Weather knowledge is essential in aviation: Visibility, air pressure, thunderstorms, temperature, clouds, rain and dew point etc.
All affect flying in one way or another. You are most affected by weather when flying VFR, but also when flying IFR, there are restrictions how bad weather can be before plans must be changed. In this section, we'll look into how weather is reported, but not how weather affects flight.
There are many acronyms and concepts in weather reporting, which we will learn throughout the rest of this document. Below we'll cover rather completely the weather report. There are many acronyms and concepts. You need not know them all from the beginning, but you must be able to get the name of the airport, wind and air pressure. Since you as a controller read the weather report to the pilots, you'll soon learn the rest of the acronyms.
6.2 METAR – Aviation Routine Weather Report [S/S+*]
* As a Student you should have knowledge about what a METAR is and recognize the most essential parts (Where, when, winds and QNH) by heart and know where to find information about the other elements. As a Senior Student you should be able to read out a whole METAR.
At major airports, weather observations are made every half hour, day and night.
The weather in the METAR is the weather observed 10 minutes before the report.
The following is part of a METAR:
The following is a brief breakdown of some of the basic METAR elements:
METAR EHAM 121755Z 21016G24KT 180V240 1500 R06/0600U -RA BR BKN015 0VC025 06/04 Q1005 BECMG 9999=
TYPE OF REPORT
Either routine (METAR) or non-routine (SPECI). En Route Facilities use either a M or S which follows the time of the report.
Four letter ICAO Code.
DATE and TIME
First two digits are the date, followed by the hour and minutes in UTC time (Z).
Normally a 5-digit grouping (6-digits if speed is over 99 knots). The first 3-digits is the direction, the next two or three is the speed. G indicates gusts with the highest gust report after it. V indicates variable wind direction. VRB indicates variable wind speed 3 knots or less Note: International stations may use meters per second (MPS) or kilometers per hour (KMH) and code the wind speed accordingly.
Prevailing visibility reported in Meters. 2800 represents 2,800 meters and 9999 represents a visibility greater than 10 km.
"R" indicates the group followed by the runway heading (06) and the visual range in meters. The report might include a "U" for increasing or "D" for decreasing values.
This example shows light rain with mist.
Shows the amount of sky cover and cloud base height.
The three digits indicate the cloud base height in hundreds of feet.
The cloud ceiling is lowest height where at least half of the sky is covered, hence the lowest BKN or OVC.
Reported in two, two-digit groups separated by a slant ("/"), in degrees Celsius. Temperature and dewpoint below zero are prefixed with a "M".
QNH reported in a four-digit format in HectoPascals (Hpa) preceded with a "Q". In i.e. USA altimeter reports are in inches of mercury.
How the weather is going to develop in the time the METAR is current (2 hours). You can here find Sky conditions, Visibility, Wind, Weather
6.2.2 Visibility [S+]
The two latter is the distance measured to a large unlit object, where the contours of this object can be seen.
Visibility is affected by moisture, ice crystals, salt or dust in the air.
Visibility in darkness is today most often measured with infrared light.
Visibility is measured in steps up to 10 km. Visibility below 50 m is written as 0000 and 10 km or more as 9999. Visibility below 5000 m is given in m, and above in km.
6.2.4 Weather Phenomena [S+]
Current weather is included in the METAR. It is abbreviated with two letters. If this isn't enough, the abbreviations can be combined. The abbreviations are listed at the end of this chapter. Some abbreviations can be preceded with a ”+” or ”-”, this lists the intensity. VC means Vicinity, which means within 8 km from the airport. Exception: thunder and cumulonimbus clouds. Sometimes the recent weather is also reported, in this case ”RE” precedes the weather condition.
The cloud base above the airport's reference height AGL (Above Ground Level) is measured in hundreds of feet. 001 means 100 ft, 012 = 1200 ft and 120 = 12000 ft. Vertical visibility is reported as VV, and if this is not measured VV///.
As described in other sections in this manual, the air pressure is vital to know, since it affects the altitude measuring system. Air pressure can be measured in different ways, and relative different levels.
The first two can be given with a time reference.
6.2.11 Runway Conditions [C]
B: TYPE OF DEPOSIT
C: EXTENT OF CONTAMINATION
0= CLEAR and DRY
1=10% or less of RWY covered
2=11-25% of RWY covered
2= WET or Water Patches
5=26-50% of RWY covered
3= RIME or FROST (<1mm)
9=51-100% of RWY covered
4= DRY SNOW
5= WET SNOW
8= COMPACTED or ROLLED SNOW
9= FROZEN RUTS or RIDGES
/= TYPE of DEPOSIT NOT REPORTED, e.g due to RWY clearance/de-icing in progress
/= NOT REPORTED e.g due to RWY clearance or de-icing in progress.
DD: DEPTH OF DEPOSIT
EE: BRAKING CONDITIONS
00= less than 1mm
01 to 90= depth in mm, e.g 23=23mm
Reported figures from 01 to 90 represent FC, e.g 05=FC 0.05, 28=FC 0.28
99= RWY not operational due to snow, slush, ice, large drifts or RWY clearance. Depth not reported.
99= UNRELIABLE, BA and FC not possible to assess, misleading, e.g in case of aquaplaning.
//= Depth operationally not significant e.g with ice or rolled snow, or not measurable e.g RWY wet.
//= RWY not operational, BA and FC not reported.
6.3 VMC Visual Meteorological Conditions [C]
Please note that the VMC minima differ between countries and you have to refer to your local vACC to get the minimums for your country. If no such values are available you can use the ones below.
Class C, D, E
At or above FL100: 8 km
At or above FL100: 8 km
Clear of clouds
Horizontal distance minimum 1500m
Class F & G
Above 3000ft MSL or 1000ft AGL whichever is higher
At or below 3000ft MSL or 1000ft AGL whichever is higher
Horizontal distance minimum: 1500m
Clear of clouds and ground in sight
Wind Gusts / Maximum
Nil significant weather
Ceiling And Visibility OK
Nil significant change