Fundamentals of Weather Flashcards
At what rate does atmospheric pressure decrease with an increase in altitude?
At a rate of approximately 1”Hg per 1,000 feet
What are the standard temperature and pressure values for sea level?
15 degrees Centigrade and 29.92” Hg
State the general characteristics in regard to the flow of air around high and low pressure systems in the northern hemisphere?
Low pressure - air flows inward, upward, and counterclockwise
High Pressure - air flows outward, downward, and clockwise
What causes the winds aloft to flow parallel to the isobars?
The coiolis force causes winds aloft to flow parallel to the isobars
Why do surface winds generally flow across the isobars at an angle?
Surface friction
When temeprature and dew point are close together what type of weather is likely?
Visibile moisture in the form of clouds, dew, or fog
What factor primarily determines the type and vertical extent of clouds?
The stability of the atmosphere
What is the difference between a stable and unstable atmosphere?
Stable resists any upward or downward displacement.
An unstable allows upward or downward disturbances to grow into a vertical or convective current.
How do you determine the stability of the atmosphere?
When temperature decreases uniformly and rapidly as you climb, you have an indication of unstable air. If the temperature remains unchanged or decreases only slightly with altitude, the air tends to be stable. When air near the surface is warm and moist, expect instability.
List the effects of stable and unstable air on clouds, turbulence, precipitation, and visibility
Clouds - S - stratiform - U - cumuliform
Turbulence - S- smooth - U - rough
Precip - S- steady - U -Showery
Vis - S - fair to poor - U - good
What are the main types of icing an aircraft may encounter?
Structural, induction, and instrument
Name the three types of structural ice
Clear - forms when large drops strike aircraft surface and slowly freeze
Rime - small drops strike and freeze rapidly
Mixed - combination of rime and clear, rough accumulation, intermingled with snow or ice
What are the intensity catatgories of aircraft structural icing?
a. Trace - rate of accumulation slightly greater than sublimation, anti ice equip. not used unless encountered for extended period of time
b. Light - rate of accumulation may create problem if flight is prolonged. Occasional use of anti ice removes or prevents accumulation
C. Moderate - the rate of accumulation is such that even short encounters become potentially hazardous. use of anti ice or diversion necessary.
D. severe - rate of accumulation is greater than anti ice ability to remove, immediate diversion necessary
What is the definition of the term freezing level and how can you determine where it is?
Lowest altitude in the atmosphere over a given location at which that air temperatures reaches freezing.
Icing forecasts and PIREPS, Area forecasts, AIRMETs, SIGMETs, and low level significant weather charts are other examples
What factors must be present for a thunderstorm to form?
A source of lift
Unstable air
High moisture content
What are squall line thunderstorms?
A non frontal, narrow band of active thunderstorms. Frequently develop ahead of cold front in moist, unstable air. Most intense weather hazard to aircraft.
State two basic ways that fog may form?
By cooling air to the dew point
By adding moisture to the air
5 types of fog?
Radiation Advection Upslope Precipitation induced fog Ice fog
What causes radiation fog to form?
Clear sky, little or no wind, small temp-dew point spread (high relative humidity). forms almost exclusively at night or near daybreak.
What is advection fog and where is it most likely to form?
Forms when moist air moves over colder ground or water. Most common along coastal areas but often develops deep in continental areas. May occur with winds, cloudy skies, over wide geographic area, at any time. It deeps as wind speed increases up to about 15 knots. Stronger wind lifts fog into layer of low stratus of stratocumulus.
Define 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. Unlike radiation fog, it can form under cloudy skies. it is most common along eastern slopes of the Rockies. Can be dense and extend to high altitudes.
Define ice fog.
Occurs in cold weather when the temperature is much below freezing and water vapor sublimates directly as ice crystals. Same conditions as radiation fog but temps a usually -25 F or colder. It occurs mostly in arctic regions, but is not unknown in middle latitudes during the cold season.
Precipitation induced fog?
When relatively warm rain or drizzle falls through cool air, evaporation from precip saturates the cool air and forms fog. Dense and can continue for extended time period. Commonly associated with warm fronts, but can occur with slow moving cold fronts and with stationary fronts.
Other than fog, what are several other examples of IFR weather producers?
Low clouds (stratus), haze, smoke, blowing obstructions to vision, and precipitation. Fog and low stratus restrict navigation by visual reference more often than all other weather phenomena.
