Meteorological Aerodrome Reports (METARs) — Page 480, Lesson 451

I want to walk you through the next part of the METAR code — the cloud and visibility group, and then the temperature, dew point, and QNH groups. Let's start with a special situation in the cloud group. If the vertical visibility cannot be assessed — that means the observer cannot determine how high you can see straight up into the cloud — then three forward slashes replace the cloud height figures. So you would see something like VV///. The VV stands for vertical visibility, and the three slashes mean the height is unknown or not measurable. Now, a very common code you will see in METARs is CAVOK. CAVOK is an abbreviation for "cloud, ceiling and visibility are OK." When CAVOK is used, it replaces the visibility group, the RVR group, the weather group, and the cloud group entirely — so you won't see separate numbers for those elements. But CAVOK is not used casually; there are four specific criteria that must all be met for CAVOK to appear. First, the visibility must be 10 kilometres or more. Second, the height of the lowest cloud must be no less than 5000 feet, or the level of the highest minimum sector altitude, whichever is greater. That means if the minimum sector altitude for that aerodrome is higher than 5000 feet, you use that higher value as the threshold. Third, there must be no cumulonimbus or 'towering cumulus' present — those are abbreviated as CB and TCU respectively. And fourth, there must be no significant weather at or in the vicinity of the aerodrome. Let me give you an example METAR that uses CAVOK: METAR EGTK 231020Z 26012G25KT 220V300 CAVOK. That decodes as: METAR for Oxford/Kidlington, observed at 1020 UTC on the 23rd of the month; the surface wind is 260 degrees True at 12 knots, gusting to 25 knots, and varying in direction from 220 degrees True to 300 degrees True; the visibility is 10 kilometres or more, with no cloud below 5000 feet; there are no CB or TCU, and there is no significant weather at or in the vicinity of the aerodrome. Now, after the wind, visibility, and cloud groups, we come to the temperature and dew point group. This is the next group in the METAR code. The temperature and dew point code is simply a two-digit number giving the air temperature, followed by a forward slash, followed by another two-digit number which indicates the dew point. Both temperatures are measured in degrees Celsius. For example, the code 10/02 indicates that the air temperature is plus 10 degrees Celsius, and the dew point is plus 2 degrees Celsius. If either figure is negative, the prefix M is used. So 10/M02 means the air temperature is plus 10 degrees Celsius, and the dew point is minus 2 degrees Celsius. There is an important note about rounding: the normal mathematical convention of rounding 0.5 to the next highest digit is used. So plus 1.5 degrees would be reported as '02', and minus 1.5 degrees would be reported as 'M01'. Minus 0.5 degrees would be reported as M00 — note that M00 is used for minus zero, essentially. Let me extend our example METAR: METAR EGTK 231020Z 26012G25KT 220V300 CAVOK 10/M02. This decodes as: METAR for Oxford/Kidlington, observed at 1020 UTC on the 23rd; surface wind 260 degrees True at 12 knots gusting 25, varying 220 to 300 True; visibility 10 kilometres or more, no cloud below 5000 feet, no CB or TCU, no significant weather; the air temperature is plus 10 degrees Celsius and the dew point is minus 2 degrees Celsius. Now we move to the next METAR code: the QNH. The QNH is represented by the letter Q, followed by a four-digit number representing the actual pressure value. If the QNH is less than 1000 hectopascals, the value will be preceded by a zero. For example, a QNH of 991 hectopascals would appear as Q0991. Here is a more complete example: METAR EGTK 231020Z 26012G25KT 220V300 9999 -RA FEW060 SCT120 10/M02 Q0991. Notice that this example does not use CAVOK — it has a visibility of 9999, which means 10 kilometres or more, and it has weather and cloud groups: -RA for light rain, FEW060 for few clouds at 6000 feet, and SCT120 for scattered clouds at 12000 feet. It is very important to note that the only pressure value given in a METAR is the QNH. The QNH is always rounded down for safety reasons, if there are digits after the decimal point. For instance, if the actual QNH were 991.7 hectopascals, the QNH would be reported as Q0991 — rounded down to the whole hectopascal, never up. This is a safety rounding to ensure you are always using a slightly lower pressure, which gives you a more conservative altitude reading.