PILOT NAVIGATION

Part 3

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 becomes 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

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 of 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

Cumulo Nimbus

At high levels the clouds (cirrus) consists of ice crystals, but most other clouds are formed

by visible droplets of water

Cumulus

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 draught 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

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 DECODE

TAFNormally published for a 24 hour period but may be shorter.Airfield Identifier.

Time Published

Time Valid For

TAFWind

Visibility

Significant Weather

Significant Changes

TAF

Cloud

TAF

TAFs do not include temperatures or pressures

The Code

TAF

CRANWELL 1306/1406 260/05 4000 HZ SCT030 BECMG CAVOK=

Cranwell/ for 0600 hrs on 13th - 0600 hrs on 14th/ 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=

METARGives the conditions at an airfield and is

recorded hourly

It is normally prefixed by a time - which is the time the conditions were observed

It is given to aircraft inbound

It can also be designated by a letter code,ATIS(Automatic Terminal Information Service)‘A’ would be the first METAR for the airfield when it opens and would move through the

alphabet hourly.

METAR

If the weather changes rapidly a SPECI – (special) report is issued

METARs & SPECIs Do not forecast conditions, but do include pressure &

temperature

METARIdentification

Wind

Visibility

METARRVR

Only given if visibility is poor

Present Weather

Cloud

METARCAVOK

Temperature and Dew Point

QNH

Recent Weather

METARWind Shear (If present)

Colour States

Runway State Group

METAR

Trend

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

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