Weather Flashcards
At what rate dues atmospheric pressure decrease with an increase in altitude?
pressure decreases by about 1” Hg per 1,000 ft
What is standard temp and pressure for sea level?
15C and 29.92” Hg
What are the general characteristics of air flow 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 wind aloft to flow parallel to isobars?
Coriolis force
Why do surface wind flow across isobars at an angle
Surface friction
When temp and dew point are within 5C, what type of weather is likely?
Visible moisture. Could be clouds, fog or dew.
What factor primarily determines the type and vertical extent of clouds?
Stability of the atmosphere
What is the difference between a stable and unstable atmosphere?
Stable atmosphere resists vertical displacement. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical or convective current
How do you determine the stability of the atmosphere
If temp decreases uniformly and rapidly as you climb (approaching 3C/1000 ft), that is an indication of unstable air. If temp changes slightly with altitude, air tends to be stable. When air at surface is warm and moist, expect instability.
What type of clouds are expected with stable and unstable air?
Stable - stratiform clouds
Unstable - cumuliform clouds
What turbulence is expected with stable and unstable air?
Stable - smooth
Unstable - rough
What precip is expected with stable and unstable air?
Stable - steady
Unstable - showery
What visibility is expected in stable and unstable air?
Stable - fair to poor
Unstable - good
What are the three main types of icing?
Structural, induction and instrument
Name the three types of structural ice
Clear, rime and mixed
When does clear ice form?
Forms when large drops strike aircraft and freeze slowly
When does rime ice form?
When small drops strike aircraft and freeze rapidly
When does mixed ice form?
Supercooled water drops of various size, intermingled with snow or ice pellets, building a rough accumulation.
What are the four intensity categories of aircraft structural icing?
Trace, Light, Moderate, Severe
What is the definition of freezing level and how can you determine where it is?
The lowest altitude where the air temp reaches 0C. It is possible to have multiple freezing levels in a temperature inversion. Use icing forecasts and PIREPS to determin approximate freezing level. AIRMETS, area forecasts and Low Level Sig Wx charts all contain icing information
What factors must be present for a thunderstorm to form?
A source of lift (heating, fast moving front) Unstable air (nonstandard lapse rate) High moisture content (temp and dew point are close)
What are squall line thunderstorms?
A narrow line of non-frontal active thunderstorms. Often develops ahead of cold fronts, but may develop far from any front. Forms rapidly. Strongest in afternoon or early evening. Single biggest 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
Name the five most common types of fog
Radiation Advection Upslope Precipitation-induced Ice fog
What causes radiation fog to form?
Clear sky, no wind, small temp-dew point spread. Forms almost exclusively at night or early morning.
What is advection fog and where does it form?
When moist air moves over colder ground or water. Common along coastal areas. Deepens in winds up to 15 knots. ABove that the wind lifts the fog to form low stratus.
Define upslope fog.
Formed when moist, stable air is cooled as it moves up sloping terrain
Define ice fog
Occurs in extreme cold weather when water vapor sublimates directly as ice crystals.
What is precipitation-induced fog?
When warm rain or drizzle falls through cool air. Evaporation from the precip saturates the air and forms fog. Can extend over large areas, completely suspending air operations. Mostly associated with warm fronts, but can occur with slow-moving cold fronts.
Other than fog, what are other IFR weather producers?
Low clouds, haze, smoke, and precip.
What is the primary means of obtaining a a weather briefing?
An individual briefing from the AFSS/FSS
What are some examples of other sources of weather information?
Telephone Information Briefing Service (TIBS)
Private industry sources
Direct User Access Terminal System (DUATS)
In Alaska, Transcribed Wx Broadcast (TWEB) and telephone TWEB (TEL-TWEB)
What pertinent information should a weather brief include?
Adverse conditions VFR flight not recommended Synopsis Current conditions Enroute Forecast Destination forecast winds aloft NOTAMs ATC delays
What is EFAS?
Enroute Flight Advisory Service
Also known as Flight Watch on 122.0
What is HIWAS?
Hazardous In-Flight Wx Advisory Service
A continuous broadcast of in-flight wx advisories.
What is a METAR?
A wx observer’s interpretation of the wx at a given point and time. Two types - a routine hourly report and a special report (SPECI) to update for rapidly changing conditions.
What are the basic elements of a METAR?
Type Identifier Date and time Modifier Wind VIs RVR Weather phenomena Sky condition Temp/Dew Point Altimeter Remarks
What are four basic weather observing programs?
