Meteorology Cloud Quiz

Aviation Weather Meteorology for Pilots

•  This is a study guide for aviation weather meteorology, specifically for pilots.
•  Chapter 6, Section A covers basic weather theory.

The Atmosphere

•  A mixture of gases surrounds the Earth.
•  The proportions of gases are fairly uniform up to approximately 260,000 feet.
•  The atmosphere is divided into layers defined by various criteria.

Atmospheric Layers

•  Troposphere
•  Tropopause
•  Stratosphere
•  Mesosphere
•  Thermosphere

Troposphere

•  Extends from the surface to approximately 36,000 feet.
•  Higher in summer than winter.
•  Higher at the equator than the poles.
•  Where most weather phenomena occur.

Tropopause

•  The top of the troposphere.
•  Contains the jet stream, turbulence, and the top of thunderstorms.

Stratosphere

•  Extends to approximately 160,000 feet.

Mesosphere and Thermosphere

•  Layers above the stratosphere.

Composition of the Atmosphere

•  Gases
  • Nitrogen (78%)
  • Oxygen (21%)
  • Other gases (1%)
  • Water vapor (0% to 4%)
•  Pollutants

Atmospheric Circulation

•  Air movement is caused by Earth's gravity and rotation.
•  Unequal temperatures on Earth's surface disrupt the equilibrium.

Circulation - Theory

•  Temperature varies with sun exposure, length of time, and angle.
•  Air compensates for unequal heating through convection. 
  • Warmer air rises at the equator.
  • Cooler air sinks at the poles.

Circulation - Reality

•  Three-cell pattern:
  • Hadley Cell
  • Ferrell Cell
  • Polar Cell

Atmospheric Pressure

•  Unequal heating causes changes in air density and circulation.
•  Altimeter settings vary based on location.

On the Weather Maps

•  Isobars connect points of equal pressure.
•  Pressure gradient represents the change in pressure over distance.
  • Close isobars imply a strong gradient (strong winds)
  • Wide isobars imply a weak gradient (lighter winds)

Isobars Identify Pressure Systems

•  High-pressure systems
•  Low-pressure systems
•  Ridges
•  Troughs
•  Cols

Air Flow

•  Air moves from high-pressure areas to low-pressure areas.
•  Pressure gradient force determines wind strength (strong isobars = strong winds).

Coriolis Force

•  Earth's rotation deflects the path of air.
  • Rightward deflection in the northern hemisphere
  • Leftward deflection in the southern hemisphere
  • No deflection at the equator.
  • Greater speed = greater deflection
•  Air moves parallel to isobars as Coriolis and pressure gradient forces equal out.

Frictional Force

•  Friction slows air near the Earth's surface
•  Reduces Coriolis Effect
•  Pressure gradient force is stronger, driving air towards low pressure

Global Wind Patterns

•  Describes global wind patterns, including prevailing westerlies, trade winds, and polar easterlies

Local Wind Patterns

•  Local terrain and water bodies influence wind patterns
• Warmer air rises and cooler air replaces it.
•  Same basic principles apply as global circulation.

Sea Breeze

•  Daytime heating of land.
•  Causes air to rise, and cooler air flows in to take its place. 
•  Return flow above the sea breeze. 
•  Winds at 10 to 20 knots to a height of 1,500 to 3,000 feet AGL.

Land Breeze

•  Land cools faster than water at night. 
•  Reverse of daytime sea breeze. 
•  Smaller temperature contrasts produce less intense winds compared to sea breezes. 
•  Winds at 1,000 to 2,000 feet AGL.

Valley Breeze

•  Mountain slopes heat adjacent air. 
•  Warmed air flows up the valley.
•  Winds at 5 to 20 knots. 
•  Greater winds several hundred feet above the surface

Mountain Breeze

•  Terrain cools at night. 
•  Cooler air flows down the slopes and valley.
•  Winds at 5 to 15 knots.
•  Higher winds (up to 25 knots) are possible

Katabatic Wind

•  Downslope winds. 
•  Stronger than mountain breezes. 
•  Potential for warm or cold winds. 

