Meteorology 1 Flashcards
Atmosphere Composition
- 78% Nitrogen
- 21% Oxygen
- 0-3% Water vapour
- 0.03% carbon Dioxide
- Argon, Helium, Neon, other rare gases
- Condensation nuclei allows water vapour to condense
Water Vapour
- Most important in context of weather
- Only component of atmosphere that is found in all 3 states under normal conditions
Changes of State
- Heat is released or absorbed when water changes state
- Melting, Evaporation, Sublimation (Heat Absorbed)
- Freezing, Condensation, Deposition (Heat Released)
Moisture Content
- Ability for atmosphere to hold moisture increases with temperature
- Weather occurs due to water vapour in the air and the changes of state it goes through
- Main sources are evaporation from oceans and lakes and transpiration from vegetation
Properties of the Atmosphere - Expansion
- As air rises, it expands and cools
- Water Vapour may condense to form clouds or precipitation
Properties of the Atmosphere - Compression
- Sinking air contracts and is heated
- Clouds dissipate
Gas Law
- Cold air is more dense and tends to sink
- Warm air is less dense and tends to rise
Properties of the Atmosphere - Dewpoint
- Temperature at a given pressure to which air must be cooled to cause saturation
- When air is saturated, water vapour will condense to form clouds or fog
- Difference between temperature and dewpoint is called the spread, provides likelihood of fog forming
- Dewpoint lapse rate is 0.5º/1000 ft (increase in spread)
Properties of the Atmosphere - Relative Humidity
- Expresses water vapour content of air as percentage of maximum possible at current temperature (saturation)
- 60% relative humidity means the air is 60% saturated
- At 100% water vapour will condense to water droplets and form clouds or fog
Divisions of the Atmosphere - Troposphere
- Varies in height from ground to 28,000’ over poles or ground to 54,000’ over equator
- Most weather occurs in troposphere
- Pressure, temp, and density all decrease with altitude
- Tropopause at top of troposphere
- Temp decrease stops and remains steady at -56ºC
Divisions of the Atmosphere - Stratosphere
- Up to 160,000’
- Temperature remains constant at -56ºC in lower portion
- Temperature rises to near -15ºC due to ozone
- Clouds are rare and visibility is excellent
Divisions of the Atmosphere - Mesosphere
- Temperature decreases to approx. 275,000’
- The mesopause is the point where temp begins to increase again
Divisions of the Atmosphere - Thermosphere
- Temperature Increases to the 1000’s of degrees
- Aurora Occur in this layer, sun’s rays cause molecules of oxygen, hydrogen, and nitrogen to fluoresce
Heating of the Troposphere - Solar Radiation
- Energy emitted from sun as short-wave UV radiation
- Ozone layer acts as filter, approximately 19% of UV rays are absorbed
- 30% of UV rays reflected by atmosphere, cloud tops, and the earth’s surface
- 51% gets absorbed by the earth’s surface
- Little direct heating of atmosphere once past ozone layer
- Absorbed UV causes surface to warm
- Air in contact with surface is warmed by conduction
Heating Processes - Conduction
Heating Through Contact
Heating Processes - Convection
- Air that is warmed becomes less dense and tends to rise
- Rising tendency continues until the density (temp) is equal to surrounding air
Heating Processes - Turbulent Mixing
- Friction causes eddies to form as air moves over surfaces
- Disruption is proportionate to terrain, rough terrain means more mixing
- In combination with convection, air can rise quite high into the troposphere
Heating Processes - Advection
- Horizontal Movement of Air Masses
- If an air mass travels over a warmer surface, it will be warmed
Heating Processes - Compression
- Descending (subsiding) air increases in pressure, in turn increasing temperature
- Can occur along ridges of high pressure or over mountain ridges
- Known as adiabatic heating
Heating Processes - Release of Latent Heat
Changing state (down and energy level) releases heat energy
Cooling Processes - Expansion Cooling
- Rising air decreases in temp
- Known as adiabatic cooling
- Lapse Rate is the rate at which air warms or cools in response to altitude (pressure) changes
- Adiabatic cooling occurs at two different rates depending on moisture content
- Dry Adiabatic Lapse Rate (DLAR) is 3º/1000 ft and occurs with less than 100% relative humidity
- Saturated Adiabatic Lapse Rate (SLAR) is 1.5º/1000 ft and occurs at 100% relative humidity
- SHOULD KNOW LAPSE RATES
Cooling Processes - Orographic and Upslope Lift
- Result of air flowing over topographical features that cause the air to rise
- Slope of rising terrain is not important
Cooling Processes - Frontal Lift
