Pressure And Density Flashcards
Explain what is meant by the partial pressure of a gas.
- The atmosphere is made up of different gases
- Each has its own partial pressure
- When they are all added together that is the pressure we feel.
Explain the significance of air pressure with reference to barometric tendency.
- Change in atmosphere pressure over a given time frame
- Caused by atmosphere/solar tide
- OR pressure change from development/decay of weather
Explain the significance of pressure with reference to altimetry.
- Altimeter reads differences in pressure
- Uses aneroid capsules
Explain the effects of temperature changes within the troposphere on the pressure lapse rates.
- Warm air needs more space than cold air
- More space means reduced pressure lapse rate
- Less space means increased pressure lapse rate as less space to do it.
X = 96 x Kelvin/Pressure
Define pressure gradient.
- Change of horizontal pressure over a distance
- Acts at 90degree angle to isobars
- Can tell the wind speed from spacing (narrow = high, wide = low)
- The steeper the pressure gradient the stronger the wind
Define QFE.
- Atmospheric pressure at the airfield
- Will read zero feet on the ground
- Will read height above ground when flying
Define QNH.
- Airfield pressure corrected to MSL using ISA conditions
- ATIS at NZCH is measured at the airport an corrected for MSL
- Indicates altitude above MSL
Define QNE:
- 1013.25hPa set on altimeter
- Pressure altitude
- Used to maintain Flight Levels
Define pressure altitude.
- Height in ISA for same atmospheric pressure in question
Define Flight Levels.
- Vertical altitude at standard pressure
- Expressed as FL255 ( 25,500ft)
Describe QNE.
- Fly at assigned elevation called a flight level above 1013 pressure level.
- Depends on where 1013 height is on the day
- Example: FL 330 is 33,000ft above the 1013 datum.
Describe pressure altitude
- What you fly at when using flight levels
- Height above a given datum which is 1013hPa level
Describe flight levels.
- If long haul flights used QNH they would have to update a lot
- By setting 1013 (QNE) there’s no more change in the cruise
- Altitude above sea-level in hundreds of feet, measured according to standard atmosphere
Describe the transition layer (as it applies to New Zealand), with reference to the transition altitude.
- 13,000ft AMSL
- When climbing through set QNE
- When flying at or below 13,000ft set zone area/aerodrome QNH
Define the transition layer (as it applies in New Zealand), with reference to the transition level.
- FL150
- Flying at or above set QNE
Define the exceptions that apply to the transition layer (in NZ) with reference to the transition level and altitude.
- No exception to transition altitude
- Transition level becomes FL160 if QNH is 980hPa or less.
- Transition level also FL160 when IMC within 20NM radius of Mt Cook
- When flying in the transition layer set as per advise by ATC
- Controlled airspace can fly at 14,000ft in emergency or due weather
- Designated NON STANDARD
- Uncontrolled IFR may use layer if unable to operate outside of range
- Still have to notify ATS unit and no traffic conflict
Explain why transition layers in other countries are found at lower and higher levels in the atmosphere.
- Based on minimum safe altitude (MSA) in country
- Takes into account the surface terrain e.g. large mountains.
- In the USA and Canada, the transition altitude is 18,000ft
Define elevation.
- How high terrain is above MSL
- Measured in feet
Explain what happens to an aircraft’s flight profile when the altimeter sub-scale is not reset during flights between areas with differing MSL pressures.
- From high pressure to low pressure, read high be low.
- From low pressure to high pressure, read low be high.
- From 1013 to 990, altimeter will read high but you will be descending
- From 990 to 1013 altimeter will read low but you will be climbing.
Describe how localised pressure changes occur in association with lee troughs.
- Low pressure created on lee of mountain
- Relative high pressure built up on windward side of mountain
- Like the high pressure on front of aircraft and low pressure at back
Describe how localised pressure changes occur in association with thermal/heat lows.
- One surface heats up more than others in the area
- Causes the hotter surface to drop in pressure due to convection
- Ocean and land heat up at different speeds
- Large shadows cast over an area by cloud/land and direct sun in another.
Describe how localised pressure changes occur in association with thunderstorms.
- Under the updrafts pressure decreases by 2-3hPa
- Under precipitation/downdrafts pressure increases by 2-3hPa
- Due to rain evaporation which cools air and makes it heavier
Describe ‘diurnal’ pressure variations.
- Measure of surface pressure level over 24hrs
- Max pressure 10am and 10pm
- Min pressure 4am and 4pm
- Lowest at 4pm and highest at 10am
State the latitudes where diurnal pressure variation is most significant.
- Larger changes at low latitudes due larger change in temperature
- Local pressure systems dominate in NZ, variation unnoticeable
- Most noticeable in tropical regions
- Between tropical of Cancer and Capricorn 23degrees25’N/S
Explain the effects of changes in pressure on air density.
- Air density is the mass per unit volume of air in terms of weight per unit volume
- Increase pressure = increase density
- Decrease pressure = decrease density
Explain the effects of changes in the temperature on air density.
- Increase temperature = decrease density
- Decrease temperature = increase density
Explain the effect of changes in altitude on air density.
- Increase altitude = decrease density
- Decrease altitude = increase density
Explain the effects of changes in moisture content of the air on air density.
- Increase moisture = decrease density
- Decrease moisture = increase density
Define density altitude.
- How dense the air is at a certain altitude
- Pressure altitude corrected for non-standard ISA temperature gives DA
- Temperature, pressure and water vapour effect density of air
State the meteorological units of pressure used in USA
- Inches of mercury (inHg)
- 1013.25hPa = 29.92inHg
Demonstrate the effect of flying at a constant indicated altitude from a cold region to a warm region, during which the surface pressure does not change.
- From low temperature to high temperature, read low be high.
- Will climb into warm air while maintaining same indicated altitude.
Describe ISA.
- Air is dry and its composition is the same at all altitudes.
- Mean sea level temp 15degrees Celsius.
- Above 36,090ft/-56degrees Celsius.
- 1.98degrees/1000ft to the tropopause.
- Mean sea level pressure 1013.25hPa
Describe Jet Standard Atmosphere (JSA).
- Does not a include a tropopause
- 2degrees/100ft to infinity
