WX301 || 100% 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 answers • 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 answers • 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
answers • 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 answers • 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 answers • 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 answers • 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 answers • 540dm (decameters, height) is a good dividing
line between rain and snow
Define what is meant by ground icing "hold-over" times. correct answers • 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 answers • 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 answers • Liquid water droplets
Identify the temperatures favorable for rime, clear and mixed icing correct answers • 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 answers • Stall speed increases
• Fuel rate consumption increases
• Service ceiling decreases
• Stall at lower angle of attack than normal
• Friction increases
Describe the effects exposure time on ice buildup. correct answers • Longer exposure time =
more icing
Describe the relationship between wing size and performance degradation due to icing correct
answers • Thin wings collect more ice than large wings
• A large transport aircraft will build proportionally less ice than a small aircraft going through
the same icing region
Identify the most critical aerodynamic factor in icing accumulation correct answers • Icing on
wings can disrupt the flow over the tops of the wings and decrease lift; stall at a lower angle of
attack
7. Describe super-cooled large droplets (SLDs) and runback icing as well the hazards they pose
correct answers • Small ice crystals form within droplet which may cause the entire drop to
freeze
• Causes a large freezing cluster of ice to form on a surface
Identify the favored areas for icing near mountains correct answers • Worst icing near the peaks
and windward side
Describe the range of temperatures and relative humidities where icing is most commonly
observed. correct answers • Most common between 0C and -40C; in-cloud humidity 70-100%
• Mountains (orographic lift), cumulus clouds and thunderstroms (strong updrafts), areas of sleet
• NW of the low, ahead of the warm front, and near the cold front
Explain why the true state of the atmosphere can never be known exactly and can only be
approximated by assimilating all available data correct answers • 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 answers • 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
answers • 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 answers • 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 answers • 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 answers • 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 answers • 540dm (decameters, height) is a good dividing
line between rain and snow
Define what is meant by ground icing "hold-over" times. correct answers • 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 answers • 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 answers • Liquid water droplets
Identify the temperatures favorable for rime, clear and mixed icing correct answers • 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 answers • Stall speed increases
• Fuel rate consumption increases
• Service ceiling decreases
• Stall at lower angle of attack than normal
• Friction increases
Describe the effects exposure time on ice buildup. correct answers • Longer exposure time =
more icing
Describe the relationship between wing size and performance degradation due to icing correct
answers • Thin wings collect more ice than large wings
• A large transport aircraft will build proportionally less ice than a small aircraft going through
the same icing region
Identify the most critical aerodynamic factor in icing accumulation correct answers • Icing on
wings can disrupt the flow over the tops of the wings and decrease lift; stall at a lower angle of
attack
7. Describe super-cooled large droplets (SLDs) and runback icing as well the hazards they pose
correct answers • Small ice crystals form within droplet which may cause the entire drop to
freeze
• Causes a large freezing cluster of ice to form on a surface
Identify the favored areas for icing near mountains correct answers • Worst icing near the peaks
and windward side
Describe the range of temperatures and relative humidities where icing is most commonly
observed. correct answers • Most common between 0C and -40C; in-cloud humidity 70-100%
• Mountains (orographic lift), cumulus clouds and thunderstroms (strong updrafts), areas of sleet
• NW of the low, ahead of the warm front, and near the cold front
