Weather Information : A - Nature Of The Atmosphere Flashcards
State the general characteristics in regard to the flow of air around high and low pressure systems in the Northern Hemisphere.
Low pressure—inward, upward, and counterclockwise.
High pressure—outward, downward, and clockwise.
AC 00-6
If your route of flight takes you towards a low-pressure system, in general, what kind of weather can you expect?
What if you were flying towards a high-pressure system?
A low-pressure system is characterized by rising air, which is conducive to cloudiness, precipitation and bad weather.
A high-pressure system is an area of descending air which tends to favor dissipation of cloudiness and good weather.
(AC 00-6)
Describe the different types of fronts (4).
Cold front — a mass of cold, dense, and stable air advances and replaces a body of warmer air.
Occluded front — a fast-moving cold front catches up with a slow-moving warm front. Two types: cold front occlusion and warm front occlusion.
Warm front — The boundary area formed when a warm air mass contacts and flows over a colder air mass.
Stationery front — When the forces of two air masses are relatively equal, the front that separates them remains stationary and influences the local weather for days. The weather is typically a mixture of both warm and cold fronts.
(AC 00-6)
What are the general characteristics and hazards of the weather a pilot would encounter when operating near a cold front? A warm front?
Cold front — As the front passes, expected weather can include towering cumulus or cumulonimbus, heavy rain accompanied by lightning, thunder and/or hail; tornadoes possible; during passage, poor visibility, winds variable and gusting; temperature/dew point and barometric pressure drop rapidly.
Warm front — As the front passes, expected weather can include stratiform clouds, drizzle, low ceilings and poor visibility; variable winds; rise in temperature.
(FAA-H-8083-25)
What is a trough?
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. (AC 00-6)
What is a ridge?
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. (AC 00-6)
What are the standard temperature and pressure values for sea level? (AC 00-6)
59°F or 15°C and 29.92” Hg or 1013.2 millibars.
What are isobars? (AC 00-6)
An isobar is a line on a weather chart which connects areas of equal or constant barometric pressure.
If the isobars are relatively close together on a surface weather chart or a constant pressure chart, what information will this provide? (AC 00-6)
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.
What is the name of the force that deflects winds to the right in the northern hemisphere and left in the southern hemisphere? (AC 00-6)
The Coriolis force. It is at a right angle to wind direction and is directly proportional to wind speed.
Why do surface winds generally flow across the isobars at an angle? (AC 00-6)
Surface friction
At what rate does atmospheric pressure decrease with an increase in altitude? (AC 00-6)
1” Hg per 1,000 feet
What does dew point mean? (AC 00-6)
Dew point is the temperature to which a sample of air must be cooled to attain the state of saturation.
When temperature and dew point are close together (within 5°), what type of weather is likely? (AC 00-6)
Visible moisture in the form of clouds, dew, or fog. Also, these are ideal conditions for carburetor icing.
What factor primarily determines the type and vertical extent of clouds? (AC 00-6)
The stability of the atmosphere.
Explain the difference between a stable atmosphere and an unstable atmosphere. Why is the stability of the atmosphere important?
The stability of the atmosphere depends on its ability to resist vertical motion.
A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear.
In an unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow and convective activity.
Instability can lead to significant turbulence, extensive vertical clouds, and severe weather.
(FAA-H-8083-25, AC 00-6)
List the effects of stable and unstable air on clouds, turbulence, precipitation and visibility. (AC 00-6)
Stable air has stratiform clouds, smooth air, steady precipitation but fair to poor visibility.
Unstable air has cumuliform clouds, rough turbulence, showery precipitation but good visibility.
When significant precipitation is occurring at the surface, how thick can you expect the clouds to be? (AC 00-6)
Significant precipitation usually requires clouds to be at least 4,000 feet thick. The heavier the precipitation, the thicker the clouds are likely to be.
During your preflight planning, what type of meteorological information should you be aware of with respect to icing?
(AC 00-6)
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.
What is the definition of the term freezing level and how can you determine where that level is? (AC 00-45)
The lowest altitude in the atmosphere over a given location where the air temperature reaches 0°C.
Sources to determine Freezing Level:
- leidos/FSS (1.800.wxbrief)
- Aviationweather.gov
- 1800wxbrief.com
- Foreflight Charts (NWS)
Products:
- freezing level graphics chart
- Winds Aloft (temp)
- GFAs, PIREPs, AIRMETs, SIGMETs
It is possible to have multiple freezing layers when a temperature inversion occurs above the defined freezing level.
What conditions are necessary for structural icing to occur?
Visible moisture and below freezing temperatures at the point moisture strikes the aircraft. (AC 00-6)
What are the two main categories of aircraft icing?
Structural - forms on aircraft surfaces and components
induction icing - forms in the engine’s induction system.
(AC 00-6)
Name the three types of structural icing that may occur in flight.
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.
What action is recommended if you inadvertently encounter icing conditions?
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. (FAA-H-8083-15)
Is frost considered to be hazardous to flight? Why?
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.
What factors must be present for a thunderstorm to form?
a. Sufficient water vapor.
b. An unstable lapse rate.
c. An initial upward boost (lifting) to start the storm process in motion.
(AC 00-6)
What are the three stages of a thunderstorm?
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.
(AC 00-6)
What is a temperature inversion?
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.
How does fog form?
Fog forms when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to a little beyond its dewpoint (producing radiation fog, advection fog, or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing frontal fog or steam fog).
Name several types of fog.
a. Radiation fog.
b. Advection fog.
c. Upslope fog.
d. Frontal fog or precipitation-induced fog.
e. Steam fog.
(AC 00-6)
What causes radiation fog to form?
The ground cools the adjacent air to the dew point on calm, clear nights.
(AC 00-6)
What is advection fog, and where is it most likely to form?
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.
(AC 00-6)
What is upslope fog?
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.
Define the term wind shear, and state the areas in which it is likely to occur.
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.
Why is wind shear an operational concern to pilots?
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.
(AC 00-6)
What sources of weather information/products will you examine to determine if wind shear conditions might affect your flight?
a. 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.
b. 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.
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.
