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METEOROLOGY ROBERT JURCA
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METEOROLOGY ATPL Exam preparation The Atmosphere • Wind • Thermodynamics • Clouds And Fog • Precipitation • Air Masses And Fronts • Pressure Systems • Climatology • Flight Hazards • Meteorological Information
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Table of Contents
THE ATMOSPHERE............................................................................................... 4 PRESSURE SYSTEMS ............................................................................................7 ALTIMETRY .............................................................................................................. 14 THERMODYNAMICS ............................................................................................ 18 WIND ........................................................................................................................22 CLOUD ....................................................................................................................29 FOG, MIST AND HAZE ....................................................................................... 37 PRECIPITATION .....................................................................................................39 FRONTAL SYSTEMS ........................................................................................... 43 JET STREAMS ........................................................................................................51 CLIMATOLOGY .....................................................................................................56 ICING ........................................................................................................................63 TURBULENCE .......................................................................................................65 THUNDERSTORMS .............................................................................................69 TROPICAL REVOLVING STORMS ................................................................... 72 WEATHER CHARTS ............................................................................................. 74 METEOROLOGICAL INFORMATION .............................................................. 78 CLIMATOLOGY .....................................................................................................84
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DEFINITIONS: ADVECTION
The horizontal motion of air
CONVECTION
The upward motion of air Air cools adiabatically in the ascent.
CONVERGENCE
The inward horizontal motion of air Convergence at the surface means instability and convective cloud Convergence at height means high pressure at the surface and little or no cloud.
DIVERGENCE
The outward horizontal movement of air. Divergence at the surface means stability and stratiform or no cloud Divergence at height means a low pressure at the surface with the likely formation of clouds.
INVERSION
An inversion is a layer of air in which the temperature increases with height This is a layer of absolute stability.
ISOBAR
Line joining points of equal pressure. On a surface weather chart the isobars are reduced to mean sea level.
ISOHYPSE
Line joining points of equal height on a particular surface, generally an isobaric surface Indicates the true altitude of a pressure level.
LABELLING
Isohypses are measured in metres or decametres. 5520 5520 metres 552 552 decametres (tens of metres) Line joining points of equal wind speed.
ISOTACH ISOTHERMAL
An isothermal layer is a layer of air in which the temperature, remains constant with height This is a layer of absolute stability.
SUBSIDENCE
Vertical downward motion of air. Associated with anticyclones. Subsiding air is cloudless due to the heating by compression.
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THE ATMOSPHERE COMPOSITION Nitrogen 78.09% Oxygen 20.95% CO2 0.03% Argon 0.93% Rest rare gases Water vapour is always present in the atmosphere. The percentage of water vapour in the air in the lower troposphere is approximately 5%.
ISA Mean Sea Level (MSL)
Temperature Pressure Density
15°C 1013.25 hPa 1225 gm-3
From MSL to 11 km (36 090 ft)
Temperature decreases at 0.65°C per 100 m (1.98°C per 1000 ft). Use -2°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)
ISA Deviation
Cold air is more dense than warm air
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EXAMPLE
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
ANSWER:
EXAMPLE
ISA Deviation -10°C In the above answer the air is denser than ISA at FL 200
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
-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
ISA Temperature (+15 – 20)
FL 80
The temperature on the 300 hPa chart is –48°C The tropopause is at FL 330. What is the most likely temperature at FL 350? The temperature becomes isothermal at the tropopause so the temperature at FL 330 is the same as at FL 350 Calculate the temperature at FL350 using the ISA lapse rate
ANSWER:
Lapse for 3000 ft (2 x 3) -6°C Temperature at FL300 -48°C -54°C
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Troposphere
Contains 80% of the atmosphere, with ½ the mass in the first 18 000 ft (5500 m). Contains 90% of the atmosphere’s water vapour, the most important constituent in regard to weather. Temperate latitudes up to 11 Km
Tropopause
Boundary between the Troposphere and the Stratosphere. Temperature becomes Isothermal Latitude Height Temperature Equator 55 000 ft – 17 Km -75°C 50° 35 000 ft – 11 Km -55°C Pole 25 000 ft – 7 Km -45°C
Stratosphere
11 – 50 Km. Initially Isothermal becoming an inversion. CB can penetrate well into the Stratosphere
Heating of the Atmosphere
The sun does not heat the atmosphere directly. The heat energy of the sun heats the earth’s surface which in turn heats the air in the atmosphere. Insolation
INcoming SOLar radiATION
Atmosphere heated 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
The daily fluctuation of temperature. The diurnal variation in temperature: • 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 Minimum Temperature KelvIn Temperature Scale
2 hours after 1200 LMT Clear sky, light wind ½ hour after dawn Clear sky, light wind ºK = ºC + 273
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PRESSURE SYSTEMS
hPA
Pressure and temperature decrease with height.