Cold Downslope Winds

•  Develop over icy or snowy terrains.
•  Develop shallow high-pressure domes, pushing cold air through mountain passes
•  High speeds are possible in narrow canyons and valleys
•  Known by local names in various regions

Warm Downslope Winds

•  Warm air masses move over mountains.
•  Develop low-pressure troughs on the lee side of mountains.
•  As the air descends, it compresses and warms.
•  Rapid temperature increases are possible (up to 20° in an hour)

Atmospheric Stability

•  Stability: resistance to vertical motion.
•  Stable atmosphere = smooth air; unstable = turbulent, rising air, significant cloud development, and hazardous weather.

Adiabatic Heating/Cooling

•  Air expands when rising, cooling adiabatically.
•  Air compresses when sinking, heating adiabatically.

Lapse Rate

•  The rate of temperature decrease with increasing altitude.
•  Average is 2°C (3.5°F) per 1,000 feet.

Water Vapor and Lapse Rate

•  Water vapor is lighter than dry air. 
•  Moist air cools at a slower rate than dry air. 
•  Dry adiabatic lapse rate is 3°C/1000 feet

Temperature and Moisture

•  Combined factors determine atmospheric stability. -Warm moist air = highly unstable -Cold dry air = highly stable

Temperature Inversions

•  Temperature usually decreases with altitude.
•  Inversion occurs when temperature increases with altitude.
•  Typically shallow layers near the surface or at higher altitudes
•  Aids in trapping weather/pollutants

Frontal Inversions

•  Cold fronts: cool air forced under warm air.
•  Warm fronts: warm air rising over cold air

Moisture

•  Significant impacts on flight hazards. -High moisture = greater weather problems. -Low moisture = good weather

State of Moisture

•  Solid, Liquid, Gas
•  Evaporation
•  Condensation
•  Sublimation
•  Deposition
•  Melting
•  Freezing

Latent Heat

•  Heat absorbed or released during phase changes. 
•  Example – 32°F water to 32° F ice

Humidity

•  Moisture in the air.
•  Relative humidity compared to total possible.

Dewpoint

•  Temperature air must cool to become saturated.
•  Used to estimate cloud base levels.

Dew and Frost

•  Surface cools below surrounding air dewpoint. -Dew: above freezing, water vapor condenses -Frost: below freezing, water vapor sublimates to ice

Frost and Airplanes

• Frost spoils airfoil surface, reducing lift and increasing drag. 

Clouds

•  Air cools to the saturation point
•  Condensation and sublimation form visible moisture
•  Clouds and fog (clouds near the surface)

Cooling of Air

•  Lifting
•  Moving over a cooler surface
•  radiation cooling

Clouds and Fog

•  Spread conditions determined by temperature and dewpoint. -Less than 4 degrees F = favorable fog/cloud conditions

Types of Clouds

•  Grouped by altitude. - Low, middle, high
•  Clouds with vertical development

Low Clouds

•  Surface to about 6,500 feet.
  • Stratus: layered, stable, uniform
  • Nimbostratus: rain-producing, widespread
  • Stratocumulus: white, puffy

Fog

•  Low cloud with base close to the ground.
  • Radiation fog: clear, calm, humid nights
  • Advection fog: warm, moist air moves over a cool surface
  • Upslope fog: moist, stable air forced up sloping terrain
  • Steam fog: cold, dry air moves over warmer water

Middle Clouds

•  6,500 to 20,000 feet AGL.  - Altostratus: flat, dense, uniform color, moderate icing possible - Altocumulus: patchy, uniform appearance, frequent from altostratus breaking up, mild turbulence.

