Know your METARs
Posted June 15, 2015
In the coded "language" of weather observations called METARs (a acronym of the French MÉTéorologique Aviation Régulière, often translated into English as Meteorological Terminal Aviation Routine Weather Report or something similar), temperatures are reported in full degrees Celsius. At 2 p.m., KRDU reported a temperature of 36°C, or 96.8°F:
However, reporting stations are capable of measuring with higher precision, and many of them include a T group in the METAR, which lists both temperature and dewpoint in degrees and tenths Celsius. When we look at that, we see the actual 2 p.m. temperature was 35.6°C or 96.08°F. In a 2-digit world, that rounds to 96°F:
This is important stuff when following temperatures hour-by-hour to see if we've broken a record high. We keep records in whole degrees Fahrenheit, but the NWS will use the highest-precision data available in determining what that whole-degree final number is. As you can see, if we used the plain METAR temperature, we'd have tied the old record high of 97°F at 2 p.m.; however, using the higher-precision data offered by some reporting stations, we know better.
We’ve gotten word from the National Weather Service that we’ve since broken the old record of 97°F no matter how you slice it, so this discussion is mostly academic for today. However, the last round of 100°+ heat in 2012 had many watching the rounded, full-degree-Celsius temperatures, and I recall this making the difference between setting a new all-time record high and tying the old one (105°F) on at least one occasion. It pays to note the details!