Air Density
High Density Low Density
Cold air, high pressure Warm air, low pressure
Pressure the dominant factor so density decreases with height Dry air more dense than moist air LOW
1004
SECONDARY
1008
1020
COL
1012
1008
High Pressure
1020
1024
RIDGE
1004
LOW
1008
1016
1024
1016
LOW
1012
HIGH
OR 1000
TROUGH 0F LOW PRESSURE
1028
1032
WEDGE OF
HIGH PRESSURE
HIGH
Pressure values decrease with distance from the centre of the high or axis of the ridge. Subsidence occurs. Divergence at the surface, convergence aloft . • Isobars widely spaced – light winds. • Subsidence inversions possible. Visibility
Winter Poor, especially where there has been radiation cooling and fog formed. Strong winds will form SC/ST Summer Moderate to poor visibility in HZ with little cloud Cloud None or patchy SC/ST Precipitation None
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Low Pressure
Roughly circular in shape. Trough a V shaped wedge away from the centre. Isobar value decreasing towards the low. Convection occurs with rising air. Convergence at the surface with widespread ascent, divergence aloft. Deepening Low Pressure
Divergence greater than convergence
Filling Low Pressure
Convergence greater than divergence
Surface winds where the isobars on the weather map are very close together are strong and flow across the isobars towards the low pressure Visibility
Good out of showers
Cloud
Convective Activity Leads to CU and CB. Over land in mid-latitudes convective activity is greatest in summer in the afternoon.
Precipitation Showers High Pressure Systems
Anticyclone (high), ridge (wedge)
Low Pressure Systems
Depression (low), cyclone or trough
Col
Region between two highs and two lows. Area of light winds due to the slack/uniform pressure gradient. Col weather can vary from: Winter Radiation fog, cold frosty nights Summer TS and showers (thermal low)
Isobars
Strong PGF - tight isobars – strong wind.
Secondary Depression
Forms in the circulation of the primary low. It tends to move round the primary low in a cyclonic sense
Wind
In the lower layers of the atmosphere due to friction the wind changes direction towards the low pressure area because wind speed decreases and therefore Coriolis force decreases.
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Because of surface friction the surface wind in the Northern Hemisphere is backed and: • Flows away from a high pressure • Flows into a low pressure In the Southern Hemisphere the wind is veered. Warm Core High Warm Core Low Cold Core High
Cold Core Low
Pressure surfaces bulge upwards at all levels, increasing intensity with height; Azores High Pressure decreases slowly with height Cold air mass where the pressure decreases rapidly, decreasing intensity with height. The lower situated pressure surfaces bulge upward and the higher situated pressure surfaces bulge downward; Siberian High Pressure lower than the surroundings at all heights
Blocking Anticyclone
A blocking anticyclone in the northern hemisphere is a warm anticyclone/quasi stationary anticyclone situated between 50ºN and 70ºN
Thermal Low
Lows formed due to temperature difference on land or sea. Common over Europe in summer during the late afternoon.
