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METEOROLOGY NOTES written by

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Meteorology Data Sheet

Chapter 1 – The Atmosphere ISA Mean Sea Level (MSL)

Temperature

15°C

Pressure

1013.25 hPa

Density

1225 gm-3

From MSL to 11 km (36 090 ft)

Temperature decreases at 6.5°C per km (1.98°C per 1000 ft)

From 11 km to 20 km (65 617 ft)

Temperature constant at –56.5°C

From 20 km to 32 km (104 987 ft)

Temperature rises with height at 1°C per km (0.3°C per 1000 ft)

Composition Nitrogen

78.09%

Oxygen

20.95%

CO2

0.03%

Argon

0.93% Rest rare gases

Troposphere

Depth varies with latitude and season. Temperate latitudes up to 11 Km

Tropopause

Boundary between the Troposphere and the Stratosphere. Temperature becomes Isothermal. The height of the tropopause varies with latitude, season and surface temperature Latitude

Height

Temperature

Equator

55 000 ft

17 Km

-75°C

50° Pole

35 000 ft 25 000 ft

11 Km 7 Km

-55°C -45°C

In Equatorial regions flights take place below the tropopause; in temperate regions flights are above the tropopause. Stratosphere

11 – 50 Km. Initially Isothermal becoming an inversion

Stratopause

Boundary between the Stratosphere and the Mesosphere

CB can penetrate well into the Stratosphere Mesosphere

50 – 80 Km

Mesopause

Boundary between the Mesosphere and the Thermosphere

Thermosphere

80 Km upwards

Troposphere

In immediate contact with the earth.

Contains 80% of the atmosphere, with ½ the mass in the first 18 000 ft Contains more than 90% of all water vapour – most of the weather found in the Troposphere. Water vapour is the most important constituent in the atmosphere from a weather stand-point. Inversion

Temperature increase with height

Isothermal

Temperature remains constant with height

Advection

The horizontal motion of air

Subsidence

The vertically downwards motion of air.

Convection

The upward motion of air

Convergence

The inward horizontal motion of air

Air warms adiabatically in the descent Air cools adiabatically in the ascent. Convergence at height means high pressure at the surface and little or no cloud. Divergence

The outward horizontal motion of air Divergence at height means falling pressure at the surface and the likely formation of convective cloud

Heating of the Atmosphere The radiation of the sun heats the surface of the earth, which heats the air in the troposphere from below. Insolation INcoming SOLar radiATION. The solar radiation striking Earth or another planet. Heat distributed by: 

Convection - Greatest overland in the mid-afternoon in summer.



Conduction



Radiation



Latent Heat of Condensation



Advection



Turbulent mixing



Absorption of long wave radiation

Two most important factors are convection and the Latent Heat of Condensation Diurnal Variation temperature:

The daily fluctuation of temperature. The diurnal variation in



Is highest when the sky is clear and the wind is weak



Is lowest when there is cloud cover and a strong wind

Maximum Temperature

2 hours after 1200 LMT

Minimum Temperature

½ hour after dawn

Clear sky, light wind Clear sky, light wind

2

Meteorology Data Sheet

Conversion of Metres per second to Knots Double the speed in metres per second to get the speed in knots 20 mps ~ 40 knots Temperature Calculations Use the ISA Lapse Rate

2°C/1000 ft or 0.65°C/100 metres

Kelvin

°K = °C + 273

Example 1

The 0ºC isotherm is forecast to be at FL 50. At what FL would you expect a temperature of -6ºC? Need to lose 6°C which is the equivalent to 3000 ft

Answer Example 2

Answer Example 3

FL 80 The temperature at 10 000 FT in the ICAO Standard Atmosphere is: Surface Temperature in ISA

+15°C

Lapse for 10 000 ft

(2 x 10)

-20°C

ISA Temperature

(+15 – 20)

-5ºC

An outside air temperature of -35ºC is measured while cruising at FL 200. What is the temperature deviation from the ISA at this level: Surface Temperature in ISA

+15°C

Lapse for 20 000 ft

(2 x 20)