High Clouds

•  Above 20,000 feet AGL. 
  • Cirrus: wispy, stable air
  • Cirrostratus: thin, white, long bands or sheets
  • Cirrocumulus: white, patchy, like cotton, mild turbulence

Clouds with Vertical Development

•  Cumulus: fair weather, little icing, mild turbulence.
•  Towering cumulus: unstable air, heavy turbulence, icing, pre-thunderstorms.
•  Cumulonimbus: thunderstorms, heavy turbulence, icing, hail

Precipitation

•  Water in liquid or solid form from the atmosphere to the ground.
•  Aviation problems: impacts on visibility, engine performance, braking, and wind
• Precipitation causes: saturation, condensation and freezing

Precipitation Causes

•  Condensation/deposition
•  Coalescence
•  Slow and inefficient

Precipitation Causes continued

• Super-cooled water droplets and water vapor lead to rapid ice crystal growth

Types of Precipitation

•  Drizzle
•  Rain
•  Virga
•  Freezing drizzle
•  Freezing rain
•  Ice pellets
•  Hail
•  Snow
•  Snow grains
•  Fallstreaks (mare's tails)

Airmasses

•  Large bodies of air
•  Uniform temperature and moisture
•  Covers hundreds of miles

Source Regions

•  Polar
•  Tropical
•  Continental
•  Maritime

Stable Air Characteristics

•  Smooth
•  Layered clouds
•  Restricted visibility

Unstable Air Characteristics

•  Cumuliform clouds
•  Showers
•  Turbulence

Modification of Air Masses

•  Airmasses adopt characteristics of the region they move over.
•  The degree of change depends on speed, nature of the region, temperature differences, and depth of the airmass.

Warming from Below

•  Causes vertical air movement.
•  Causes instability. 
•  Lake effect.

Cooling from Below

•  Vertical movement inhibited.
•  Increased air stability.
•  Fog can develop.
•  Temperature inversions are possible.

Fronts

•  Boundaries between air masses
  • Cold front
  • Warm front
  • Stationary front
  • Occluded front

Discontinuities

•  Identifying frontal passage: temperature/pressure/wind changes

Cold Front

•  Characteristics, including cloud formations, typical weather conditions.

Cold Front Weather

•  Prior, during, and after cold-front passage conditions for clouds, precip, winds, visibility, temperature, and dewpoint. 

Warm Front

•  Characteristics, including cloud formations, typical weather conditions
•  Weather changes: prior to, during, and after warm-front passage

Stationary Front

•  Relative balance of opposing air masses, extended duration.
•  Mix of cold and warm front characteristics.

Occluded Front

•  Fast-moving cold front catches up to slower-moving warm front.
•  Two types of occlusions
  • Cold front occlusions
  • Warm front occlusions

Occluded Fronts continued

• Characteristics, typical weather conditions.

Occluded Front Weather

•  Prior, during and after passage conditions for clouds, precipitation, visibility, winds, temperatures, and dewpoint

Thunderstorms

•  Needed for development: unstable conditions, lifting force, high moisture levels
•  Two types: airmass and severe
•  T-storm types: single-cell, super-cell, multi-cell, squall line, frontal
•  Life cycle: cumulus, mature, dissipating.

T-storm Hazards - Turbulence

•  Cumulonimbus clouds are highly turbulent.
•  Turbulence between updrafts and downdrafts, and low-level turbulence from downdrafts spreading. 

T-storm Hazards - Lightning

•  Lightning always associated with thunderstorms.
  • In-cloud
  • Cloud-to-cloud
  • Cloud-to-ground
  • Cloud-to-clear air
•  Risks of damage to aircraft or personnel

T-storm Hazards - Hail

•  Can happen at all altitudes, either inside or outside of clouds. 
•  Potential for damage to aircraft

T-storm Hazards - Tornadoes

•  Funnel cloud descending from cloud base, impacting land or water.
•  Strong winds possible

Turbulence

•  Turbulence in and near thunderstorms, low-level, clear air, mountain wave. 
•  Pilot actions during turbulent conditions.