Extensive cloud and precipitation is often associated with a non frontal thermal depression because of surface convergence and upper level divergence causing widespread ascent of air in the depression PRESSURE DEFINITIONS QFE
Pressure at an airfield reference point/field elevation using ISA values 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
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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
Pressure Altitude/Density Altitude/True Altitude - Pressure altitude equals density altitude equals true altitude when ISA conditions occur. At altitude, the atmospheric pressure in a column of warm air is likely to be higher than at the same height in a column of cold air. Lowest True Altitude - Cold temperature, low pressure. Altimeter will indicate a higher altitude than that flown.
True altitude Given atmospheric pressure (pressure altitude)
Indicated altitude 3000 ft
2000 ft 1520 ft
2000 ft 1000 ft
High OAT
Standard OAT
Low OAT
Highest True Altitude - High temperature, high pressure. Altimeter will indicate a lower than flown altitude.
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Height Change for 1 hPa Mean Sea Level 18 000 ft 38 000 ft
5500 m
27 ft 50 ft 100 ft
8m 15 m 32 m
Equivalent Levels PRESSURE LEVEL 150 hPa 200 hPa 250 hPa 300 hPa 400 hPa 500 hPa 700 hPa 850 hPa 1000 hPa
FLIGHT LEVEL FL 450 FL 390 FL 340 FL 300 FL 240 FL 180 FL 100 FL 50 Sea Level
QFE/QNH Relationship
The difference between QFE and QNH is always the same.
To Calculate QNH from QFE or Vice Versa
Elevation only is required. The difference between QFE and QNH at an airfield is always the same. Above msl QNH = QFE + hPa difference QFE < QNH
QFF and QNH Relationship
Below msl QNH = QFE - hPa difference QFE > QNH
The difference between QFE and QNH is always the same.
Warmer than ISA Colder than ISA
Above msl
Below msl
QFF < QNH QFF > QNH
QFF > QNH QFF < QNH
If the air temperature is not available then the calculation is not possible. At 0 ft amsl QFF = QNH = QFE Over land in mid-latitudes convective activity is greatest in summer in the afternoon.
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Flying Towards Pressure Systems IN THE NORTHERN HEMISPHERE: Towards a Depression Towards a High
Right drift – true altitude decreases Left drift – true altitude increases
IN THE SOUTHERN HEMISPHERE: Towards a Depression Towards a High
Left drift – true altitude decreases Right drift – true altitude increases
NORTHERN HEMISPHERE
L
H
SOUTHERN HEMISPHERE
L
H
For both hemispheres, flying along the isobars/isohypse between a low and high pressure systems the true altitude will remain the same. Cold Pool
A cold air pool can be best identified by means of the isohypses on an upper air chart, and seen as a low pressure area aloft either on the 500 hPa or 700 hPa chart. Most evident in the circulation and temperature fields of the middle troposphere and may show little or no sign on a surface chart. The direction and speed of movement of cold air pools is difficult to forecast. The weather encountered during the summer, over land, in the centre of a cold air pool is showers and thunderstorms. The weather activity within a cold air pool is usually greatest in the afternoon.
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Flight Over High Ground
Flying over a mountain range on a cold day the altimeter will indicate a higher altitude than the top of the mountain due to the pressure dip at the top of the mountain. The converse on a hot day.
EXAMPLE
An aircraft is flying over the Alps on a very cold winterâ&#x20AC;&#x2122;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
Flying over a mountain range on a cold day the altimeter will indicate a higher altitude than the top of the mountain due to the pressure dip at the top of the mountain.
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ALTIMETRY When calculating true altitude use the following: + ISA add
Pressure greater than 1013 hPa Temperature deviation greater than ISA
- ISA subtract Pressure less than 1013 hPa Temperature deviation less than ISA Where a question asks for something other than true altitude then reverse the corrections. • Correct for barometric error first 1 hPa ≈ 27 ft ≈ 8 m • 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
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:
6240 ft
EXAMPLE
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:
A
Paris B
The true altitude will be higher at A than at B
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EXAMPLE
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 ISA –15°C Subtract Temperature Error 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 -551 ft ANSWER:
8639 ft
EXAMPLE
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:
+916 ft 16 186 ft
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