Wx Watch: Mega METARs!

Web site of the month

We all learned about METARs during our pilot training, but for anyone wanting a comprehensive review, go online. The site’s content is by Troy M. Kimmel Jr., a lecturer at the University of Texas-Austin and a certificated weather observer. In addition to dissecting the basic METAR, Kimmel goes into all the groups mentioned in the accompanying article, and even delves into cloud coverage abbreviations using the “octas” (eighths of the sky) method of sky observation. Numerical cloud codes are also explained. — TAH

There you are, tooling across the sky, basking in the wonderful situational awareness and plentiful weather graphics afforded by your GPS navigator, when, what’s that? You call up the latest METAR (aviation routine meteorological reports) for your destination airport for an update, but up pops what looks like the longest, strangest METAR you’ve ever seen. You understand the first half of the report. That contains the surface winds, visibility, current weather, temperature, dew point, and any remarks. But then comes a string of baffling codes and numbers. The odd thing is that you’ve seen these sorts of suffixes before—but not every time you called up a METAR. What’s going on?

Those extra codes and number groups are used by meteorologists and posted for certain designated airports at certain intervals. Some of these postings come every three hours, some every six. Sometimes, this extra information is missing. Which explains why those suffixes didn’t appear each time you bring up a METAR.

Let’s take a look at what these abbreviations represent, starting with the “SLP” notation that comes after any comments in the “remarks” section of a METAR:

• SLP stands for sea-level pressure, with the pressure expressed in millibars. The reading is abbreviated. “SLP201” translates to 1,020.1 millibars. Below 1,000 millibars of pressure the first number will be a “9,” so 995.4 millibars, for example, will be posted as “SLP954.” Incidentally, because this pressure is corrected for sea level it can be used as an altimeter setting in altimeters equipped with Kollsman windows having values in millibars. (Even so, the “A”-prefixed altimeter setting used earlier in a METAR is the altimeter reading for the United States. For example, “A2992” stands for “altimeter 29.92” inHG (inches of mercury).
• The “4” group refers to snow depth on the ground, in inches. “4/036” means 36 inches of snow were measured. Of course, this group won’t appear during the warmer months of the year—unless it snows!
• The next group of numbers is the “P,” or precipitation sequence. This is the hourly liquid precipitation, shown in hundredths of an inch. So “P0011” means 0.11 inches of rain fell in the hour ending at the METAR post time.
• Next comes the “6” group of numbers. This stands for the last three- or six-hours’ worth of precipitation—again, expressed in hundredths of an inch. The 6 group is posted every three or six hours, which explains why this group often does not show up in METARs. Three-hour totals appear at 03Z, 09Z, 15Z, and 21Z. Six-hour totals are posted with the METARs for 00Z, 06Z, 12Z, and 18Z.
• The “7” group of numbers appears once a day, on the METAR for 12Z. The numbers represent the total liquid precipitation for the 24 hours before the report—again, in hundredths of an inch.
• The “T” group is for the hourly temperature and dew point—expressed to the nearest tenth of a degree Celsius. A “0” (zero) after the T means temperatures and dew points above 0 Celsius. A “1” after the “T” means below-zero Celsius temperatures and dew points.
• The next is the “1” group. It lists the highest temperature (in tenths of a degree Celsius) during a six-hour period. “1-group” reports come at 00Z, 06Z, 12Z, and 18Z.
• Next is the “2” group of numbers. This is for the lowest temperatures in the previous six-hour period. “2-group” reports come at the same intervals as those of the 1-group.
• Finally, there’s the “5” group. This covers pressure trends. The first number after the “5” indicates the pressure trend in the last three hours. (Numbers from 0 to 3 indicate that the current atmospheric pressure is higher than three hours ago; numbers from 0 to 5 mean that the pressure is the same; and numbers 5 through 8 mean the pressure is lower than three hours ago.) The next three numerals show the amount of pressure change, in tenths of a millibar. So a report of “57020” means that pressure has decreased by 2.0 millibars in the past three hours.

By now, you probably understand why these extra groups appeal mainly to meteorologists. All that data is fed into computer forecast models, used to verify past forecasts, compile record high and low temperatures, precipitation levels, and much more. But still, there are times when pilots may be interested.

Now let’s translate a full-blown METAR (page 106). This one’s the 12Z report for Traverse City, Michigan, from December 4, so it has all the data groups except for the three-hour precipitation levels. Information and translation is from Aviation Digital Data Service (ADDS) online. Hmmm, sure looks like a wintry day, complete with lake-effect snow. But notice that the “translation” only goes so far. Let’s take on the full-blown translation, beginning with the remarks (RMK) group:

• RMK A02 SNB17 = automated weather observation equipment with a precipitation sensor
• SNB17 = snow began 17 minutes after the hour
• SLP178 = sea level pressure is 1,017.8 millibars
• 4/006 = six inches of snow on the ground
• P0000 = no liquid precipitation fell in the last hour
• 60000 = a trace (0000) of liquid precipitation fell in the last six hours
• 70029 = 0.29 inches of liquid precipitation fell in the last 24 hours
• T10501083 = hourly temperature was –5 degrees C.; hourly dewpoint was –8.3 degrees C.
• 11039 = the highest temperature in the last six hours was -3.9 degrees Celsius
• 21050 = the lowest temperature in the last six hours was –5 degrees Celsius
• 53014 = pressure is higher than three hours ago, by 1.4 millibars

Now you know how to sort out the nitty-gritty details the next time a “super-METAR” lights up your display screen. Too much detail for most of us? Perhaps, but more information is always better than less when it comes to weather flying.

E-mail the author at [email protected].