Transcript Chapter 1 PowerPoint

Introduction and Chapter 1
CPL MET
Aim
To introduce basic meteorology and to provide a
sufficient level of understanding on Chapter 1 of
ATC MET
Objectives
1. State the composition of the atmosphere
2. State what ISA is and the values of ISA
3. Explain the significance of atmospheric
temperature, density and pressure.
4. Explain the diurnal and seasonal circulations
5. State and explain the different types of inversion
1. The Atmosphere
State the composition of the atmosphere
The atmosphere is divided into layers, these layers are based on
temperature.
These layers are:
• The troposphere
• The stratosphere
• The mesophere
• The thermosphere
1. The Atmosphere
The Troposphere
In the troposphere temperature usually falls with an increase in height
until the tropopause is reached
The rate at which the temperature falls is called the Environmental Lapse
Rate (ELR).
Altitude
Tropopause
ELR
Earth
Temperature
1. The Atmosphere
The Troposphere
The height of the tropopause varies with latitude
• At the equator, the tropopause can be as high as 60,000’
• At the poles the tropopause can be as low as 20,000’
Most weather occurs in the troposphere including:
• Clouds
• Precipitation
• wind.
1. The atmosphere
The Stratosphere
Altitude
The stratosphere extends up above the tropopause up to a height of
approximately 50km AMSL.
It is characterised by a temperature that remains relatively constant in its
lower layers up to a height of about 35km AMSL.
This area is called an isothermal layer
Temperature then increases to about 0 degrees.
ELR
Stratopause
Isothermal layer
Tropopause
Temperature
2. ISA
The International Standard Atmosphere (ISA)
ISA was designed to act as a reference point to compare the atmosphere
at any point and at any time.
The standard atmosphere assumes sea level to be:
• A pressure of 1013.25 hPa
• A temperature of 15°C
• A density of 1.225 kg/m³
2. ISA
The International Standard Atmosphere (ISA)
The temperature is assumed to fall at 1.98°/1000ft up to a theoretical
tropopause of 11km (36,090ft)
From this height up to 20km (65,617ft) the temperature is assumed to be
constant at -56.5°C
From 20km to 32km the temperature rises at a rate of 1°C/km or
0.3°/1000ft
ISA is only a model, it is seldom that the
conditions stated actually apply.
It is used as an aviation standard, for
example in the calibration of altimeters.
3. Pressure, Temperature, Density
Significance of atmospheric temperature
It is important to know about temperature,
pressure and density in aviation as it is these
three things that apply most to aircraft
performance and performance charts.
In aviation, the unit of Celsius is standard, but
we may find that some US manufactured
aircraft use Fahrenheit in the Pilot Operating
Handbook.
Conversion scales in temperature can be found
in most Pilot Operating Handbooks
3. Pressure, Temperature, Density
Significance of atmospheric temperature
As mentioned in previous slides, the temperature falls at a constant rate of
1.98/1000’ up to an altitude of 36,090’. For ease of mental arithmetic we
can say 2/1000’ up to 36,000’ (the tropopause) at which the temperature
is then constant at -57C.
We can now work out an ISA temperature at any height.
• Since the sea level temperature in ISA is +15C, we can say:
• ISA temperature = [+15 – (2x feet in thousands)]C
• Eg ISA temperature at 10000’ = 15 – (2 x 10) = 15 – 20 = -5C
3. Pressure, Temperature, Density
Significance of atmospheric temperature
Temperature deviation is the difference between the actual temperature
at a given height and the ISA temperature.
For example what is the temperature deviation at 10000’ if the actual
temperature is -10C?
• The ISA temperature is -5C (from previous slide)
• Actual temperature is -10C
• Therefore temperature deviation is = -5C (ISA -05)
3. Pressure, Temperature, Density
Significance of atmospheric pressure
Atmospheric pressure is a measure of weight of
air above the surface per unit area.
Generally speaking, when we talk about
pressure we are talking about force exerted on
a certain area. The standard unit for pressure is
the Newton.
In aviation however, we talk about
Hectopascals (hPa) and Inches of Mercury (Hg)
Most Pilot Operating Handbooks have
conversion tables between these 2 scales.
