Aviation Weather Flashcards
When does radiation fog form ?
Evening or Daybreak.
What is true about advection fog ?
more extensive and persistent than radiation fog. can move more rapidly.
Virga is described as what ?
streamers of precipitation trailing beneath clouds which evaporate before reaching the ground.
Standard Temperature (Sea Level ?)
15 C
Standard Temperature Lapse rate ?
2 C per 1000 ft
What lifts advection fog into low stratus clouds?
Wind 15 knots or stronger.
What type of fog does adiabatic cooling produce ?
Upslope fog.
What is fog produced by frontal activity known as ?
Precipitation-induced fog.
A convective cloud with virga below it could indicate what ?
A microburst.
Describe Radiation Fog
a warm, moist air mass settling over a cool surface under no-wind conditions.
Describe Advection Fog
forms when moist air moves over colder ground or water.
Dewpoint Lapse Rate ?
.5 deg C
Adiabatic lapse rate ?
3 deg C
Rate at which temperature and dewpoint converge ?
2.5 deg c
What is Radiation fog is restricted to ?
land because water surfaces cool little from nighttime radiation
When does Advection fog form ?
when moist air moves over colder ground or water.
What is Orographic lifting ?
lifting is caused by mountain forces, mountain winds, etc.
What determines the structure or type of clouds which will form as a result of air being forced to ascend?
The stability of the air before lifting occurs.
What is a temperature inversion ?
when warm air exists over cooler air.
Which are characteristics of a cold air mass moving over a warm surface?
Cumuliform clouds, turbulence, and good visibility.
Which in-flight hazard is most commonly associated with warm fronts?
Precipitation-induced fog arises from drops of warm rain or drizzle evaporating as it falls through cool air.
Whats the most easily recognized means of being alerted to possible wind shear on approach ?
power and vertical velocity required to remain on the proper glidepath.
When can a strong wind shear can be expected
When the speed of the jet stream is in excess of 110 knots, strong wind shears can be expected on the lower-pressure side.
What minimum distance should exist between intense radar echoes before any attempt is made to fly between these thunderstorms?
40 NM
Where is a common location of clear air turbulence ?
an upper trough on the cold (polar) side of the jet stream.
What are the three stages of a thunderstorm ?
cumulus, mature, and dissipating
Describe moderate turbulence
Changes in altitude and/or attitude occur, but the aircraft remains in positive control at all times.
Describe severe turbulence
Large, abrupt changes in altitude and/or attitude and the aircraft may be momentarily out of control.
What are the 2 greatest threats in thunderstorms ?
Hail and Turbulence
When flying into a low-pressure area in the Northern Hemisphere, the wind direction and velocity will be from which direction ?
left and increasing.
The general circulation of air associated with a high-pressure area in the Northern Hemisphere is
Air flows outward from a high-pressure area, causing a descending column of air within the high. As the air moves outward, it is deflected to the right by the Coriolis force, resulting in a clockwise rotation.
What should you be prepared for when landing in calm wind under clear skies within a few hours after sunrise ?
A temperature inversion near the ground and wind shear if the winds are 25kts or greater between 2000 and 3000 ft AGL
What is anticyclonic circulation ?
In the Northern Hemisphere, the flow of air from an area of HIGH to LOW pressure is deflected to the right and produces a clockwise circulation around an area of high pressure.
Which feature is associated with the tropopause?
Abrupt temperature changes.
When flying low over hilly terrain, ridges, or mountain ranges, the greatest potential danger from turbulent air currents will usually be encountered on the
leeward side when flying into the wind.
One of the most dangerous features of mountain waves is the turbulent areas in and where ?
below rotor clouds.
During the winter months in the middle latitudes, the jet stream shifts toward which cardinal direction ?
south and its speed increases.
When does an occluded front occur ?
When a cold front overtakes a warm front.
What are the conditions most favorable to wave formation over mountainous areas ?
stable air at mountaintop altitude and a wind of at least 20 knots blowing across the ridge.
How is moisture is added to a parcel of air ?
Evaporation or Sublimation. (i.e when liquid water or ice are changed into water vapor.)
