Aviation and Snow Sports Weather: Clouds, Visibility, and Hazards

Posted on Feb 1, 2025 in Food Science and Technology

Flying Weather: Clouds and Ceiling

Normal Clouds:

  • Convective/Cumuliform clouds (Cu): Look like stacked cotton balls, associated with updrafts.
  • Layer/Stratiform clouds (St): Look like sheets/blankets, extending horizontally.
    • Nimbostratus: Precipitating.
    • Stratus: Not precipitating.

From increasing height: stratus/nimbostratus, altostratus/cumulus, cirrostratus/cumulus, cirrus.

Special Clouds:

  • Clouds in unstable air aloft:
    • Castellanus: Small castle turrets, indicate potential thunderstorms.
    • Billow (K-H wave): Indicate wind shear and clear-air turbulence (CAT).
  • Clouds associated with strong winds across mountains:
    • Lenticular (mountain-wave): Indicate vertical wind oscillations and possible mountain-wave turbulence.
    • Rotor: Indicate severe turbulence at low altitudes due to mountain waves.
    • Banner: Form on the downwind side of mountain peaks, indicating strong turbulence.
  • Clouds formed by extra heat, updrafts, or turbulence:
    • Pyrocumulus: Form over forest fires and volcanoes.
    • Pileus: Form over fast-growing cumulus clouds.
    • Fractus/Scud: Form in turbulent humid air near the ground, indicating high humidity.
  • Clouds formed by humans:
    • Fumulus: Water-droplet clouds over cooling towers.
    • Contrails: Aircraft condensation trail, indicating turbulent wing-tip vortices.

Sky Coverage: Measured in eighths (oktas): 0/8=clear, 1-2/8=few, 3-4/8=scattered, 5-7/8=broken, 8/8=overcast, unknown=obscured.

Cloud Ceiling: Height (AGL) of the lowest cloud base below 20,000 ft (6,000 m) covering more than half the sky.

Visibility Measurement

  • Transmissometers: Measure light dimming over a known path.
  • Ceiling Measurement: Laser ceilometers, ceiling balloons, pilot reports, and weather-observer estimates.
  • Runway Visual Range (RVR): Automated sensors near runways.
  • Vertical Visibility: Used when ceiling is indefinite.

Visibility

  • Horizontal Visibility: How far you can see a black object/bright light.
  • Runway Visual Range: How far a pilot can see along a runway centerline.
  • Vertical Visibility: Height of the cloud base above ground.

Flight Regulations

  • Visual Flight Rules (VFR): Fly by looking out the window, need good visibility, stay out of clouds.
  • Visual Meteorological Conditions (VMC): Weather good enough for VFR.
  • Rules for VFR: Ceiling >3000 ft AGL and visibility >5 Statute Miles.
  • Marginal Visual Flight Rules (MVFR): VFR allowed but with bad visibility.
  • Rules for MVFR: Ceiling between 1000 ft–3000 ft, and/or visibility between 3&5 SM.
  • Instrument Flight Rules (IFR): Navigate using instruments, not looking out the window.
  • Rules for IFR: Ceiling <1000ft and/or Visibility <3 SM.

Fog

Fog is a cloud that touches the ground.

  • Formation:
    1. Water added to unsaturated air.
    2. Unsaturated air cooled to dew-point temperature.
  • Freezing Fog: Supercooled liquid water droplets that freeze instantly.
  • Ice Fog: Tiny ice crystals.
  • Types:
    • Radiation Fog: Forms during clear, calm nights.
    • Advection Fog: Forms when humid air blows over a colder surface.
    • Upslope Fog: Forms when wind blows air against a hill slope.
    • Precipitation/Frontal Fog: Formed by evaporation from warm rain.
    • Steam Fog: Forms when cold air moves over warm humid surfaces.

Obscurations

Mist [BR], fog [FG], smoke (FU), volcanic ash (VA), sand (SA), haze (HZ), spray from breaking waves (PY), and widespread dust (DU).