3. Pressure, Temperature, Density
Significance of atmospheric pressure
As we gain altitude, the weight of air above us decreases so therefore the
pressure also decreases.
We can assume for practicality (and mental arithmetic purposes) that
pressure falls at a constant rate of 1 hPa for ever 30ft gained in altitude up
to an altitude of 5000’. Therefore in ISA conditions the pressure at 5000’
will be approximately 845 hPA.
Above this height, the rate of change decreases with an increase in
altitude.
At approximately 30000’ the pressure is said to be approximately 300 hPa.
3. Pressure, Temperature, Density
Significance of atmospheric density
Density is by definition the mass per unit volume. In aviation, we are talking
about the mass of air, per unit volume.
Under ISA conditions, the density of air at seal level is 1.225kg/m^3
Density is important for us because without air, we would not be able to
operate. Our wings rely on lift which is generated by air flow around them,
our engine power is generated by burning a mixture of fuel and air, without
air, we couldn’t breathe.
If the air is more dense:
• The required lift can be achieved at a lower TAS
• Engine power output is greater due to the greater mass of fuel-air
• Breathing is easier due to a greater mass of oxygen taken into the
lungs per breath.
4. Seasonal and diurnal circulations
Seasonal Circulation
Throughout the year, we experience different weather in the different
seasons. This is due to the earth orbiting around the sun once a year.
Because the earths axis is tilted, some of the earth is presented to the sun
for a longer period. This is why we have summer and winter and longer
days in summer.
4. Seasonal and diurnal circulations
Diurnal Variation
Diurnal Variation is the heating and cooling of the earths surface over a 24
hour period.
Solar heating of the surface occurs only by day, but terrestrial radiation of heat
energy from the earth happens continually.
This combination of heating and cooling causes the maximum temperature of
the day to be around 3pm and the minimum temperature generally an hour
after sunrise.
5. Inversions
Inversions
As previously discussed, usually temperature in the lower atmosphere
decreases with an increase in height. This is not always the case.
Frequently warmer air sits above a colder layer. As we ascend through this
atmosphere and encounter the warmer layer, temperature actually
increases. - This is known as a temperature inversion.
5. Inversions
Inversions
Because the colder air is underneath, and colder air is more dense than
the warmer air, it is more than happy just to sit there.
This is an indication of a stable atmosphere.
There are three types of inversion:
• Surface (radiation) inversions
• Subsidence inversions
• Frontal inversions
5. Inversions
Surface Inversions
Form on clear, light wind nights and are strongest around sunrise
After sunset, the earth cools by terrestrial radiation. As the surface cools,
the layer of air just above it also cools by contact with the cold surface.
Because air is a very poor conductor of heat, only a thin surface layer
becomes cold whilst the warmer air above it is almost unaffected
On cloudy or windy nights, the
surface is not cooled as much
because the cloud layer inhibits
terrestrial radiation. Strong
winds mix the air so that a
surface layer doesn’t get the
chance to become cold enough.
5. Inversions
Subsidence Inversions
Subsidence inversions are caused when sinking air warms adiabatically as
it descends into regions of higher pressure.
The air in a high pressure system is sinking therefore warming until it
reaches the surface.
The air at the surface then moves out and suffers no more adiabatic
heating. The air above is still warming due to compression. This is called
subsidence and can cause the air above to be warmer layers, therefore
producing an inversion.
5. Inversions
Frontal Inversions
Frontal inversions are caused when cold air moves into a region
previously occupied by warm air.
The cold air is more dense therefore remains at the surface, pushing the
warm air aloft.
5. Inversions
Flying conditions in an inversion
Due to convection (as a mass of air is heated, it will expand, become less
dense and rise), dust and smoke particles are disrupted from the surface.
They generally rise only up to the inversion layer as convection tends only
to happen in the cold layer.
This causes turbulence and poor visibility below the inversion, and
smooth flying conditions above.
References
Aviation Theory Centre, Meteorology for the CASA PPL/CPL Day VFR
Syllabus, 2012
Bob Tait CPL Meteorology, Issue 3, 2008
Australian Government, Manual of Aviation Meteorology, 2003
Questions?