Ice pellets encountered during flight normally are evidence of what ?
Either a warm front or a cold front is about to pass.
What does a curving jetstream indicate ?
Abrupt weather system changes, which lend themselves to more violent turbulence.
In general what do more pronounced differences in weather systems indicate ?
greater the potential for very strong turbulence.
Which is true with respect to a high- or low-pressure system?
A high-pressure area or ridge is an area of descending air.
High-pressure air descends because it is heavier than low-pressure air. Ridge refers to an elongated area of high pressure.
What does a Trough refer to ?
An elongated area of low pressure.
Why does low pressure rise ?
Because it weighs less than high-pressure air.
The minimum vertical wind shear value critical for probable moderate or greater turbulence is what ?
Moderate or greater turbulence should be expected where vertical wind shears exceed 5 knots per 1,000 feet.
What prevents air from flowing directly from high-pressure areas to low-pressure areas?
The Coriolis force
When is Hazardous wind shear commonly encountered ?
in areas of temperature inversion and near thunderstorms.
What is the minimum vertical wind shear value critical for probable moderate or greater turbulence ?
5 knots per 1,000 feet.
A situation most conducive to the formation of advection fog is
an air mass moving inland from the coastline during the winter.
Which type of pressure system has heavier air ?
High-pressure air descends because it is heavier than low-pressure air. Ridge refers to an elongated area of high pressure.
In what ways do advection fog, radiation fog, and steam fog differ in their formation or location?
Radiation fog is restricted to land areas; advection fog is most common along coastal areas; steam fog forms over a water surface.
The minimum vertical wind shear value critical for probable moderate or greater turbulence is ?
5 knots per 1,000 feet.
Fog produced by frontal activity is a result of saturation due to ?
evaporation of precipitation.
Which combination of weather-producing variables would likely result in cumuliform-type clouds, good visibility, and showery rain?
Unstable, moist air and orographic lifting.
On a flight from west to east, what side of a high-pressure system would have the most favorable winds. ?
North-side, due to anticyclonic circulation. (clock-wise circulation around high-pressure)
The wind system associated with a low-pressure area in the Northern Hemisphere is
a cyclone and is caused by Coriolis force.
Which is true regarding actual air temperature and dew point temperature spread? The temperature spread
decreases as the relative humidity increases. (i.e As relative humidity increases to 100%, the dew point approaches the temperature and the spread approaches zero.)
The wind system associated with a low-pressure area in the Northern Hemisphere is…
a cyclone and is caused by Coriolis force.
The conditions most favorable to wave formation over mountainous areas are a layer of
stable air at mountaintop altitude and a wind of at least 20 knots blowing across the ridge.
Convective circulation patterns associated with sea breezes are caused by
land absorbing and radiating heat faster than the water.
Which statement is true regarding squall lines?
They are nonfrontal and often contain severe, steady-state thunderstorms.
Streamlined, windswept cirrus clouds always indicate what ?
very strong upper winds.
A common location of clear air turbulence is
in an upper trough on the polar side of a jet stream.
Which would increase the stability of an air mass?
Cooling from below.
From which measurement of the atmosphere can stability be determined?
The ambient lapse rate.
What is an important characteristic of wind shear?
It can be present at any level and can exist in both a horizontal and vertical direction.
During departure, under conditions of suspected low-level wind shear, a sudden decrease in headwind will cause
a loss in airspeed equal to the decrease in wind velocity.
Fog produced by frontal activity is a result of saturation due to
evaporation of precipitation.
Which is true regarding the development of convective circulation?
Cool air must sink to force the warm air upward.
What should you be prepared for when landing in calm wind under clear skies within a few hours after sunrise ?
A temperature inversion near the ground.
Why does high-pressure air descend ?
Because it is heavier than low-pressure air.
The difference found by subtracting the temperature of a parcel of air theoretically lifted from the surface to 500 millibars and the existing temperature at 500 millibars is called what ?
The lifted index.
What does the ambient lapse help determine ?
Stability of the atmosphere.
Advection fog has drifted over a coastal airport during the day. What may tend to dissipate or lift this fog into low stratus clouds?
Wind 15 knots or stronger.
