WX 301 ACTUAL Questions and
CORRECT Answers
Explain why the true state of the atmosphere can never be known exactly and can only be
approximated by assimilating all available data - CORRECT ANSWER✔✔- • Atmosphere is
too complex and changing constantly
• We cannot make up observations at all altitudes, places, most accurate, etc
• Our technology cannot keep up and continuously report everything that goes on
• D, all of the above
Explain the fundamental difference between ensemble forecast models and deterministic
forecast models - CORRECT ANSWER✔✔- • Deterministic: start with an initial state and
generate a single forecast, high resolution
• Ensemble: utilize multiple forecasts for the same period of time, faster but not as good of a
forecast, forecasting the uncertainty of the forecast
3. Determine the analysis time, valid time, forecast length given numerical model output. -
CORRECT ANSWER✔✔- • Given a map, on map, know:
• time when you started collecting data (analysis time)
• ___ for Month, Day, Zulu (valid time, convert to local by subtracting 5 hours)
• How many hours between valid and analysis time (forecast length)
Identify the favored regions around a low pressure system the formation of snow and other
types of frozen precipitation - CORRECT ANSWER✔✔- • Snow and freezing precipitation
located to the northwest of the low, can have it north of warm front
Describe how the temperature must change with height to produce rain, snow, sleet, or
freezing. - CORRECT ANSWER✔✔- • Depends on thickness of the cold layer
• Rain is melted snow crystals
• In snow, crystals remain below freezing all the way to the ground
• Freezing rain is rain that falls into shallow layer of cold surface air
• Sleet is rain that falls into deeper layer of cold surface air
, Define critical thickness - CORRECT ANSWER✔✔- • Thickness at which air is cold
enough to snow
• Thickness is proportional to the average temperature of the layer
• Larger thickness = warmer; smaller thickness = colder
Analyze a 1000-500mb thickness chart to determine the likelihood of rain versus snow.
(Recall that the 540dam line only identifies where snow is possible, the lower the thickness
values the greater the likelihood of snow.) - CORRECT ANSWER✔✔- • 540dm
(decameters, height) is a good dividing line between rain and snow
Define what is meant by ground icing "hold-over" times. - CORRECT ANSWER✔✔- •
Hold-over time: length of time deicing fluid will prevent ice build-up on an aircraft
• Time ranges from 45 minutes for frost to less than a couple minutes for freezing rain
Explain why equivalent liquid water precipitation rate is important in aviation winter weather
decision-making and how it affects hold-over times. - CORRECT ANSWER✔✔- • Basing
precipitation intensity on visibility alone does not adequately measure true rate of liquid
water equivalent accumulation
• Accurate, real-time measurements of liquid-equivalent snowfall rates need to be made and
reported to pilots and ground operations personnel, not just visibility measurements.
• Accurately predicting deicing hold-over times remains a significant aviation safety factor
Identify the fundamental cause of icing - CORRECT ANSWER✔✔- • Liquid water droplets
Identify the temperatures favorable for rime, clear and mixed icing - CORRECT
ANSWER✔✔- • Rime: small droplets, freeze rapidly cause trapped air, opaque appearance,
forms at temps below -15C
• Clear: larger droplets with little trapped air, forms at 0 to -5 C, equivalent to freezing rain
• Mixed: drop size varies, forms at intermediate temps -5 to -15C
Describe the impacts of icing on aircraft performance (stall speed, fuel consumption, service
ceiling, stall angle, etc) - CORRECT ANSWER✔✔- • Stall speed increases
• Fuel rate consumption increases
• Service ceiling decreases
CORRECT Answers
Explain why the true state of the atmosphere can never be known exactly and can only be
approximated by assimilating all available data - CORRECT ANSWER✔✔- • Atmosphere is
too complex and changing constantly
• We cannot make up observations at all altitudes, places, most accurate, etc
• Our technology cannot keep up and continuously report everything that goes on
• D, all of the above
Explain the fundamental difference between ensemble forecast models and deterministic
forecast models - CORRECT ANSWER✔✔- • Deterministic: start with an initial state and
generate a single forecast, high resolution
• Ensemble: utilize multiple forecasts for the same period of time, faster but not as good of a
forecast, forecasting the uncertainty of the forecast
3. Determine the analysis time, valid time, forecast length given numerical model output. -
CORRECT ANSWER✔✔- • Given a map, on map, know:
• time when you started collecting data (analysis time)
• ___ for Month, Day, Zulu (valid time, convert to local by subtracting 5 hours)
• How many hours between valid and analysis time (forecast length)
Identify the favored regions around a low pressure system the formation of snow and other
types of frozen precipitation - CORRECT ANSWER✔✔- • Snow and freezing precipitation
located to the northwest of the low, can have it north of warm front
Describe how the temperature must change with height to produce rain, snow, sleet, or
freezing. - CORRECT ANSWER✔✔- • Depends on thickness of the cold layer
• Rain is melted snow crystals
• In snow, crystals remain below freezing all the way to the ground
• Freezing rain is rain that falls into shallow layer of cold surface air
• Sleet is rain that falls into deeper layer of cold surface air
, Define critical thickness - CORRECT ANSWER✔✔- • Thickness at which air is cold
enough to snow
• Thickness is proportional to the average temperature of the layer
• Larger thickness = warmer; smaller thickness = colder
Analyze a 1000-500mb thickness chart to determine the likelihood of rain versus snow.
(Recall that the 540dam line only identifies where snow is possible, the lower the thickness
values the greater the likelihood of snow.) - CORRECT ANSWER✔✔- • 540dm
(decameters, height) is a good dividing line between rain and snow
Define what is meant by ground icing "hold-over" times. - CORRECT ANSWER✔✔- •
Hold-over time: length of time deicing fluid will prevent ice build-up on an aircraft
• Time ranges from 45 minutes for frost to less than a couple minutes for freezing rain
Explain why equivalent liquid water precipitation rate is important in aviation winter weather
decision-making and how it affects hold-over times. - CORRECT ANSWER✔✔- • Basing
precipitation intensity on visibility alone does not adequately measure true rate of liquid
water equivalent accumulation
• Accurate, real-time measurements of liquid-equivalent snowfall rates need to be made and
reported to pilots and ground operations personnel, not just visibility measurements.
• Accurately predicting deicing hold-over times remains a significant aviation safety factor
Identify the fundamental cause of icing - CORRECT ANSWER✔✔- • Liquid water droplets
Identify the temperatures favorable for rime, clear and mixed icing - CORRECT
ANSWER✔✔- • Rime: small droplets, freeze rapidly cause trapped air, opaque appearance,
forms at temps below -15C
• Clear: larger droplets with little trapped air, forms at 0 to -5 C, equivalent to freezing rain
• Mixed: drop size varies, forms at intermediate temps -5 to -15C
Describe the impacts of icing on aircraft performance (stall speed, fuel consumption, service
ceiling, stall angle, etc) - CORRECT ANSWER✔✔- • Stall speed increases
• Fuel rate consumption increases
• Service ceiling decreases
