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PILOT NAVIGATION
Senior/Master Air Cadet
THE WEATHER
Introduction
Previously you have learnt how the weather affects the hill walker, and the
aviator in the circuit
We will now see how the weather affects aircraft en route between
airfields
The Air
In order to understand the reactions involved when the air is in motion we must consider
its constituents
The major variable is water as we shall see
Pure air consists of :
OXYGEN20%
NITROGEN 79%NITROGEN 79%
OTHER1 %
(POLLUTION,OZONE,
CO 2)
Temperature & Pressure
From your physics you will remember Boyles’s gas laws. If you don’t you should
know that up high in the atmosphere (such as on top of a mountain) it is very cold and climbers need to carry oxygen
This is because the air pressure is very low at high altitudes
Temperature & Pressure
The air pressure at sea level is caused by the weight of the air above us. The higher you go,
the less the weight becomes
If a gas is compressed it become hotter, & so the less compressed it is the colder it is
Water Vapour
A certain volume of air, under fixed conditions of pressure and temperature, can only hold a certain amount of water
vapour, which is an invisible gas
If this air becomes cooler it will not be able to hold as much moisture and will eventually
become saturated with water vapour
Water Vapour
When the air cools to this point it is known as the dew point
It is at this point that dew , mist or fog will form. If the temperature falls below
freezing then frost or freezing fog will occur
Cloud
When unsaturated air rises the temperature drops & the dew point is
reached, and cloud is formed
D P
AIR RISINGDUE TO
TRIGGERACTION
TEMPERATUREDROPPING
Updraft
Updraft is the vertical motion in the air
Trigger Actions
There are four trigger actions that force air to rise:
Turbulence - air currents coming together
Convection - heating of the air by the sun
??????
Trigger Actions
Orographic uplift (mountain effect) air hitting mountain ranges
Trigger Actions
Frontal uplift - when 2 fronts meet
Cool Air
Warm Front
Warm air
The height of the cloud will depend on the amount moisture in the air, and the
strength of the uplift
In extreme cases Cumulo Nimbus thunder clouds form, and these are a
severe hazard to aircraft
At high levels the clouds (cirrus) consists of ice crystals, but most other clouds are formed
by visible droplets of water
Thunderstorms
A large thunderstorm presents a variety of hazards, both in the cloud and surrounding
areas, to aircraft and are best avoided
Precipitation - all types are present - the most dangerous being hail
Icing - as previously covered
but the main danger is to electrical systems- radar, radios etc, and temporary blinding of
the crew !
Lighting & thunder
the main effect is psychological. Lighting often strikes aircraft
-
Turbulence
the air in a thunder cloud can be in vertical motion up or down in excess of
50 knots and can change from up to down and back again in seconds. This
can destroy aircraft
Landing hazards
all of the above hazards exist under the base of the thunderstorm - the most
significant being the risk of a severe down drought just as the aircraft is landing
Most modern aircraft carry weather radar’s for detecting thunderstorms &
turbulent air
Isobars
When watching the weather forecasts on TV you will have noticed that the air
pressure changes from place to place
The normal range of pressure is
930 millibars
to 1050 millibars
The pressure is shown on the chart by
Linking areas of
equal pressure by a line called an Isobar
1016shown thus
Pressure in the Atmophere
This is measured using Millibars in the UK and Hectopascals in
Europe
The isobars surround areas of high or low pressure, and show us how the wind is moving - the wind velocity
(WV)
1016
1000992
L
The WV is always expressed as where it is coming from in degrees and its
strength in knots
For example a WV of 200/25 is a wind coming from 200° at 25 knots
In the northern hemisphere the wind circulates clockwise around anti cyclones (high
pressure areas) & anti clockwise around cyclones ( low pressure areas)
1016
1000992
L
WIND
To remember this stand with your back to the wind, and the area of low
pressure is to your left
This is reversed in the southern hemisphere
Isobars – facts and figures
Isobar patterns represent the wind at 2000’ above the
surface
Isobars – facts and figures
The direction of the lines give the direction the wind
The closer the lines the stronger the wind
Isobars – facts and figures
At the surface the wind is about 25% weaker than at 2000’ as a result of surface friction.
It is also backed by about 25°
For example: a wind at 2000’ of 270/20 will be 245/15 at the surface
TAFs & METARs
The weather forecaster uses many charts & symbols to convey the details of the weather
over the whole country
For an aviator, who receives info from radio, it must be coded & is standardized world wide
(except Canada !)
There are 2 formats:T A F
Terminal Aerodrome Forecast
M E T A R
Meteorological Actual Report
one records a forecast and the other reports actual conditions
TAF
Is published for a 9 hour period, and starts with 4 figures
eg 0615 introduces a forecast valid from 0600 to 1500 hrs
TAF
TAFs do not include temperatures or pressures
But may include information on expected changes
TAF
These are prefixed :
TEMPO - Temporary
BECMG - Becoming
TAF
If the forecaster is unsure then he gives a probability:
PROB 30TS
means 30% chance of thunderstorm
METAR
Gives the conditions at an airfield and is recorded hourly
It is normally prefixed by a time - which is the time the conditions where observed
It is given to aircraft inbound
METAR
If the weather changes rapidly a SPECI – (special) report is issued
METARs & SPECIs Do not forecast conditions, but do include pressure &
temperature
The Code
TAF
CRANWELL 0615 260/05 4000 HZ SCT030 BECMG CAVOK=
Cranwell/ for 0600 - 1500 hrs/ surface wind 260° true at 5 knots/visibility 4000 meters in haze/scattered cloud base at 3000’/becoming “cloud and visibility okay” that is the visibility will be at least 10km and no cloud below 5000’
This translates as
CRANWELL 0615 260/05 4000 HZ SCT030 BECMG CAVOK=
The Code
METAR
CRANWELL 0900 250/07 8000 FEW 035 +17/+13 1028=
This translates as
CRANWELL/ AT 0900 Z HRS/ SURFACE WIND 250° TRUE AT 7 KNOTS/VISIBILITY 8000 METERS/LOWEST CLOUD - few (EIGHTS) AT 3500’/TEMPERATURE +17°C, DEW POINT + 13°C/PRESSURE 1028
CRANWELL 0900 250/07 8000 FEW 035 +17/+13 1028=
In the USA temperature is give in °F
= Means End Of Message
Pressure is the QNH at the airfield
TAF and METAR codes
Time: A 4 Figure Group In Hours/Minutes
Wind: Wind Speed In Knots & Direction In Degrees True. In gusty conditions a “G” is added with A higher figure to indicate range – 18G28
Visibility:
Weather:
2 letter groups to indicate type of weather
4 Figures From 0000 To 9999. This Is The Visibility In Meters & Km
BR Mist
FZ Freezing
DZ Drizzle
SN Snow
FG Fog
FU Smoke
HZ Haze
HZ Haze
RA Rain
SH Shower
TSThunderstorm
- Slight
+ Heavy
These can be mixed in any combination:
RASN Rain & snow mixed
+SHRA Heavy rain shower
-FZDZ Light freezing drizzle
Cloud: 6 item code giving cloud amount & height of cloud base
Cloud Amounts:
FEW 1 or 2 eighths coverage
SCT Scattered 3 or 4 eighths
BKN Broken 5 or 7 eighths
OVC 8 eighths coverage
Cloud Base:
3 numbers to indicate cloud base height above airfield in hundreds of feet, eg. 018 = 1800 feet
Check of understanding
What is a TAF?
Terminal Aerodrome Forecast
What is a METAR?
Meteorological Actual Report
What is a SPECI?
If the weather changes rapidly a SPECI – (special) report is issued