-40°C

ISA Temperature

(+15 – 40)

-25ºC

Actual temperature

-35°C

Temperature is 10°C lower than ISA Answer Example 4

ISA Deviation

-10°C

The temperature on the 300 hPa chart is -48ºC, the tropopause is at FL 330. What is the most likely temperature at FL 350? Remember that the temperature becomes isothermal at the Tropopause 300 hPa ~ 30 000 ft Actual temperature at 30 000

-48°C

Calculate to FL 330 using ISA lapse rate Answer

3 x -2

-6°C

Temperature

-54°C

3

Chapter 2 – Atmospheric Pressure Pressure decreases with height. Definitions QFE

Pressure at an airfield reference point On the airfield, the altimeter reads zero with QFE set With QFE set reference is - Height

QFF

QFE reduced to mean sea level using the actual temperature Used on Synoptic Charts – surface weather charts Measured to one decimal place

QNH

QFE reduced to mean sea level using ISA On the airfield, the altimeter reads airfield elevation with QNH set Rounded down to the nearest hPa With QNH set reference is - Altitude

QNE

Landing altimeter setting 1013 hPa set ATC gives the pilot the reading of the altimeter when landing

SPS

1013.25 hPa With SPS set refer to – Flight Level

Isallobar An imaginary line or a line on a chart connecting the places of equal change of atmospheric pressure within a specified time Isallobaric Low

A pressure fall centre

Isallobaric High

A pressure rise centre

QFF and QNH Relationship Above msl

Below msl

Warmer than ISA

QFF < QNH

QFF > QNH

Colder than ISA

QFF > QNH

QFF < QNH

If the air temperature is not available then the calculation is not possible. At 0 ft amsl QFF = QNH = QFE Air Density High Density

Cold air, high pressure

Low Density

Warm air, low pressure

Pressure the dominant factor so density decreases with height Dry air more dense than moist air

4

Meteorology Data Sheet

Pressure and Height Equivalent Levels Pressure Level

Flight Level

1000 hPa

Sea Level

850 hPa

FL 50

700 hPa

FL 100

500 hPa

FL 180

400 hPa

FL 240

300 hPa

FL 300

250 hPa

FL 340

200 hPa

FL 390

150 hPa

FL 450

Height Change for 1 hPa Mean Sea Level 18 000 ft

5500 m

38 000 ft

27 ft

8m

50 ft

15 m

100 ft

32 m

Pressure Systems Isobar

Lines of equal pressure at a given level. On a surface chart reduced to msl - QFF

High

Anticyclone Pressure at its highest in the centre Wind clockwise round a high

Low

Depression or cyclone Pressure at its lowest in the centre Wind anticlockwise round a low

Ridge

Extension of a high

Trough

Extension of a low

Col

Region between two highs and two lows Area of light winds due to the slack pressure gradient

5

To Calculate QNH from QFE or Vice Versa between QFE and QNH is always the same.

Elevation only is required. The difference

Above msl

Below msl

QNH = QFE + hPa difference

QNH = QFE - hPa difference

QFE < QNH

QFE > QNH

Example 1

If the QFE at Locarno (200 metres above sea level) is 980 hPa, what is the approximate QNH? Calculate the hPa difference for Locarno (200 ÷ 8) 25 hPa Airfield is above sea level so QNH = QFE + hPa difference

Answer Example 2

980 + 25 =

1005 hPa

An aircraft lands at an airport (airport elevation 1240 FT, QNH 1008 hPa). The altimeter is set to 1013 hPa. The altimeter will indicate: Think of the altimeter, you increase pressure you add altitude. If you landed with 1008 hPa set the altimeter would read the elevation. 1240 ft The aircraft landed with 1013 hPa set. 5 hPa difference which is an increase on the QNH. 5 hPa ~

+135 ft

Answer Example 3

1375 FT. The QFF at an airfield located 400 metres above sea level is 1016 hPa. The air temperature is 10ºC higher than a standard atmosphere. What is the QNH? Use the table on Page 4

Answer

More than 1016 hPa

6

Meteorology Data Sheet

Chapter 3 – Altimetry Pressure Altitude and True Altitude The pressure altitude is equal to the true altitude only if standard atmospheric conditions occur. The density altitude is equal to the true altitude only if standard atmospheric conditions occur. High to Low look out below Example

An aircraft is flying over the Alps on a very cold winter's day. The regional QNH is 1013 hPa. During the flight, The aircraft circles around a mountain at an altitude of its summit. What reading will the aneroid altimeter give, compared to the elevation of the summit?

