Ground lesson 6 - Weather theory

Weather theory

• The atmosphere 
• Composition of air
• 78% nitrogen
• 21% oxygen
• 1% other
• Troposphere 
• Surface to 23000 - 65000
• Height varies with the season and position on the globe
• Higher where it's warmer 
• Decrease in temperature with altitude
• Almost all weather occurs here
• Stratosphere 
• Temperature increase with altitude
• This is because the ozone layer is placed here and absorbs solar radiation
• About 80000 feet
• Mesosphere / thermosphere
• Almost no gases
  
  
  

• Atmosphere circulation
• The uneven heating of the earth’s surface is the driving force of weather
• Equator receives the most sunlight, poles the least

• Pressure and wind patterns
• Lines of equal pressure are called Isobars
• Labeled in millibars
• Wind is created as air flows from high pressure to low pressure
• The closer the isobars, the stronger the wind
• Coriolis force
• If it wasn't for Coriolis force, the wind would blow straight from high to low pressure
• Coriolis force is strong aloft and makes the wind flow parallel to the isobars
• Friction at the surface weaken Coriolis force and makes the wind cross at an angle 
• For this reason, wind tends to shift directions below 2000 feet AGL
  
• High pressure
• Wind flows clockwise, outbound, and downward 
• Wind speed is dependent on gradient force
• The stronger the pressure gradient, the stronger the wind
• Shorter distance between isobars also means stronger winds 
• Associated weather (generally)
• Dissipating clouds, or clear skies
• Good visibility
• Calm or light winds
• Low pressure
• Wind flows counterclockwise, inbound, and upward
• Wind speed is dependent on gradient force
• The stronger the pressure gradient the stronger the wind
• Usually close isobars
• Associated weather
• Clouds
• Steady precipitation
• Poor visibility 
• Gusty winds and turbulence 

• Land breeze
• During night, land cools down faster than water
• Warm air over water rises and gets replaced by cold air over land
• Sea breeze
• During day, land is heated faster than water
• Warm air over land rises and is replaced by colder air from the sea
  
  
• Valley winds 
• During day, mountain slopes get heated up faster than the flat valley 
• Warm air over the slope rises and is replaced by colder air from the valley
• Mountain winds 
• During night, mountain slopes cools down faster than the flat valley
• Warm air rises and is replaced by cold air from the mountain slopes
• Katabatic winds
• Any wind blowing down an inclined slope 
• Mountain winds is one type of katabatic winds
• Cold dense air blowing down slope to replace warm air in front
• May have a dramatic effect on temperature

Weather patterns

• Stability 
• Resistance of vertical motion in a parcel of air 
• Stability of an air mass is determined by its temperature and moisture 
• Warm and moist air is unstable, cold and dry air is stable
• Stability then determines what kind of weather is developing

  	Stable air	Unstable air
  Clouds	Layered, fog, stratus	Cumulus
  Precipitation	Wide spread,steady	Heavy rain showers
  Icing	Rime,clear, freezing rain	Moderate/heavy clear ice
  Visibility	May be restricted for long periods	Good when no precipitation
  Turbulence	No, or little	Heavy with gusty winds
  Other	Frost, dew, temp. Inversion	Thunderstorms

• Temperature inversion 
• Normally temperature decreases with altitude
• In a temp. Inversion, the temp will increase before decreasing
• Moisture 
• The fuel of weather
• Without moister, weather would not change so dramatically 
• 3 physical states → solid, liquid, gas
• Latent heat of condensation 
• The heat transaction , in the change of state
• As heat is taken in, it cools down the source it comes from
• Relative humidity 
• The actual amount of moisture in the air compared to the amount that it can hold
• Higher temperature holds more moisture
• Dewpoint
• Dew Point is the temperature that air must be cooled to become saturated 
• Reveals how close the air is to saturation 
• When temperature and dewpoint are the same, the air is saturated 
• Higher temp - dewpoint spread → low humidity
  Low temp - dewpoint spread → high humidity
  
  
• Precipitation
• Happens when condensed water droplets grow to a size where the atmosphere can no longer support their weight
• During low relative humidity, rain may evaporate before it reaches the ground
• Freezing rain
• Water cooled below freezing that still remains liquid 
• Will freeze upon impact
• Ice Pellets 
• Freezes as they fall and bounce of your aircraft rather than freeze to it
• Normally indicates the existence of freezing rain and warmer air at higher altitudes
• Hail
• Build up in strong vertical current
• Drop when the air current cannot support them anymore
• Clouds 
• Physical visible weather phenomenon 