- Expansion cooling occurs when warm air is forced up along a frontal surface
- Cold Fronts are when advancing cold air displacing warmer air, causing it to rise quickly
- Warm Fronts are when warm air cannot displace the cold air, so warm air gradually rises up and over cold air
Cooling Processes - Mechanical Turbulence
Same mechanism as the heat distribution process, can result in cooling
Cooling Processes - Convection
Rising air cools according to the appropriate lapse rate
Cooling Processes - Convergence
- Air at the centre of a low pressure area rises, causing cooling
- Air spins counter-clockwise inwards
Cooling Processes - Non-Expansional Cooling
Advection, an air mass moving over a cool surface will cause cooling of the air mass
Cooling Processes - Non-Expansional Cooling
Evaporation, opposite of condensation, as liquid turns to gas, heat is absorbed
Atmospheric Stability
- Stability in the air refers to the resistance to vertical movements of air parcels
- Stability is determined by temp difference between the rising air parcel and the surrounding air
- An air parcel will continue to rise as long as its temp remains above the temp of the surrounding air
Stability Definitions - Steep Lapse Rate
Temperature decreases rapidly with altitude, this implies unstable air
Stability Definitions - Shallow Lapse Rate
Temperature decreases little with altitude, this implies stable air
Stability Definitions - Isothermal Layer
- Temperature does not change with an increase in altitude
- More stable than a shallow lapse rate
Stability Definitions - Inversion
Temperature increases with height, this indicates extremely stable air
Stability Definitions - Environmental Lapse Rate
The actual change of temperature with an increase in altitude
Pressure Terminology - Low (L)
- Area of pressure lower than that which surrounds it
- Centre of low pressure area marked by an L
- Scale ranges from 1 km to hundreds of km
- Lower the pressure, deeper the L
- Air flow tendency is counter-clockwise and toward the centre
Pressure Terminology - High (H)
- Area of pressure higher than that which surrounds it
- Centre of high pressure area is marked by an H
- Also known as an anticyclone
- Typically larger than low pressure systems
- Airflow tendency is clockwise and outward from centre
Pressure Terminology - Trough
- Line of low pressure extending outwards from a low pressure area
- Produces convergence, resulting in cooling and promotes low ceilings and visibilities with sufficient moisture
Pressure Terminology - Ridge
- Line of high pressure extending outwards from a high pressure area
- Produces subsidence, resulting in compression heating and clear skies
Pressure Terminology - Col
- Neutral areas that exist between two highs and two lows
- Geographically small, rarely significant impact on the weather
Air Pressure
- Weight of an imaginary column of air
- Two common methods of measurements, mercury barometer and aneroid barometer
Air Pressure Measurements - Mercury Barometer
- Consists of an open dish of mercury into which the open end of and evacuated glass tube is placed
- Atmospheric pressure causes mercury to rise
Air Pressure Measurements - Aneroid Barometer
Consists of a flexible metal cell that contracts with increasing pressure and expands with decreasing pressure
Station Pressure
The Weight of a column of air lying above the reporting station
Mean Sea Level Pressure
- Station Pressure plus the weight of an imaginary column of air between the station and sea level
- Weight of this column of air is calculated using the average temp over the previous 12 hours
Altimeter Setting
- Station Pressure plus the weight of an imaginary column of air between the station and sea level
- Weight of this column of air is based on the ICAO Standard Lapse Rate of 1.98ºC /1000’
Flying with Pressure Systems
- Aircraft altimeters do not automatically detect changes in pressure
- Altimeter settings must be adjusted in the cockpit to ensure accurate altitude indications
Air Circulation - Isobars
- Lines joining places of equal barometric pressure
- Spaced 4 hectopascals apart
Air Circulation - Pressure Tendency
The change of pressure over time
Air Circulation - Pressure Gradient Force (PGF)
- Air moves from areas of high pressure to areas of low pressure
- Intensity of PGF can be determined by spacing between isobars (close together means strong winds)
Coriolis Force
- Earth’s Rotation plays a significant part in movement of air, particularly when moving north or south
- Air moving from high pressure to low pressure does not flow directly from one to the other
- The Earth rotating under the moving air creates an apparent deflection to the right (Fennel’s Law)
- Coriolis force is deflecting force