Answer

A higher altitude than the elevation of the summit due to the pressure dip at the top of the mountain.

Minimum Usable Flight Level Lowest usable flight level when conditions are 

Lowest value of QNH and the highest negative temperature deviation from ISA.



Safest when the temperature is ≥ ISA; QNH ≥ 1013 hPa

Lies at the same height or greater than the minimum safe altitude when the conditions are: 

At a temperature greater than or equal to that of the ISA and where the QNH is greater than or equal to 1013.25 hPa

Altimeter Readings 

After a front has passed pressure increases – altimeter reading decreases



Before a front passes the pressure falls slightly – altimeter reading increases



Where there is a weak pressure gradient there will be no change in the altimeter reading

Altimeter Calculations + ISA add

Pressure greater than 1013 hPa

- ISA subtract

Pressure less than 1013 hPa

Temperature greater than ISA (+15°C) Temperature less than ISA (+15°C)

7

For all calculations: 1 hPa ≈ 27 ft ≈ 8 m



Correct for barometric error first



If QNH is the start point then barometric error does not need to be corrected for



For Temperature Error use a correction of: 4% for every 10°C deviation

Example 1

If atmospheric conditions exist such that the temperature is ISA +10ºC in the lower troposphere up to 18 000 FT, what is the actual layer thickness between FL 60 and FL 120? In ISA conditions the layer thickness 6000 ft Deviation is ISA +10, 4% correction needed + ISA so add the correction

+240 ft

Answer Example 2

6240 ft Whilst flying at FL 180 in the northern hemisphere an aircraft experiences right drift. What effect, if any, will this have on the aircraft's true altitude? Use Buy’s Ballots Law to determine where the low pressure is.

LOW

Right Drift

HIGH

High to Low – Look out below Answer

The True Altitude decreases

8

Meteorology Data Sheet

Example 3

An aircraft is flying from Point A to Point B on the upper level contour chart. The altimeter setting is 1013.2 hPa.

First decide on the pressure systems at A and B With the direction of the wind both are Low Pressure Elongate the wind arrow – It is obvious now that A is a higher pressure than B. Answer

Example 4

The true altitude will be higher at A than at B

The QNH of an airport at sea level is 983 hPa and the temperature deviation from ISA is -15ºC below FL 100. What is the true altitude of FL 100? 983 hPa less than 1013 hPa

Subtract Barometric Error

Temperature deviation –15°C

Subtract Temperature Error

FL 100

10 000 ft

983 hPa 30 hPa ~ 810 ft 1013 hPa

9

Correct for Barometric error first 10 000 ft hPa difference, 30 hPa ~

-810 ft 9190 ft

Correct for Temperature Error ISA –15°C ~ 6% correction of 9190 ft Answer

-551 ft 8639 ft

Example 5

You plan a flight over a mountain range at a true altitude of 15 000 FT/AMSL. The air is on an average 15ºC colder than ISA, the pressure at sea level is 1003 hPa. What approximate indication should the altimeter (setting 1013.2 hPa) read? Read the question!! It is not asking for the True Altitude but the reading on the altimeter with 1013 hPa set for the aircraft to be safe. Reverse the corrections for Barometric and Temperature Error. Correct for Barometric Error

15 000 ft

1003 hPa is 10 hPa difference to 1013 hPa Changing from 1003 to 1013 will add altitude +270 ft 15 270 ft Correct for Temperature Error -15°C deviation is a 6% correction Answer Example 6

+916 ft 16 186 ft

During a flight at FL 100 from Marseille (QNH 1012 hPa) to Palma de Mallorca (QNH 1015 hPa), an aircraft remains at a constant true altitude. The reason for this is that: In ISA conditions the column of air at Marseille would be of less height than the column of air at Palma. The only way that column of air can change height is by increasing or decreasing the temperature. Answer

The air at Marseille is warmer than that at Palma de Mallorca or the air at Palma de Mallorca is colder than the air at Marseille.

