MET Flashcards
Basic composition of the atmosphere
78% Nitrogen
21% Oxygen
1% water vapour, argon, and other gases
Layers of the atmosphere
Troposphere (tropopause) Stratosphere (stratopause) Mesosphere (mesopause) Thermosphere (and ionosphere) Space (above 90 miles AGL)
Troposphere
- 28,000 ft at poles, 54,000 ft at the equator
- Majority of weather happens here due to the presence of water vapour
- Pressure, density, and temp decrease with height
- (-56) degrees celsius marks the beginning of tropopause where temp pauses then starts to increase
Stratosphere
- Extends from tropopause for about 50,000 ft
- Pressure continues to decrease
- Very little water vapour
- Air currents are minimal
- Thicker over poles and thinner over the equator due to varying height of tropopause
- Temp constant (-56) degrees celsius in lower levels, then gradually increases to 0 celsius
- Temp in stratopause begins to fall again
What layer of the atmosphere is the Ozone present?
Stratosphere: absorbs the sun’s UV rays causing temp to increase
Mesosphere
- Temp decreases
- Meteorites typically burn up in this layer
- Top of the layer is the mesopause: temp drops to -100 degrees celsius then rises again with thermosphere
Thermosphere
-Temp increases indefinitely into space
Ionosphere
- 50 to 250 miles within the thermosphere
- Affects radio communication due to electron density
- Northern lights happen here
Exosphere
- Thin layer beyond thermosphere
- Temp has no meaning
- Pressure drops to a little more than a vacuum
The limit of national sovereignty
90 miles above earth
ICAO Standard atmosphere
- No water vapour
- 29.92 inHg
- (15) degrees celsius
- Lapse rate: 1.98/1000 ft
Properties of the Atmosphere
- Mobility
- Expansion
- Compression
Cloud ingrediants
High relative humidity, condensation nuclei, and a cooling process
Expansion
- most important property of the atmosphere*
- A parcel of air rises, therefore pressure decreases, and the air will expand and cool
- Condensation may occur
- Clouds and precip may result
Compression
- As air descends pressure will increase, air volume decreases, and temp will increase
- The increase in temp will cause any water droplets to evaporate and prevent clouds from forming
Solar Radiation
Heat transferred from the sun to the Earth
Terrestrial radiation
Earth returns radiated energy back into the atmosphere
How is the earth heated?
Terrestrial radiation
from below, not above
Diurnal Variation
-During the day the Earth becomes warmer
solar radiation > terrestrial radiation
Nocturnal
-During night the Earth becomes cooler
no solar radiation, only terrestrial radiation
Topography (in terms of radiation)
-Land surfaces absorb more solar radiation than water
day: warmer over land (but cools faster)
night: warmer over water (cools slower)
Seasonal Variation
-Angle at which solar radiation strikes the Earth varies from season to season
Clouds effect on surface temp
- Day: stops solar radiation and reflects it back to space (cooler)
- Night: stops terrestrial radiation and reflects it back to the earth (warmer)
How is the atmosphere is heated?
- Convection
- Advection
- Turbulence
- Compression
- terrestrial radiation is distributed through these methods*
Convection
- The surface layer of air heated by conduction becomes buoyant and rises up as the convective current carries it into the atmosphere
- Colder air descends to take its place, and a vertical circulation develops which distributes the heat through the upper layers
- during summer time air mass thunderstorms can develop when there is a lot of convective heating
Advection
Horizontal movement of COLD air over a warm surface warms up the cool air parcel
advective warming
Turbulence (in terms of heating)
Mechanical turbulence: mixes surface air that has been heated by conduction with the unheated air aloft and spreads the heat upwards
Compression
As the air descends pressure increases and temp increases
How is the atmosphere cooled?
- Radiation cooling
- Advection cooling
- Expansion
Radiation Cooling
Terrestrial radiation only at night (Earth cools approx 1 degree C per hour)
Advection cooling
Horizontal movement of WARM air over a cold surface
Expansion cooling/ adiabatic process
- air forced to rise (over mountain etc…)
- When air rises pressure decreases, expands, temp lowers
Is cold or warm air denser?
