be encountered below these clouds

Question 6

CHART WITH CLOUD GROUP WEATHER

Answer 6

Question 7

DESCRIBE the relationship between air temperature and dew point temperature with respect to saturation

Answer 7

• This spread provides a good indication of how close the atmosphere is to the point of saturation. When the dew point depression reaches about 4° F, the air is holding close to the maximum amount of water vapor possible. If this spread continues to decrease, moisture will begin to condense from the vapor state to the liquid (or solid) state, and become visible. This visible moisture can form dew or frost on exposed surfaces, fog near the ground, or clouds higher in the atmosphere.
• Beyond 100% saturation, precipitation will result

Question 8

DESCRIBE sea breezes

Answer 8

• During the day, the pressure over the warm land becomes lower than that over the colder water. The cool air over the water moves toward the lower pressure, replacing the warm air over the land that moved upward
• speeds sometimes reaching 15 to 20 knots
• shallow winds
• should be considered when taking off or landing near water

Question 9

EXPLAIN the term pressure gradient

Answer 9

• The rate of pressure change in a direction perpendicular to the isobars (horizontal distance)
  
  • isobar spacing represents the size of the pressure gradient force (PGF). The PGF is steep, or strong, when isobars are close together, and is shallow, or weak, when the isobars are far apart–the steeper the gradient, the stronger the winds.

Question 10

DESCRIBE land breezes

Answer 10

• At night, the circulation is reversed so that the air movement is from land to sea
• not as strong as sea breezes
• shallow wind, should be considered for takeoff and landing near water

Question 11

DESCRIBE valley winds

Answer 11

• mountain slopes are heated by the Sun’s radiation, and in turn, they heat the adjacent air through conduction
• air usually becomes warmer than air farther away from the slope at the same altitude, and, since warmer air is less dense, it begins to rise
• cools while moving away from the warm ground, increasing its density. It then settles downward, towards the valley floor, completing a pattern of circulation
• important to small aircraft

Question 12

DESCRIBE the jet stream

Answer 12

• narrow band of strong winds of 50 knots or more that meanders vertically and horizontally around the hemisphere in wave-like patterns
• jet streams (polar and subtropical) have a profound influence on weather patterns
• average about 100-150 knots but may reach speeds in excess of 250 knots
• rarely encircles the entire hemisphere as a continuous river of wind. More frequently, it is found in segments from 1000 to 3000 miles in length, 100 to 400 miles in width, and 3000 to 7000 feet in depth
• The average height of jet stream winds is about 30,000 feet MSL
  
  • During the winter, the position of the jet stream is further south, the core descends to lower altitudes, and its speed is faster than in the summer
• possible wind shear

Question 13

DEFINE dew point depression

Answer 13

• difference between the air temperature and the dew point temperature
• AKA Dew Point Spread
• Dew point can never be higher than air temp
• When the spread is within 4 degrees F, clouds and fog will form

Question 14

Describe the weather conditions associated with Special Clouds

Answer 14

• nimbostratus, produces continuous rain, snow, or ice pellets
• fog is often present
• poor visibility and low ceilings with very slow clearing
• Towering cumulus are clouds nearing the thunderstorm stage. They can produce heavy rain showers and moderate turbulence in and near the cloud. Icing is common above the freezing level
• Cumulonimbus clouds are thunderstorm clouds
  
  • exceedingly dangerous cloud, with numerous hazards to flight such as severe to extreme turbulence, hail, icing, lightning

Question 15

DEFINE saturation

Answer 15

• air reaches saturation when it contains the maximum amount of water vapor it can hold for that temperature
• higher the temperature, the more water vapor the air can hold

Question 16

EXPLAIN and identify the surface wind direction with respect to the gradient winds in a pressure system in the Northern Hemisphere

Answer 16

• Surface winds: below 2,000 ft AGL, have to include friction
• air blows at an angle across the isobars from high pressure to low pressure. This angle varies as a result of the type of terrain, but for our purposes, we will assume a 45° angle
• blow across the isobars at a 45° angle, they also have a component of motion that moves air out of the high pressure and into the low.