High Altitude Flight

  • Above 13,000 ft: Use oxygen mask to avoid hypoxia.
  • Above 40,000 ft: Use a mask with pressurized oxygen.
  • Above 62,000 ft (Armstrong limit): Bodily fluids begin to boil.

Standard Air/Density/Winds

  • Density Altitude: Altitude the aircraft feels, affected by temperature and pressure.
  • Lower air density: Decreases aircraft performance.
  • Crosswind Graph: Interpreting wind speeds and directions.

Updrafts For Soaring

  • Mountain Waves: Occur when stable air flows perpendicular to a mountain.
  • Wind Shear: Change of wind speed/direction with altitude.
  • Anabatic Winds: Warm air moving up mountain slopes during daytime.
  • Katabatic Winds: Cold air draining down mountain slopes at night.
  • Orographic Thunderstorms: Form when anabatic cumulus clouds turn into thunderstorms.
  • Thermals: Warm air rising, creating cloud streets.

Layers in Standard Atmosphere

Troposphere, Stratosphere, Mesosphere, Thermosphere. Temperature decreases in troposphere, increases in stratosphere, decreases in mesosphere, increases in thermosphere.

Atmospheric Stability

  • Static Stability: Depends on temperature layering.
  • Adiabatic Lapse Rate: Temperature change for vertically moving air parcels (-10ºC/km).
  • Statically Stable Conditions: Occurs at night under clear skies, non-turbulent air.
  • Statically Neutral Conditions: Typically occurs in overcast conditions, slight wind shear can create turbulence.
  • Statically Unstable Conditions: Occurs in sunny days over land, thermals occur.

Thunderstorm Indices

  • CAPE (Convective Available Potential Energy): Measure of buoyant energy.
  • K Index: Views temperature and dew-point humidity.
  • Sounding: Weather conditions at different heights.
  • Rawinsondes: Weather balloons.

Turbulence

  • Turbulence: Random gust fluctuations of wind.
  • Eddies: Swirls of air motion.
  • Formation: Convection, wind shear, obstacle turbulence.
  • Mountain Waves: Froude Number (Fr) determines wave intensity.
  • Clear Air Turbulence (CAT): Wind shear outside thunderstorms, near jet streams.
  • K-H waves: Occur when warmer air moves faster than cooler air below.
  • Atmospheric Boundary Layer (ABL): Bottom 300 m to 4 km of the atmosphere, often weak to moderate turbulence.
  • Obstacle/Mountain Wake Turbulence: Occurs downwind of large obstacles.

Icing

  • Supercooled Droplets: Liquid between -40ºC to 0ºC, freeze instantly.
  • Clear Ice: Larger raindrops freeze slowly, difficult to remove.
  • Rime Ice: Smaller droplets freeze instantly, brittle.
  • Mixed Ice: Mixture of clear and rime ice.

Fronts

  • Front: Boundary separating air masses.
  • Wind Shift: Change in wind direction at a front.
  • Frontogenesis: Birth of a new front.
  • Frontolysis: Death of an old front.
  • Squall Line: Line of thunderstorms not along a front.
  • Frontal Zone: Transition zone between warm and cold fronts.
  • Dry Air: Boundary between dry and humid air.
  • Frontal Hazards: Cold fronts (cumuliform clouds), warm fronts (stratiform clouds), stationary fronts, occluded fronts, dry lines.
  • Frontal Hazards to Pilots: Clouds, precipitation, poor visibility, strong winds, thunderstorms, supercooled rain, heavy snowfall.
  • Reading Front Diagrams: Warm front (half circle), cold front (triangle), stationary front (circle and triangle on opposite sides), occluded front (circle and triangle on same side), squall line (line of thunderstorms).

Thunderstorms/Tornado Cells

  • Stages of Thunderstorms: Cumulus, Mature, Dissipating.
  • Basic Storms: Single-Cell, Multicell, Orographic, Mesoscale Convective Systems.
  • Mesoscale Convective Systems: Squall line, Bow echo, Mesoscale Convective Complex, Mesoscale Convective Vortex.
  • Supercells: Low Precipitation, Classic Supercell, High Precipitation.
  • Thunderstorm Hazards: Convective Turbulence, Downbursts, Microbursts, Gust Fronts, Low Level Wind Shear, Lightning, Precipitation Static, Hail.
  • Avoiding Thunderstorm Hazards: Stay > 20 nautical miles away.
  • Tornadoes: Violently rotating columns of air, avoid supercell thunderstorms.