- Results in winds around a high pressure area to move clockwise around the centre of the high pressure area
- Results in winds around a low pressure area to move counter-clockwise around the centre of the low pressure area
Buys Ballot’s Law
“If you stand with your back to the wind in the northern hemisphere, low pressure will be on your left”
Geostrophic Wind
- Occurs when Pressure Gradient Force equals Coriolis Force
Friction Effect
Topographical features affect the movement of air, reducing wind speeds at low level
- This occurs between the surface and 2000’-3000’ AGL
Localized Effects - Sea Breeze
A breeze blowing toward the land from the sea, especially during the day owing to the relative warmth of the the land
Land Breeze
A breeze blowing toward the sea from the land, especially during the night owing to the relative warmth of the the sea
Anabatic Wind
- Warm Wind which blows up a steep slope or mountain side
- Driven by heating of the slope through insolation
- Also known as an upslope flow
- These winds typically occur during daytime in calm sunny weather
Katabatic Wind
- Also called downslope wind or gravity wind
- Blows down a slope because of gravity
- Occurs at night, when highlands radiate heat and are cooled
Low Lovel Jet Stream (LLJ) - Frontal
- Narrow band of strong winds in warm air just ahead of a cold front
- 500’-5000’ AGL, hundreds of feet wide, up to 100 knots
- Wind shear is primary risk
Low Level Jet Stream (LLJ) - Nocturnal
- Requires inversion and flat terrain, no barriers to wind speed
- inversion is a very stable condition typically found after sunset
- 700’-2000’ AGL, can reach 60 knots
- Dissipates as sun rises
Turbulence Definition
- Unpredictable and irregular motion of air
- Can be caused by wind shear, frontal air, hot air, or surface texture
Convective Turbuelnce
- Hot air rising quickly
- Caused by daytime heating
- Different surfaces reflect different amounts of heat
- Generally only within 2000’ of ground
Mechanical Turbulence
- Air moving around obstructions is broken up into a series of complicated and chaotic eddies
- Stronger the wind and the more severe the obstructions, the stronger the turbulence
- Turbulence carried downwind and is affected by stability
- Eddies do not grow in stable air, but dissipate slowly
- Eddies can grow in unstable air, but dissipate quickly
Mountain Waves
- Also known as lee waves
- Can be very dangerous
- Occurs as airflow is forced up the windward side of a mountain, crossing over the top and dropping back down the leeward side
- This sets up a wave pattern extending downwind up to 500 km
- Occur with lots of wind flowing 30º off perpendicular to ridge line, and stable air above mountaintops
- Can break if strength is great enough
- Strongest during winter and early spring
- Can extend 6000’ above height of mountain with wavelength ranges of 3-15 miles
- Risk is severe turbulence with up and downdrafts up to 4500 ft/min
Mountain Wave Clouds - Cap Clouds
- Forms on the windward side as air flows up and cools
- Usually indicates strong downdraft on the leeward side
Mountain Wave Clouds - Lenticular Clouds
- Form downwind of mountain range
- Clouds form in the wave crest
- Indicates high wind speeds and clear air turbulence (CAT)
Mountain Wave Clouds - Rotor Clouds
- A line of stratocumulus a few miles downwind and parallel to mountain range
- Form at and below mountaintop heights
- Forms in vortex below the wave crests
Low Level Wind Shear
- Created when there is a sudden change in temp in the vertical plane
- Often associated with approaching frontal depression
Low Level Wind Shear Alerts
- Surface to 500’ AGL with wind vector changes by 25 knots or more
- Surface to 1000’ AGL with wind vector changes by 40 knots or more
- Surface to 1500’ AGL with wind vector changes by 50 knots or more, PIREP’s report airspeed fluctuation of 20 knots or more
Frontal Zone Wind Shear
- Sudden wind shifts can occur across a front
- Turbulence will usually be found along the slope of the front
- Usually fairly narrow band, so it may be passed through quickly
Wing Tip Vortices
- Created by the passage of a wing through air
- Pressure differential on upper and lower
Vertical Wind Shear
Moderate or greater turbulence can occur when the wind changes by 5 knots or more per 1000 feet of altitude
Horizontal Wind Shear
- Occur when wind speed differs dramatically between two adjacent areas
- Can form in the vicinity of troughs and ridges
- Same effect as a river flowing rapidly around a corner
Clear-Air Turbulence (CAT)
- Turbulent movement of air masses in the absence of any visual clues
- Caused by when bodies of air moving at widely different speeds meet
- Cirrus clouds can also indicate high probability of CAT
