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Mystery METARS

METARs, by the numbers

Textbooks, self-study programs, and instructors have done a pretty good job of passing along the basics of METAR interpretation.

Illustration by Leigh Caulfield
Illustration by Leigh Caulfield

Most of us can rattle off a raw METARs without resorting to the decoded version of these hourly reports of the weather at many—not all—airports. It wasn’t always that way. When METARs replaced the earlier surface reports (SAs) in 1996 there was a lot of brouhaha and a steep learning curve. Same thing with the switch from terminal forecasts (FTs) to today’s TAFs. All of this in the name of establishing a world standard of weather reports.

However, most pilots haven’t been educated in some METARs’ more infrequently used coded abbreviations. Particularly those from sites having automated weather observing capability. You know, the additional reporting codes that follow the core report of date and time, surface wind, visibility, current weather, cloud conditions, temperature/dew point, and altimeter setting. Sometimes instructors will blow these off by saying, “Oh, you don’t need to know them. It’s for meteorologists.” True, in the strictest sense. Most of us accept this advice. Others are curious about these seemingly odd collections of numbers. So, let’s delve into them.

Let’s identify them by their prefix letters. There are the P, 6, 4, T, 1, 2, and 5 groups of codes.

The P-group code gives any hourly precipitation amounts, in tens, tenths, and hundredths of an inch since the last METAR. So, if you see a P0050 code, you know that a half-inch of rain has fallen. A trace of rain—meaning an unmeasurable amount of rain less than 0.01 inch—gets a P0000 code.

The 5 group is used to show three-hour pressure tendencies. For example, is the atmospheric pressure higher, the same, or lower than three hours ago? And if so, did it decrease first, then increase?The 6-group reports three- and six-hour precipitation amounts at three-hour intervals (shown in METARs posted for the 03, 09, 15, and 21 Z observations) and six-hour intervals (in 00, 06, 12, and 18 Z observations). Snow gets its own, three-digit 4-group. This shows snow depth in inches on the ground in six-hour intervals—at 00, 06, 12, and 18 Z. So, a 4021 report means 21 inches of snow fell in the previous six hours.

The T-group shows the hourly METARs’ temperature and dew point to the nearest tenth of a degree Celsius. So, a report of T00710054 translates to a temperature of 7.1 degrees Celsius and a dew point of 5.4 Celsius. Temperatures and dew points below freezing? Then a “1” is posted before the temperature and dew point numbers.

The 1 group is for showing the maximum temperature, in Celsius, for the past six hours. Like the 6 group, it posts on METARs for 00, 06, 12, and 18 Z.

The 2 group also comes out at the same intervals, but it gives the previous six-hour minimum temperatures.

The 5 group is used to show three-hour pressure tendencies. For example, is the atmospheric pressure higher, the same, or lower than three hours ago? And if so, did it decrease first, then increase? Or decrease steadily or unsteadily? This is rarified stuff, and to decode it you need to refer to charts in the Federal Meteorological Handbook No. 1, or FMH1. This is the weather observer’s Bible and is gospel for everything weather. Like, what exactly is “moderate” rainfall? Answer: It’s a situation where “individual drops are not clearly identifiable; spray is observable just above the pavement and other hard surfaces.”

There are other codes and terms for various weather conditions, but like the ones we just reviewed they may not appear in some METARs—things like duration of sunshine and water equivalent of snow on the ground. Or the dollar sign ($) at the end of a report, meaning that the weather observing equipment needs maintenance; maybe the equipment for measuring the precipitation amount is not operating. In which case, you’d see a “PNO $.”

But for now, I think we’ve done a pretty good job of pegging the weather geek-meter. At least you now know something about those arcane codes that pop up every three or six hours on many METAR printouts.

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Thomas A. Horne

Thomas A. Horne

AOPA Pilot Editor at Large
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.

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