10

Meteorology Data Sheet

Example 7

You are flying at FL 130, and your true altitude is 12 000 ft. What is the temperature deviation from the standard atmosphere at FL 130 (QNH 1013,2 hPa)? A reversal of the Temperature Error will give the answer Difference between FL 130 and 12000 ft 1000 ft Work out what percentage 1000 ft is of 12 000 ft 8.33% 4% is 10°C deviation 8% is 20°C deviation Because the True altitude is less than the Pressure Altitude it must be -ISA Answer

ISA -20ºC

11

Chapter 4 – Thermodynamics, Adiabatic Processes and Stability of the Atmosphere Adiabatic Process Dewpoint saturation.

During an adiabatic process heat is neither added nor lost

The temperature to which a mass of air must be cooled in order to reach

To get dew, fog or cloud the temperature must be cooled further. Dewpoint can only be equal to, or lower than, the temperature of the air mass Dewpoint Spread

The difference between the OAT and dewpoint

Large Spread Low humidity Small Spread High humidity The amount of water vapour that air contains depends upon air temperature. Humidity Mixing Ratio (HMR) The HMR is the ratio of the mass of water vapour present relative to the mass of dry air in the air parcel. Expressed as g/kg of dry air. The HMR will remain constant as long as the moisture content of the air parcel does not change. Even as the parcel expands or contracts the total mass of water vapour remains the same thus the HMR will remain the same. Relative Humidity the air.

Relative Humidity depends on moisture content and temperature of



Ratio between the actual mixing ratio and the saturation mixing ratio x 100



Constant pressure and temperature, RH increases with increasing water vapour content. RH decreases with decreasing vapour content



Increase in temperature decreases RH if no other variables change. Dewpoint will remain the same.



Decrease in temperature increases RH if no other variables change. Dewpoint will remain the same



Descending air warms adiabatically – Relative humidity decreases as the moisture content remains the same

High Relative Humidity means:

Low Relative Humidity means:

Low evaporation rate

High evaporation rate

Reduced Latent heat absorption

Increased Latent Heat absorption

Small wet/dry bulb difference

Large wet/dry bulb difference

Relative Humidity Changes Morning

High RH

Afternoon

Low RH

Descending Air

Warms on descent – reduces RH

Ascending Air

Cools on ascent – increase in RH

12

Meteorology Data Sheet

Stability If a parcel of unsaturated air is lifted to just below the condensation level and then returned to its original level the surface temperature will return to the same starting temperature. Stable Air

The vertical motion of rising air tends to become weaker and disappears

Stability When

Unstable Air



Warm air is advected into the upper part of a column of air



Cold air is present in the lower layer.

The vertical motion of rising air tends to become stronger

Dry Adiabatic Lapse Rate

DALR

3°C/1000 ft

1°C/100 m

Saturated Adiabatic Lapse Rate

SALR

1.8°C/1000 ft

0.6°C/100 m

Dewpoint Lapse Rate

DPLR

0.5°C/1000 ft

ISA Lapse Rate

2°C/1000 ft

Environmental Lapse Rate

ELR

0.65°C/100 m

Varies with time and gives the actual conditions within the atmosphere

SALR is less than DALR due to the release of Latent Heat due to condensation

Height SALR DALR

1.8°C/1000 ft 0.6°C/100 m

3°C/1000 ft 1°C/100 m

U

S – Stable

CI

S

U – Unstable CI – Conditional Instability

Surface Temperature Temperature

A layer is conditionally unstable if the air is unstable for saturated air and stable for dry air. Stable Air

Stratiform clouds

Unstable Air

Convective clouds

The ELR is less than the SALR The ELR is greater than the DALR 13

Conditional Instability
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