*2A Weather - Nature of the Atmosphere

Question 0

1. State the general characteristics in regard to the flow of air around high and low pressure systems in the Northern Hemisphere.
   (AC 00-6A)

Answer 0

Low Pressure—inward, upward, and counterclockwise

High Pressure—outward, downward, and clockwise

Question 1

2. What is a “trough”?

AC 00-6A

Answer 1

A trough (also called a trough line) is an elongated area of relatively low atmospheric pressure. At the surface when air converges into a low, it cannot go outward against the pressure gradient, nor can it go downward into the ground; it must go upward. Therefore, a low or trough is an area of rising air. Rising air is conducive to cloudiness and precipitation; hence the general association of low pressure and bad weather.

Question 2

3. What is a “ridge”?

AC 00-6A

Answer 2

A ridge (also called a ridge line) is an elongated area of relatively high atmospheric pressure. Air moving out of a high or ridge depletes the quantity of air; therefore, these are areas of descending air. Descending air favors dissipation of cloudiness; hence the association of high pressure and good weather.

Question 3

4. What are the standard temperature and pressure values for sea level?
   (AC 00-6A)

Answer 3

15°C (59°F) and 29.92” Hg

Question 4

5. What are “isobars”?

AC 00-6A

Answer 4

An isobar is a line on a weather chart which connects areas of equal or constant barometric pressure.

Question 5

6. If the isobars are relatively close together on a surface weather chart or a constant pressure chart, what information will this provide?
   (AC 00-6A)

Answer 5

The spacing of isobars on these charts defines how steep or shallow a pressure gradient is. When isobars are spaced very close together, a steep pressure gradient exists which indicates higher wind speeds. A shallow pressure gradient (isobars not close together) usually means wind speeds will be less.

Question 6

7. What causes the winds aloft to flow parallel to the isobars?
   (AC 00-6A)

Answer 6

The Coriolis force.

Question 7

8. Why do surface winds generally flow across the isobars at an angle?
   (AC 00-6A)

Answer 7

Surface friction.

Question 8

9. At what rate does atmospheric pressure decrease with an increase in altitude?
   (AC 00-6A)

Answer 8

1” Hg per 1,000 feet.

Question 9

10. What does “dew point” mean?

AC 00-6A

Answer 9

Dew point is the temperature to which a sample of air must be cooled to attain the state of saturation.

Question 10

11. When temperature and dew point are close together (within 5°), what type of weather is likely?
    (AC 00-6A)

Answer 10

Visible moisture in the form of clouds, dew, or fog. Also, these are ideal conditions for carburetor icing.

Question 11

12. What factor primarily determines the type and vertical extent of clouds?
    (AC 00-6A)

Answer 11

The stability of the atmosphere.

Question 12

13. How do you determine the stability of the atmosphere?

AC 00-6A

Answer 12

Unstable air is indicated when temperature decreases uniformly and rapidly as you climb (approaching 3°C per 1,000 feet). If temperature remains unchanged or decreases only slightly with altitude, the air tends to be stable. Instability is likely when air near the surface is warm and moist. Surface heating, cooling aloft, converging or upslope winds, or an invading mass of colder air may lead to instability and cumuliform clouds.

Question 13

14. List the effects of stable and unstable air on clouds, turbulence, precipitation and visibility.
    (AC 00-6A)

Answer 13

Stable
     Stratiform clouds
     Smooth turbulence
     Steady precipitation 
     Fair to poor visibility
Unstable
     Cumuliform clouds
     Rough turbulence
     Showery precipitation
     Good visibility

Question 14

15. At what altitude above the surface would the pilot expect the bases of cumuliform clouds if the surface temperature is 82° and the dew point is 62°?
    (AC 00-6A)

Answer 14

You can estimate the height of cumuliform cloud bases using surface temperature/dewpoint spread. Unsaturated air in a convective current cools at about 5.4°F (3.0°C) per 1,000 feet; dew point decreases at about 1°F (5/9°C). Thus, in a convective current, temperature and dew point converge at about 4.4°F (2.5°C) per 1,000 feet. You can get a quick estimate of a convective cloud base in thousands of feet by rounding the values and dividing into the spread. When using Fahrenheit, divide by 4 and multiply by 1,000. This method of estimating is reliable only with instability, clouds and during the warmer part of the day.

Temp - Dew Point
——————— x 1,000 = Base of clouds
4

82 – 62 = 20
20 ÷ 4 = 5
5 x 1,000 = 5,000 feet AGL

Question 15

16. During your preflight planning, what type of meteorological information should you be aware of with respect to icing?
    (AC 91-74)

Answer 15

a. Location of fronts—A front’s location, type, speed, and direction of movement.
b. Cloud layers—The location of cloud bases and tops, which is valuable when determining if you will be able to climb above icing layers or descend beneath those layers into warmer air; reference PIREPs and area forecasts.
c. Freezing level(s)—Important when determining how to avoid icing and how to exit icing conditions if accidentally encountered.
d. Air temperature and pressure—Icing tends to be found in low-pressure areas and at temperatures at or around freezing.

Question 16

17. What is the definition of the term freezing level and how can you determine where that level is?
    (AC 00-6A)

Answer 16

The freezing level is the lowest altitude in the atmosphere over a given location at which the air temperature reaches 0°C. It is possible to have multiple freezing layers when a temperature inversion occurs above the defined freezing level. A pilot can use current icing products (CIP) and forecast icing products (FIP), as well as the freezing level graphics chart to determine the approximate freezing level. Other potential sources of icing information are: area forecasts, PIREPs, AIRMETs, SIGMETs, surface analysis charts, low-level significant weather charts, and winds and temperatures aloft (for air temperature at altitude).

