Pilots use METAR codes to communicate weather conditions to other pilots and air traffic controllers. These codes are defined by the World Meteorological Organization (WMO) and International Civil Aviation Organization (ICAO).METAR Codes describe the weather conditions of the air around us. It contains information about wind direction, speed, visibility, cloud base, temperature, dew point, pressure, etc of the aerodrome. The Weather Service maintains a network of observation stations around the world to provide continuous and accurate weather information. This information is then relayed to air traffic control centers, where it is used to make decisions regarding aircraft operations. Air traffic control centers have access to weather forecasts and advisories provided by the Meteorological department/NWS. These forecasts and advisories are based on observations made by the various weather stations around the country.
Present weather is a very important phenomenon with regard to aviation. It is the present weather that enables smooth flight operations. In this post, we will discuss how to report weather phenomena in METAR and SPECI.
How to report weather phenomena using METAR codes?
Present weather (w’w’) in METAR/SPECI is reported just after the visibility and before clouds reporting. i.e METAR LULX 260000Z 36005KT 1000 BR NSC T 20/19 Q 1011 RH95%. Here BR (Mist) is a weather phenomenon that is reported after visibility (1000m) and before clods(NSC).
What are the WMO and ICAO Guidelines for Present Weather?
When visibility is 5000 m or less, one of the phenomena FU, HZ, DU, SA, or BR is
reported in the METAR/SPECI. When visibility is above 5000 m, the phenomena
FU, HZ, DU, SA, or BR are not present by definition and are therefore not reported.
For instance, if visibility is 5000 m, it will be encoded as 5000 together with the
phenomena FU, HZ, DU, SA, or BR, causing this reduction in visibility.
When visibility is less than 5000 meters or 5000 meters, there is a phenomenon like FU, HZ, DU, SA, or BR in the METAR/SPECI report. Visibility of more than 5 000 meters has nothing to do with the phenomenon FU, HZ, DU, SA, or BR. Therefore, they are not reported. For instance, if the visibility is 5000 meters, it will be encoded as 5000 together with the phenomena FU, HZ, DU, SA, or BR, causing poor visibility. The phenomena FU, HZ, DU, SA, and BR will not appear if visibility is 5001 to 5999 m, but if it is 5000 (rounded down to the nearest 1 000 m) in the METAR/SPECI.
If more than one weather phenomena are present then separate groups are used to report in METAR/SPECI. If More than one form of precipitation is there then both are reported in a single group reporting the most dominant first and then the next. i.e if there is snow and rain at the same time it will be reported as SNRA. If It is heavy rain or snow it will be written as +SNRA.
What is the METAR Codes for the Present Weather?
WMO has clearly defined all the METAR codes for all kinds of weather phenomena that are to be reported in the METAR according to its predefined criteria.
The intensity of precipitation is indicated with +,- signs.- for light and + for heavy.
VC is an indicator of any phenomena which is not at the station but in the vicinity.
Important weather phenomena and their codes are:
PR Partial(covering part of the aerodrome)
DR Low drifting
FZ Freezing (supercooled)
SG Snow grains
PL Ice pellets
GS Small hail and/ or snow pellets
UP Unknown precipitation
VA Volcanic ash
DU Widespread dust
PO Dust/sand whirls (dust devils)
FC Funnel cloud (tornado or waterspout)
Source: WMO Manual on codes.
In conclusion, I hope that you have gained a lot from reading this post and that it has helped you to understand weather phenomena reporting and decoding METAR/SPECI in a better way. In case you are still confused about something or have more questions, feel free to drop me a message.