• Formation requirements 
• Moisture
• Condensation nuclei
• Air must get saturated - temp. Cooled down to dewpoint
• Types of cooling process
• Convective current
• Result in different surface temp.
• Cool heavy air forces light warm air up
• Continues as long as uneven surface temp continues
  
  
• Orographic lifting
• Result of wind blowing warm air up side of a ridge into cooling surrounding air
• Radiation cooling
• Cool ground will cool surrounding warm air
• Best on calm clear nights
• Clouds will reflect heat back 
• Surface cooling
• Terrestrial radiation cools the ground due to lack of sun
• Warmer air contacts with colder surface and gets saturated 
• Can create → advection fog, sea breeze, and land breeze
• Frontal lifting 
• Cold front pushes warm air up
• Cloud families 

Name	Means	Air is	Association
Stratus	layer	Stable	Smooth air, low vis
Cumulus	Accumulation	Unstable	Turbulence, good vis
Nimbo	Rain	Saturated	Turbulence, downdraft,precipitation, icing
Cirrus	ice	Very cold	Restricted visibility
Alto	Altitude	X	Cloud in middle family

• Other cloud types 
• Rotor clouds 
• Forms on lee side of mountain peak due to mountain waves
• Heavy turbulence
• Lenticular clouds 
• Lense shaped clouds 
• Stations at high altitudes when little movement on lee side of mountain peak
• Stable air flowing around and air forced up and cooled to dewpoint
• Fog
• Basically a low cloud classified in the way it’s formed 
• Radiation fog
• Forms over low lying flat surfaces 
• On clear, calm humid nights 
• Surface is cooled at night
• Air gets cooled, saturated, and creates fog
• Common in mornings, if rain in evening/night
• Created in stable air with high pressure systems 
• As temperature increases, fog gets burned off
• Advection fog
• Caused when warm humid air flows over a cold surface
• Cooled from below and gets saturated
• Most common along coast lines
• More extensive and longer lasting than radiation fog
• Steam fog
• Often called steam smoke
• Cold dry air moves over warm water
• Water evaporates and saturates the air forming fog
• Low level turbulence and icing
• Orographic fog 
• Also called up-slope fog
• When air is lifted up a mountain slope 
• Cools down and saturates
• Precipitation induced fog 
• Usually forms with warm fronts, also slow moving cold fronts
• When warm air falls down into cold air
• Haze 
• Dust,smoke,and other dry particles obscure the clarity of the sky

• Air masses
• Large body of air, with particular characteristics in temperature, moisture, lapse rate, and pressure
• Source region
• The area over which the air-mass gets its characteristics
• Characteristics/classifications 
• To identify temperature 
• Polar → Cold
• Tropical → Hot
• To identify moisture content
• Continental → Dry
• Maritime → moist
• Air mass modification 
• Air-masses are modified as they move out of their source region 
• Depends on several factors 
• Speed
• Nature
• Temperature
• Moisture
• Warming from below
• Air-mass moves over warm surface
• Vertical movement of air develops 
• Stability decreased 
• Cumulus clouds, turbulence and good visibility
• Cooling from below 
• Air-mass moves over cold surface
• Stability of air is increased 
• Smooth air, low stratus clouds or fog may develop
• Can create a temp. Inversion
• Fronts 
• Cold front 
• When a cold air-mass overtakes a warm air-mass
• Cold air-mass has a greater density → warm air-mass is lifted 
• Associated weather 
• Cumulus clouds
• Thunderstorms 
• Shower precipitation 
• Good visibility/ clear skies afterwards
• Warm front
• When a warm air-mass overtakes a cold air-mass
• Warm air is less dense and is lifted over cold air 
• Associated weather
• Poor visibility 
• Wide area of steady precipitation 
• Freezing rain
• Stratus clouds
• Stationary front
• Neither air-mass is moving or overtaking 
• Can last for many days
• Weather likely to be a mix of both fronts
• Cold front occlusion 
• Fast moving cold front is cooler than the air ahead of the slow moving warm front 
• Cold air replaces the cool air at the surface and forces the warm air aloft
• Warm front occlusion
• Develops when the air ahead of the warm front is colder than the fast moving cold front

