Meteorological Aerodrome Reports (METARs) — Page 473, Lesson 442

I want to walk you through the Meteorological Aerodrome Report — the METAR — starting with the date-time group and then moving into wind and visibility. This is the core of how we encode the observed weather at an aerodrome for flight operations. Let’s begin with the third group in the METAR, which is the date-time group. This group simply gives the date of the actual weather observation. The first two digits represent the day of the month — so if you see "25", that means the 25th day of the month. After that comes the time in hours and minutes. Time is always given as Coordinated Universal Time, abbreviated UTC. For all practical purposes, UTC is the same as Greenwich Mean Time, or GMT — that’s the local time at Greenwich, London. In the METAR itself, UTC is indicated by the code letter Z, which is pronounced "Zulu". So a date-time group like "251200Z" means the 25th day of the month at 1200 Zulu — 12:00 UTC. Now we move to the wind information. The next items in the METAR — labelled d, e, f, and g in the example — are the observed wind information. First, we have the direction of the wind, given in degrees true. That direction is rounded up or down to the nearest 10 degrees. So if the wind is actually 263 degrees, it would be reported as 260 degrees. That’s group (d). Next comes the wind speed in knots, group (e). This speed is a mean speed taken over a 10-minute period — not an instantaneous reading. However, if a gust is observed that is at least 10 knots more than the mean wind speed, then a gust figure appears after the mean wind. That gust figure is group (g), and it is preceded by the letter G, which is group (f). So a wind group like "27015G28KT" means the wind is from 270 degrees true, mean speed 15 knots, with gusts up to 28 knots — because 28 minus 15 is 13, which is at least 10 knots above the mean. The next code-group, labelled (h), may or may not appear depending on the directional variability of the wind. Variability of direction is included when the wind direction, over the preceding 10 minutes, has changed by 60 degrees or more. When that happens, you will see two extreme directions separated by the letter V. For example, "240V300" means the wind direction has varied between 240 degrees and 300 degrees over the last 10 minutes. If there is no wind at all, the coding "00000KT" is used — that’s five zeros followed by KT. If the wind direction cannot be defined — for example, in very light and variable conditions — then the code VRB, for "variable", replaces the direction. So you might see "VRB02KT" meaning variable at 2 knots. Now let’s talk about visibility. Visibility in the METAR is represented by the next group, which is depicted in red in the example. In the METAR, the reported visibility is the prevailing visibility. Prevailing visibility is the greatest visibility value that is reached over at least half of the horizon circle — not necessarily in all directions. Under certain conditions, the METAR may also include the minimum visibility. In the example given, the prevailing visibility is reported as 0800 metres — that’s 800 metres. So to summarise: the date-time group gives the day and UTC time with a Z suffix; the wind group gives direction in degrees true, mean speed in knots over 10 minutes, gust if the gust is at least 10 knots above the mean, and variability if the direction has changed by 60 degrees or more; and the visibility group gives the prevailing visibility in metres, with the possibility of including minimum visibility under certain conditions.