Meteorological Aerodrome Reports (METARs) — Page 480, Lesson 447

I want to walk you through how weather phenomena and cloud coverage are reported in a METAR. Let's start with the structure of a weather group. The first part of a weather group tells you the intensity or proximity of the phenomenon. There are three possibilities. A minus sign (–) means light. A plus sign (+) means heavy. The letters VC mean in the vicinity of, but not at the observation point itself — so the phenomenon is happening near the aerodrome, not directly overhead. If there is no qualifier — no plus or minus — in front of precipitation, then that precipitation is considered moderate. So moderate is the default; you only write a symbol if it's light or heavy. Next comes the descriptor. This is a set of letters that adds detail to the weather phenomenon. For example, BC means patches, and it's frequently used to describe fog — so "patches of fog." SH means showers. TS means thunderstorm. These descriptors refine what kind of phenomenon we're dealing with. Then we have the actual weather phenomenon codes, which fall into three columns. The precipitation column contains codes for drizzle, rain, snow, hail, and so on. The obscurations column covers phenomena that reduce visibility — things like mist, fog, smoke, and ash. The last column contains weather phenomena that haven't been mentioned in the other columns — these are the more unusual events that are rarely reported in the United Kingdom. Let's apply this to a real example. Earlier in the chapter we saw the partially complete METAR with the group +SHRA. Breaking that down: the + means heavy, SH is the descriptor meaning showers, and RA is the code for rain. So +SHRA means heavy showers of rain. Now let's talk about thunderstorms. A thunderstorm report will appear in a METAR if thunder has been heard within the last 10 minutes. That's the criterion — the observer must have heard thunder in that window. The code for a thunderstorm is TS. If there is no precipitation associated with it, the letters TS appear on their own. But if there is precipitation, two additional letters are inserted after TS to indicate the type of precipitation. For example, if rain is observed from the thunderstorm, the code becomes TSRA. If hail is observed, the code could be TSGR or TSGS, where GS means small hail. Now let's move to the cloud coverage group. This is the next code-group in the METAR, and it gives detail about the cloud. Let me read you an example METAR: METAR EGTK 211020Z 26012G25KT 0800 R30/1100 +SHRA OVC020CB. The highlighted part is OVC020CB. That breaks down as: overcast sky, base 2000 feet, with cumulonimbus clouds. Cloud coverage is reported using three-letter prefixes. Let me go through each one. FEW means one to two eighths of cloud coverage. SCATTERED, abbreviated SCT, means three to four eighths. BROKEN, abbreviated BKN, means five to seven eighths. And OVERCAST, abbreviated OVC, means complete cloud coverage — that is, eight eighths. So the sky is completely covered. After the coverage code, you get a three-digit number that gives the cloud base in hundreds of feet. In our example, 020 means 2,000 feet. And then, if applicable, you get a suffix for the cloud type — in this case CB for cumulonimbus. So to summarise: the weather group starts with intensity or proximity, then a descriptor, then the phenomenon code. Thunderstorms use TS, with precipitation codes added if needed. And the cloud group gives you coverage in eighths, base height in hundreds of feet, and optionally the cloud type.