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

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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

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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