Cold
Density Altitude
Pressure altitude corrected for temp
Adiabatic
-No transfer of heat between parcel and surrounding environment
Dry Adiabatic Lapse Rate (DALR)
3°/1000ft
air not saturated, cools by expansion or warms by compression
Saturated Adiabatic Lapse Rate (SALR)
- 5°/1000ft
* air is saturated, cools with a slower rate because latent heat is released during process of condensation which will heat the surrounding air*
Standard Adiabatic Lapse Rate
- 98°/1000ft
* ICAO standard*
Environmental Lapse Rate
Actual lapse rate calculated using ground temp and upper air temp
What # do you use to calculate cloud base?
2.5° for TC
3° for MFC
Inversion
- Temp increases with altitude
- Called negative lapse rate
- Caused by warm air moving over warm air (ex: cold air trapped in a valley, terrestrial radiation cooling the bottom few 100 feet, snow covered area in spring)
Isothermal layers
- Temp is neither increasing or decreasing with height
- Indicates very stable air
ex: stratosphere
Dewpoint
Temp at which unsaturated air must be cooled to become saturated
What comes with small temp/dew point spread?
Higher relative humidity: formation of cloud, fog, or precip
Relative humidity
Ratio of water vapour present in the air divided by the amount of water vapour the same volume of air could hold if it were saturated
(100%=saturated air) (0%=dry air)
Condensation
- Process by which the water vapour changes into water droplets (gas to liquid)
- During process is a release in energy (responsible for change in lapse rate in clouds)
- Causes visible cloud or fog
- Since warmer air can hold more moisture than cold air more violent weather will occur in very warm moist air
What are clouds made of?
Water droplets, ice crystals, or both depending on temp
What determines the type of cloud?
Stability
How much does a column of air weigh?
14.7 lbs/square inch
Altimeter
- Aneroid barometer that indicates the pressure it senses as an altitude in feet
- pressure decreases with height so it is calibrated to sense a pressure and display an altitude
In the lower 5,000 ft of the atmosphere, what is the relationship between inHg and altitude?
-pressure changes about 1 inHg for every 1,000 ft of altitude
1.0”Hg=1000 ft
.1”=100 ft
1hPa= 30 ft
* as you increase altitude the difference becomes smaller*
(at 20,000 ft its approx .5in for every 1000 ft)
Density effects on performance and altimeters at different levels
- Variations in temp from ISA will effect a/c performance MORE at low levels than high levels
- Variations in temp from ISA will effect altimeter errors MORE at high levels than low levels
If surface pressure has increased to 30.02” what will the altimeter read on the surface?
-100 ft
when it reads 1000 ft you are actually at 1100ft
What altitude does high level airspace begin?
18,000 ft
Which direction does air circulate in low and high pressure
low= counter clockwise high= clockwise
Temp error
When climbing through colder air the lapse rate is steeper, therefore pressure change is steeper and your altimeter thinks you’re higher than you are
formula: 4x(altitude/1000)x(STD-OAT)
* height between aerodrome and a/c)*
Ex: An aircraft with an indicated altitude of 9,000 feet ASL is using an altimeter setting from an aerodrome at 1,000 feet and has an OAT 30 degrees C. below ISA.
The column of air from the aerodrome to the aircraft is 8,000 feet deep so the error is……
4 X 30 X 8 = 960 feet.
If a surface inversion exists, what should be done to the calculated temp error?
increase by 50% for a safety margin
True Altitude
Indicated altitude corrected for fl temp
only correct when fl temp is standard
Density Altitude
pressure altitude corrected for temp
formula: PA+(120x(OAT-STD))
Virtual Temperature
The temp dry air would have so that its density would be the same as the moist as the moist air being considered
will always be higher than the temp of the air since moist air is less dense than dry air
Pressure Altitude
Reading on altimeter when 29.92 is set
formula: (29.92-altimeter setting)x1000
What does air parcel do in areas of low pressure?
it rises into low pressure areas and expands & cools
What happens during expansion?
Air parcel cools, can reach point of condensation, clouds can form and
Convergence and divergence
- Upward motion over surface low pressure area due to convergence of wind
- Downward motion over surface high pressure area due to divergence of wind
Latent heat
water vapour carried into the earth’s atmosphere is frequently carried aloft and if it should condense than its latent heat is released and distributed to higher levels of the atmosphere
Sublimation
water vapour to ice crystals
What is stable air?