Manual
AWOS
AWOS Broadcasts
ASOS/AWSS (Auto Sfc Observing System/Auto Weather Sensing System)
What are PIREPS and what are the required elements?
Pilot Wx Report. Type Location Time FL Acft type At least one wx element
What are radar weather reports?
Contains information about precip observed by radar. Resolution is course and can be up to 80 mins old. Should be used only if no other information available.
What is a TAF?
Terminal Aerodrome Forecast
Expected wx conditions within 5SM radius from the center of an airport’s RWY complex during a 24 hr period. Large ARPTs go out to 30 hours.
What is an aviation area forecast?
A forecast of VMC, clouds and general ex conditions over a several state area. Issued three times per day.
What info is provided by an FA (area forecast)?
Synopsis - 18 hour valid period Clouds and Wx Cloud coverage Cloud bases and tops Precip Vis Sustained sfc winds 12-18hr categorical outlook (IFR, MVFR, VFR)
What is a convective SIGMET?
Severe turbulence, severe icing and low-level wind shear. Issued hourly at H+55. Special bulletins issued at any time. Valid up to 2 hrs.
What is a SIGMET?
Non convective weather that is potentially hazardous to all aircraft not associated with a thunderstorm. Max forecast period is 4 hrs. Can be severe icing, turbulence, dust storms, volcanic ash.
What is an AIRMET?
Significant wx phenomena with conditions lower than that for a SIGMET. Issued every 6 hours. Can be for IFR, Mtn obscuration, turbulence, strong sfc winds, icing and freezing levels.
What are the three types of AIRMETs
Sierra - IFR and/or mtn obscuration
Tango - Mod turb, sfc winds >30 knots, low level wind shear
Zulu - Moderate Icing and freezing levels
What is a wind a temp aloft forecast (FB)?
Wind direction, speed and temp at specified altitudes. Produced 4x/day.
What information can be determined from a wind a temp aloft forecast
Most favorable altitude
Areas of possible icing
Temp inversions
Turbulence
What is a Center Wx Advisory (CWA)?
Aviation warning for use to anticipate and avoid adverse weather conditions. Valid for up to 2 hrs.
Name seven examples of current weather charts
Sfc Analysis Wx depiction Radar summary Sig Wx Prog Short-range Sfc Prog Convective Outlook Constant pressure analysis
What is a surface analysis chart?
Computer prepared chart covering lower 48 states. Transmitted every 3 hours. Depicts pressure systems and fronts. Overview of winds, temps and dew points.
What info does a wx depiction chart provide?
Computer generated from METARs. Transmitted every 3 hours. Includes sky cover, cloud height, wx, visibility, obstructions to visibility.
Define LIFR, IFR, MVFR, and VFR
LIFR - CLG 3000, VIS >5SM
What are radar summary charts
Computer generated display of automated weather reports, displaying precip. Includes type, intensity, configuration, coverage, echo tops and cell movement of precip. Does not display clouds or fog.
What are short-range prognostic charts?
Provide a forecast of surface pressure systems. Includes valid periods of 12-, 24-, 36- and 48 hrs.
Describe a US low-level significant weather prognostic chart
Significant wx for surface to FL240. Contains four panels with forecast period of 12- and 24 hrs. Issued 4x/day. Upper panels are the Sig wx progs, the lower panels are the surface progs.
Describe a mid-level significant weather (SIGWX) chart
Covers 10,000ft to FL450. Includes snapshot of wx at valid time and shows elements that could be hazardous to aviation. Issued 4x time a day for 24hr ahead.
What is the convective outlook chart?
Depicts areas to have severe and non-severe convection weather threats for the following three days. Has areas of slight risk moderate risk or high risk. Covers 24hr period.
What are constant pressure analysis charts.
An upper air weather map for the specified pressure of the chart. Depicts highs, lows and troughs and ridges. Issued twice daily at five pressure levels - 850mb (5000ft) 700mb(10,000ft) 500mb (18,000ft) 300mb (30,000ft) 200mb (39,000ft)
What significance do height contour lines have on a constant pressure chart?
Heights of the specified pressure through the use of solid lines called contours to give a height pattern. Depicts highs, lows, troughs and ridges. Closely spaced contours mean strong winds.
What significance do isotherms have on a constant pressure chart?
Isotherms drawn at 5C intervals show temperature variations. You can see if you will be travelling to colder or warmer temps. Temps and dewpoint spreads of less than 5C suggest icing.
What is the significance of the isotach lines on a constant pressure chart?
Isotachs are lines of constant wind speed.
