The Weather

Despite a fine looking late summer day I arrived at Blackbushe to be greeted by Tony telling me we weren't flying. There was a lot of turbulence, wind was very gusty and veering all over the place. Tony thought it was a good time to do a lesson on weather, specifically METAR, TAF and what Tony calls Wind Checks.

I'd been looking at forecasts for a while via AeroWeather and was familiar with the kind of information passed in TAFs and METARS but it was very useful to go though what I should actually get from them. I'm going to include a write up of what Tony said here for my own learning purposes.

METAR - Met Actual Report

Weather report from the Met office reporting current conditions.

The current report for Farnborough (the local service for Blackbushe) is as follows:-

EGLF 111250Z 21016KT 180V250 9999 SCT025 19/13 Q1005

EGLF - The ICAO airport code for the station.

111250Z - Date and Time. 11th, 12:50, ZULU time (GMT)

21016KT - Wind and direction. Bearing 210 16 knots. May include G for gusts and a speed.

180V250 - Wind direction variable between 180 and 250.

9999 - Visibility. 9999m or 10km. This is as good as they measure.

SCT025 - Scattered cloud at 2500ft. Just add two zeros as Tony said to Nicholas Lyndhurst. Sometimes two cloud entries when there are layers.

19/13 - Temperature and Dew Point. Tony's tip: difference x400 for visible moisture. Today 2400ft. 

Q1005 - Qnh is 1005.

Other weather types that appear:-

VCXX - Vicinity XX

RA - Rain

SH - Showers

SHRA - Showers and rain, can include be + or -

DZ - Drizzle

RADZ - Rain and drizzle

MI - Mist

BR - Fog (Brume)

SN - Snow

GR - Hail (Tony says they say Granite Rain in the US - Grêle)

RERA - Recent rain

FU - Smoke (Fumée)

CB - Cumulo Nimbus

CU - Cumulus

TCU - Towering Cumulus

NSW - No significant weather

This guy has a nice article on the subject.

Cloud types are measured in 1/8ths of the sky so:-

FEW - Few clouds 1/8 to 2/8

SCT - Scattered clouds 3/8 to 4/8

BKN - Broken clouds 5/8 to 7/8

OVC - Overcast 8/8

TAF - Terminal Aerodrome Forecast

This is a weather forecast and has a stated valid time.

EGLF 111101Z 1112/1120 21016KT 9999 SCT020 TEMPO 1112/1120 20018G28KT

Station, date & time, wind, visibility, cloud are the same at the METAR

1112/1120 - valid time. In this case 11th 12:00 to 20:00.

TEMPO - Temporary condition which is no more than half of the forecast time

BECMG - Becoming, condition will change to.

PROB30 - Might happen

PROB40 - Probably happen. 

Wind Check

How we calculate the cross wind component. 

This is very important and based on the aircraft type we're flying. The ratings for familiar aircraft are:-

PA28 17-20 kts

C152 12 kts

We use the Clock Code to get the cross wind amount. Using runway 25 i.e. bearing of 250±10°

Wind 220 @ 40 kts. 30° difference. 30 is half clock face, so cross wind is half ∴ 20 kts.

Wind 210 @ 30 kts. 40° difference. 40 is 2/3 clock face, so 2/3 cross wind ∴ 20 kts.

Approach speed

If the wind is gusting we add 1/2 the difference in wind to the approach speed.

Thus 220 20 G30. Difference 10 means our approach speed needs 5 extra knots.

This is important on shorter runways. A PA28 has Vapp  of 70kts and Vatt 65kts. 1 knot difference is about 100ft stopping space on the runway.