Air that resists vertical displacement and tends to return to its horizontal level
What is unstable air?
-Air that will continue rising vertically if disturbed
Relationship between lapse rate and stability
-shallower lapse rate = more stable
If the lapse rate if a parcel of air is greater than the ELR will it be more or less stable?
- more stable
- air parcel cooler than surrounding air and wont continue to rise
Absolute Stability
- ELR is less than SALR
- Any parcel of air will eventually become cooler and sink
Absolute Instability
- ELR is greater than DALR
- Any air parcel will quickly rise since its warmer than surrounding air
Conditional Instability
SALR
Potential Instability
Stable air becomes unstable as the whole air mass is forced to rise until it becomes saturated
This is what happens with frontal lift and convergence
Main method of developing instability
daytime heating
What is a super adiabatic lapse rate?
intense daytime heating causes a lapse rate greater than the DALR
Subsidence
- A stable layer or inversion results from this
- air put under pressure is forced to sink and the temp will increase
What causes the atmosphere to become unstable?
- Heating of the surface
- Advection at the surface
- Cold advection aloft
- Large scale rising of the air
- Convective cells
What causes the atmosphere to become stable?
- Radiation cooling
- Cold advection at the surface
- Warm advection aloft
- Large scale sinking of air
Characteristics of Stable air
- poor vis
- layered cloud
- steady precip
- steady winds
- smooth flying cond.
Characteristics of unstable air
- good vis
- heap type cloud
- showery precip
- gusty winds
- turbulent flying cond.
Convective Cells
- Vertical currents that develop in unstable air in the form of either a shaft of rapidly rising air or a large bubble
- surrounded by slowly descending air
- they can occur at any height, in clear air or embedded in cloud
- strength increases with daytime heating
- associated with strong turbulence and possible wind shear
What 3 types of sky coverage are considered ceilings
-BKN, OVC, VV
Middle Clouds
AS, AC, & ACC
Altostratus, altocumulus, and altocumulus castellanus
High Clouds
CI, CS, & CC
cirrus, cirrostratus, and cirrocumulus
Low Clouds
CB, TCU, CU, CF, SC, NS, ST, SF
Contrails
formed by exhaust gases of aircraft; increasing the relative humidity of air in its wake
*if relative humidity is already high contrails will linger
**if humidity is low, supercooled water droplets can evaporate into the surrounding air and disappear
***ice crystals may persist for hours and combine with other contrails to form a cirrus layer
Continental Arctic (cA)
Dry, very cold, very stable, very low tropopause
Continental Polar (cP)
Dry, very cold, fairly stable, low tropopause
Maritime Arctic (mA)
Moist, cold, unstable in low levels, low tropopause
Maritime Polar (mP)
Moist, cold, unstable, medium tropopause
Maritime Tropical (mT)
moist, hot, very unstable, high tropopause
Continental Tropical (cT)
dry, very hot, very unstable, very high tropopause
What factors determine weather?
moisture content, cooling process, stability
What causes wind?
pressure differential
What factors lead to variations in the height of the tropopause?
- warmer troposphere =higher tropopause
- height is greatest at the equator because they receive the most heat
How can you recognize the Tropopause?
- haze layer with a definite top
- anvil tops from ts spread out
- wind strength increases with height and is strongest at tropopause
- temp stops decreasing
Which way does wing flow in the lower levels?
- according to pressure differences
- parallel to the isobars
- clockwise around high, counterclockwise around low
- speed proportional to the pressure gradient
Thermal Wind Component
-wind created by the temperature gradient
(low pressure over cold air/high over warm)
-
Difference between pressure decrease in cold & warm air
rate of decrease is greater in cold air
Jet stream
- at least 60 kts
- narrow, rapid flowing, ribbon like stream of air
- forms if there is a strong temp gradient at a front
- normally form between polar and tropical air masses
- fronts don’t reach stratosphere so jet streams lie just under the tropopause
- if moist, warm side will cause cirrus clouds to form
- wind shear strongest on cold side
What happens to subpolar jet streams in the winter?
stronger, lower, and moving south