Low-level Turbulence

•  Below 15,000 ft.
•  Associated with surface heating or friction
•  Four types: mechanical, convective, frontal, wake. 

Mechanical Turbulence 

•  Obstacles interfering with wind flow
•  Edities around obstacles (trees, buildings).
•  Downwind of obstructions

Convective Turbulence

•  Thermal turbulence during daytime
•  Warmer surface heats air
•  200 to 2000 FPM updrafts.
•  Towering cumulus clouds signify convective activity.
•  Stable layer above can produce haze/dust
•  Potential for turbulence.

Frontal Turbulence

•  Ahead of cold fronts, moderate to severe. 
•  Up to 1000 feet per minute vertical. 

Wake Turbulence

•  Wingtip vortices generated during lift.
•  Intensity depends on aircraft weight, speed, and configuration
•  Large planes at low speeds generate significant wake turbulence. 

Wake Turbulence continued

•  Vortices sink below the flight path. 
•  Most dangerous just after/below the aircraft. 
•  ATC separation procedures for avoiding wake turbulence.

Avoiding Wake Turbulence

•  Recommended procedures: following distances behind aircraft, considerations for direction/speed differences, recommended flight paths based on wind conditions

Jet Engine Blast

•  Hazard for small planes behind jet aircraft
•  Stay several hundred feet away.

Clear Air Turbulence (CAT)

•  Above 15,000 feet.
•  Often occurs around jet streams or high-wind areas near the tropopause
•  No immediate visual warnings.

Mountain Wave Turbulence

•  Stable air over mountains
  • Smooth over the windward side 
  • High winds perpendicular to ridgelines create disturbance. 
  • Significant wave extension downwind. 
•  Upper air turbulence as air flows around high peaks

Mountain Wave Turbulence continued

•  Signature clouds form as evidence.  -Rotor clouds
  • Lenticular clouds
•  Avoiding mountain wave turbulence 
• Approach at angles (45 degrees)
• Consider prevailing wind conditions.

Wind Shear

•  Sudden shift in speed or direction. 
•  Frontal systems, thunderstorms, temperature inversions, clear air turbulence, jet streams are associated. 
•  Potential dangers are loss of control/altitude and increased difficulty for aircraft.

Microburst

•  Strong downdraft.
•  Horizontal spread is less than a nautical mile often. 
•  Significant vertical spread 

Low-Level Wind Shear Alert Systems (LLWSAS)

•  Wind sensors at airports detect wind differences.
•  Computer analysis detects wind shear.
•  ATC gives alerts and information.

Terminal Doppler Weather Radar (TDWR)

•  Radar for higher resolution of thunderstorms.

Visual Indications of Wind Shear

•  Rain shaft
•  Virga
•  Dust ring on ground

Icing

•  Visible moisture necessary for structural icing.
  • Freezing rain generates accumulation quickly.
•  Aircraft surface at 0° Celsius or below increases icing risk.
  • Effects in terms of lift and drag
•  Icing on aircraft can drastically reduce performance and cause risk of control issues/damage

Types of Ice

•  Rime ice – typically white
  • Forms in stratus clouds
  • Supercooled water droplets
•  Clear ice – typically clear
  • Freezing rain under a warm front inversion.

Types of Icing continued

•  Mixed ice  -Combo of rime and clear ice
• Important to consider for pilots in icy weather to prevent damage/loss of control in flight.

Restrictions to Visibility

•  Haze: fine particles, stable atmosphere, decreased visibility compared to stable conditions above
•  Smoke: airborne combustion particles, orange/reddish colors
•  Smog: combo of fog and smoke, trapped in valleys or regions of stable air, reduced visibility
•  Dust: fine soil particles, strong winds, unstable atmosphere, poor visibility

Volcanic Ash

•  Highly abrasive. 
•  Impacts windscreens and landing lights. 
•  Clogs pitot/vent systems, potentially damaging control surfaces
•  Engines are at greater risk of damage.