Question 17

18. What conditions are necessary for structural icing to occur?
    (AC 00-6A)

Answer 17

Visible moisture and below freezing temperatures at the point moisture strikes the aircraft.

Question 18

19. Name the main types of icing an aircraft may encounter in-flight.
    (AC 00-6A)

Answer 18

Structural, induction system, and instrument icing.”

Question 19

20. Name the three types of structural icing that may occur in flight.
    (AC 00-6A)

Answer 19

Clear ice—forms after initial impact when the remaining liquid portion of the drop flows out over the aircraft surface, gradually freezing as a smooth sheet of solid ice.
Rime ice—forms when drops are small, such as those in stratified clouds or light drizzle. The liquid portion remaining after initial impact freezes rapidly before the drop has time to spread out over aircraft surface.
Mixed ice—forms when drops vary in size or when liquid drops are intermingled with snow or ice particles. The ice particles become imbedded in clear ice, building a very rough accumulation.

Question 20

21. What action is recommended if you inadvertently encounter icing conditions?
    (FAA-H-8083-15)

Answer 20

The first course of action should be to leave the area of visible moisture. This might mean descending to an altitude below the cloud bases, climbing to an altitude above the cloud tops, or turning to a different course.

Question 21

22. Is frost considered to be hazardous to flight? Why?

AC 00-6A

Answer 21

Yes, because while frost does not change the basic aerodynamic shape of the wing, the roughness of its surface spoils the smooth flow of air, thus causing a slowing of airflow. This slowing of the air causes early airflow separation, resulting in a loss of lift. Even a small amount of frost on airfoils may prevent an aircraft from becoming airborne at normal takeoff speed. It is also possible that, once airborne, an aircraft could have insufficient margin of airspeed above stall so that moderate gusts or turning flight could produce incipient or complete stalling.

Question 22

23. What factors must be present for a thunderstorm to form?
    (AC 00-6A)

Answer 22

a. Sufficient water vapor
b. An unstable lapse rate
c. An initial upward boost (lifting) to start the storm process in motion

Question 23

24. What are the three stages of a thunderstorm?

AC 00-6A

Answer 23

Cumulus stage—Updrafts cause raindrops to increase in size.
Mature stage—Rain at earth’s surface; it falls through or immediately beside the updrafts; lightning; perhaps roll clouds.
Dissipating stage—Downdrafts and rain begin to dissipate.

Question 24

25. What is a “temperature inversion”?

AC 00-6A

Answer 24

An inversion is an increase in temperature with height—a reversal of the normal decrease with height. An inversion aloft permits warm rain to fall through cold air below. Temperature in the cold air can be critical to icing. A ground-based inversion favors poor visibility by trapping fog, smoke, and other restrictions into low levels of the atmosphere. The air is stable, with little or no turbulence.

Question 25

26. State two basic ways that fog may form.

AC 00-6A

Answer 25

a. Cooling air to the dew point.

b. Adding moisture to the air near the ground.

Question 26

27. Name several types of fog.

AC 00-6A

Answer 26

a. Radiation fog
b. Advection fog
c. Upslope fog
d. Precipitation-induced fog
e. Ice fog

Question 27

28. What causes radiation fog to form?

AC 00-6A

Answer 27

The ground cools the adjacent air to the dew point on calm, clear nights.

Question 28

29. What is advection fog, and where is it most likely to form?
    (AC 00-6A)

Answer 28

Advection fog results from the transport of warm humid air over a cold surface. A pilot can expect advection fog to form primarily along coastal areas during the winter. Unlike radiation fog, it may occur with winds, cloudy skies, over a wide geographic area, and at any time of the day or night.

Question 29

30. What is upslope fog?

AC 00-6A

Answer 29

Upslope fog forms as a result of moist, stable air being cooled adiabatically as it moves up sloping terrain. Once the upslope wind ceases, the fog dissipates. Upslope fog is often quite dense and extends to high altitudes.

Question 30

31. Define the term “wind shear,” and state the areas in which it is likely to occur.
    (AC 00-6A)

Answer 30

Wind shear is defined as the rate of change of wind velocity (direction and/or speed) per unit distance; conventionally expressed as vertical or horizontal wind shear. It may occur at any level in the atmosphere but three areas are of special concern:

a. Wind shear with a low-level temperature inversion.
b. Wind shear in a frontal zone or thunderstorm.
c. Clear air turbulence (CAT) at high levels associated with a jet stream or strong circulation.

Question 31

32. Why is wind shear an operational concern to pilots?

AC 00-6A

Answer 31

Wind shear is an operational concern because unexpected changes in wind speed and direction can be potentially very hazardous to aircraft operations at low altitudes on approach to and departing from airports.

Question 32

33. What types of weather information will you examine to determine if wind shear conditions might affect your flight?
    (AC 00-54)

Answer 32

a. Terminal forecasts—any mention of low level wind shear (LLWS) or the possibility of severe thunderstorms, heavy rain showers, hail, and wind gusts suggest the potential for LLWS and microbursts.
b. METARs—inspect for any indication of thunderstorms, rain showers, or blowing dust. Additional signs such as warming trends, gusty winds, cumulonimbus clouds, etc., should be noted.
c. Severe weather watch reports, SIGMETS, and convective SIGMETS—severe convective weather is a prime source for wind shear and microbursts.
d. LLWAS (low level windshear alert system) reports—installed at 110 airports in the U.S.; designed to detect wind shifts between outlying stations and a reference centerfield station.
e. PIREPs—reports of sudden airspeed changes on departure or approach and landing corridors provide a real-time indication of the presence of wind shear.