Aviation Weather Services

Use METAR and TAF.

Snow Sports Weather

Winds on Pressure-Level Maps

Half barbs (10 km/h), full barbs (20 km/h), triangles/flags (100 km/h).

Isobars

Connect locations with same pressure.

Interpreting Moisture Pressure-Level Maps

  • Relative Humidity: Measures moisture, condensation begins at 100% RH.
  • Low Pressure (Cyclone): Associated with bad weather.
  • Low Pressure Centers: Areas of low pressure surrounded by higher pressure.
  • Low Pressure Troughs: Areas of lower pressure not completely surrounded by high pressure.
  • High Pressure (Anticyclone): Accompanies good weather.
  • Dangers of High Pressure: High solar radiation, flat light.

Identifying Fronts

Transition zones where warm air meets cold air.

  • Cold Front Temp. Trends: Colder with time, wind chill.
  • Precipitation: Cumulonimbus clouds, convective precipitation.
  • Visibility: Reduced by winds and precipitation.
  • Warm Fronts Temp. Trends: Around 0ºC, snow changes to rain.
  • Winds: East ahead, southerly behind.
  • Precipitation and Clouds: Weather deteriorates, snow gets wetter.

Predicting Large-Scale Surface High Winds

Close isobars signify a strong pressure gradient.

Predicting Areas of Light Winds

Far apart isobars signify a weak pressure gradient.

Weather Resources

Past (avalanche blog), Current (webcam, satellite imagery), Future (forecast maps).

Satellite Imagery

  • Visible: Sunlight illuminates cloud tops, not useful at night.
  • Infrared: Shows cloud top temperature, can infer storm locations.
  • Water Vapor: Helps determine large-scale movement and circulations.

Orography/Interpolation

Cold Air Pooling

Cold air drains into valleys, forming clouds or fog.

Effects of Cold Air Pooling

Misleading temps, fogs at lower elevations, dangerous night skiing.

Diurnal Process

Slope flow induced by temperature changes.

  • Daytime slope flows: Upslope flow or anabatic winds.
  • Nighttime slope flows: Downslope winds (katabatic winds).

Temperature Inversion

Temperature increases with altitude in the troposphere.

  • How inversions occur: Sinking due to high pressure, radiative cooling of the ground.
  • Valley Cloud/Fog Formation: Moisture condenses in the air within the valley.
  • Valley cloud and fog dissipation: Dissipates from bottom up.

Orographic Uplift

Air lifts up and over mountains, cools adiabatically.

Lee Shadowing

Drier air on the leeward side of the mountain.

Wind Exposed Locations

Mountain peaks and ridgetops.

The Venture and Bernoulli Effect

Convergence of wind into a constricted area accelerates flow.

Wind Sheltered Areas

Mountain valleys and treed areas.

Determining Temperature at Your Elevation

  • Vertical Interpolation: Estimating quantity between locations.
  • Free air temperature: Temperature at a particular level of the atmosphere.
  • Dry adiabatic conditions: Temperature decreases 10C per 1000m.
  • Wet adiabatic conditions: Temperature decreases 6C per 1000m.

Factors that Affect Surface Heating and Cooling

  • Sun angle: Lower in polar latitudes, perpendicular in the equator.
  • Cloud Cover: Reduces incoming solar radiation.
  • Wind Speed: Reduces solar radiative heating effect.

Arctic Outflow/Wind Channels

  • Arctic air-mass: Cold, dense, shallow, and stable.
  • Dangers to skiers: Bitterly cold air, outflow winds.
  • Wind Chill: Air combines with cold temps to make it feel colder.
  • Gap winds: Occur when prevailing wind is perpendicular to mountain range.

Snow Conditions

  • Rain-snow line: Elevation at which precipitation transitions from rain to snow.
  • Snow density: Amount of ice and liquid water per volume.