Question 17

DESCRIBE the weather conditions associated with High Clouds

Answer 17

• High clouds have little effect on flying except for moderate turbulence and limited visibility associated with dense jet stream cirrus
• Since high clouds are composed mostly of ice crystals, they have no precipitation and do not constitute an icing hazard
• Severe or extreme turbulence is often found in the anvil cirrus of thunderstorms

Question 18

DESCRIBE the flight conditions associated with a stable and unstable atmosphere

Answer 18

Question 19

DESCRIBE the types of precipitation

Answer 19

1. Drizzle – Very small droplets of water that appear to float in the atmosphere.
2. Freezing drizzle – Drizzle that freezes on impact with objects.
3. Rain – Precipitation in the form of water droplets that are larger than drizzle and fall to the ground.
4. Freezing rain – Rain that freezes on impact with objects.
5. Hail or graupel – A form of precipitation composed of irregular lumps of ice that develop in severe thunderstorms, consisting of alternate opaque and clear layers of ice in most cases. Water drops, which are carried upward by vertical currents, freeze into ice pellets, start falling, accumulate a coating of water, and are carried upward again, causing the water to freeze. A repetition of this process increases the size of the hailstone. It does not lead to the formation of structural ice, but it can cause structural damage to aircraft.
6. Ice pellets or sleet – Small translucent and irregularly shaped particles of ice. They form when rain falls through air with temperatures below freezing. They usually bounce when hitting hard ground and make a noise on impact. Ice pellets do not produce structural icing unless mixed with super-cooled water.
7. Snow – White or translucent ice crystals, usually of branched hexagonal or star-like form that connect to one another forming snowflakes. When condensation takes place at temperatures below freezing, water vapor changes directly into minute ice crystals. A number of these crystals unite to form a single snowflake. Partially melted, or “wet” snow, can lead to structural icing.
8. Snow grains – Very small white, opaque grains of ice. When the grains hit the ground, they do not bounce or shatter. They usually fall in small quantities from stratus-type clouds, never as showers.

Question 20

DESCRIBE the four methods of lifting

Answer 20

1. Convergence: Convergence of two air masses, or parts of a single air mass, force the air upward because it has no where else to go
2. Frontal: because of the shape of cold fronts, as they move through an area, they will lift the air ahead of the cold air mass
3. Orographic: indicating that the force of the wind against a mountainside pushes the air upward
4. Thermal (convective): caused when cool air is over a warm surface, and it is heightened by intense solar heating

Question 21

DESCRIBE mountain winds

Answer 21

• At night
• air in contact with the mountain slope is cooled by outgoing terrestrial radiation and becomes more dense than the surrounding air
• denser air flows downhill, from the top of the mountain,
• oppisite to valley winds
• important to small aircraft close to the ground

Question 22

DESCRIBE the weather conditions associated with low clouds

Answer 22

• Low Clouds:
  
  • mainly composed of water droplets
  • nimbo or nimbus appears, beware that these clouds are producing violent, or heavy, precipitation
  • frequently present serious hazards to flying
  • low cloud types hide hills, making a collision with the terrain a very real danger, and visibility within low clouds is very poor
  • may also hide thunderstorms
  • icing may result, accumulates faster in low clouds since they are generally denser than middle and high clouds
  • Turbulence varies from none at all to moderate turbulence.
  • turbulence in and below the clouds
  • Precipitation from low clouds is generally light rain or drizzle.

Question 23

DESCRIBE the three characteristics of precipitation

Answer 23

1. Showers - Characterized by a sudden beginning and ending, and abruptly changing intensity and/or sky conditions. Showers are associated with cumuliform clouds.
2. Continuous - Also known as steady (not showery). Intensity changes gradually, if at all. Continuous precipitation is associated with stratiform clouds.
3. Intermittent - Stops and restarts at least once during the hour. Intermittent precipitation may be showery or steady, and therefore may be associated with cumuliform or stratiform clouds.

Question 24

DEFINE dew point temperature

Answer 24

• The dew point temperature (TD) is the temperature at which saturation occurs
• dew point is a direct indication of the amount of moisture present in the air. The higher the dew point, the greater chances for clouds, fog, or precipitation.

Question 25

DESCRIBE the four principal cloud groups

Answer 25

1. Low clouds, ranging from just above the surface to 6500 feet AGL. No special prefix.
2. Middle clouds with bases between 6500 and 20,000 feet AGL. Prefix alto-
3. High clouds found above 20,000 feet AGL. Prefix cirro- or the word cirrus
4. Special clouds with extensive vertical development.

• Height of base determines classification

Question 26

FATVIPWC- Fat V I P With Chow