Weather hazards

• Towering cumulus 
• When air becomes unstable, convection current can cause cumulus clouds tops to go beyond the low cloud altitude into the middle cloud altitude
• Thunderstorm
• The ultimate manifestation of atmosphere instability 
• Can reach heights of 6000ft
• Conditions necessary for formation of a thunderstorm
• Lifting force
• Unstable airmass
• Moisture
• Condensation nuclei
• Formation and life cycle - thunderstorms have three predominant stages
• Cumulus stage
• Building stage of the thunderstorm
• Strong updrafts, rapid vertical growth
• Occasional lighting 
• Last about 10 minutes
• Mature stage
• The classic thunderstorm
• Rain begins
• Updrafts and downdrafts 
• Severe weather such as 
• Hail
• Turbulence 
• Heavy rain
• Wind shear
• Lighting
• Dissipating stage
• Updrafts weaken
• Downdrafts become predominant 
  
  
• Different types of thunderstorms
• Isolated (single cell)
• Single thunderstorm
• Going through the life cycle without producing new thunderstorms
• A thunderstorm can be embedded within clouds and cannot be seen
  
  
• Multi-cell
• A thunderstorm that often forms in clusters within numerous cells in various stages of development merging together
• Multi-cells can last for several hours producing
• Large hail
• Damaging winds
• Isolated tornadoes
• Floods
• Super-cell
• Special kind of single cell thunderstorms
• Responsible for nearly all of the significant tornadoes in the US
• Also known to produce extreme winds and flash floods
• They are characterized as a rotating updraft
• Squall-line
• Most hazardous to aircraft 
• Often forms ahead of fast moving cold front
• Single cell thunderstorms will form in a line which can extend for hundreds of miles
• Persists for many hours and produces damaging winds and hail
  
  
  
• Turbulence
• Thunderstorm turbulence
• develops when air currents change direction or velocity rapidly over a short distance
• Clear air turbulence 
• Usually encontered above 15,000 feet, but can take place at any altitude
• Often present with no visual warning 
• Often developed near or in the Jet stream
• Mountain wave turbulence 
• When stable air crosses a mountain barrier, the air is smooth on the windward side
• Forms layers or waves when going over the ridge and the wind is 40Kt or higher
• May extendo for 100 miles and as high as 100,000 feet
• Low level turbulence 
• Mostly due to surface heating
• Includes mechanical, convective, frontal, and wake turbulence
• Mechanical turbulence
• When obstacles such as buildings and terrain interfere with the wind flow
• Convective turbulence 
• Caused by currents, or thermals, which develop in air heated by contact with the warm surface
• Frontal 
• Occurs in the narrow zone just ahead of a cold front where updrafts can reach 1000 f.p.m
• Wake turbulence
• As the wings generate lift, they produce wing tip vortices (wake turbulence)
• Intensity depends on aircraft weight, speed, and configuration
  
  
• Wind shear
• A sudden, drastic shift in wind speed and/or direction
• Associated with temp. Inversion, jetstream,thunderstorm,frontal inversion
• May cause loss of lift, large changes in airspeed/altitude
• Use caution during max performance operations and landings
• Use faster than normal approach

• Microburst
• Localized column of sinking air
• Producing damaging winds in all directions on the surface
• Short lasting, up to 15 minutes
  
• Avoidance procedure
• Do not fly within 20 nm of a storm
• Precautionary landing
• ICING
• Induction icing / carb ice
• Icing of air intake system
• Impact Ice: Formed by the impact of moist air at temperatures between -10°C and 0°C on air scoops, throttle plates, etc
• Fuel Ice:  Fuel is introduced into the carburettor when the moisture content of the air freezes as a result of the cooling caused by fuel vaporisation
• Throttle Ice: formed at or near a partly closed throttle valve  The water vapour in the induction air condenses and freezes due to the venturi effect cooling the air as it passes the throttle valve
• Structural icing 
• Ice builds on any exposed surface of the aircraft
• Rime ice
• Encountered from stratus clouds 
• Tiny ice droplets hit the aircraft surface and freeze instantly 
• It will change the profile of your blade
  
• Clear ice
• Encountered from cumulus clouds or from freezing rain
• Water droplets slowly freeze
• Clear and heavy
• Changes profile and weight of blade
• Mixed ice
• Combination of rime and clear ice
• Frost
• Most common cause
• Airframe temp is below freezing
• Moist air
• Freezing rain
  
  
• Restricted visibility
• Haze
• Concentration of a very fine dry particle
• Smoke
• Suspension of combustion particles in air
• Smog
• Combination of fog and smoke
• Dust
• Fine particles of soil suspended in air
• Volcanic ash
• Can pit the aircraft's windscreen and landing lights to the point they are rendered useless
• Under severe conditions, they can clog pitot and ventilation systems as well as damage aircraft control surfaces .