Eyes in the Sky 1: METAR

I’m currently studying for my CAA permission for commercial operations (PfCO) – what is commonly thought of as the UK drone pilot’s license. Flying small unmanned aerial systems (SUAS) is an increasingly common part of field science especially in polar science where a) scaling in-field observations over space is critical, b) we rely heavily on satellite observations that require sub-pixel validation, and c) it is often hazardous to manually survey areas that can be easily surveyed using a UAS. We an achieve so much more when we have eyes in the sky as well as feet on the ground. Legislation covering SUAS users is also changing rapidly and is likely to (rightly) become much stricter in the near future. I plan to write about several aspects of the PfCO that are relevant to UAS flights in polar regions, partly for interest and partly as a revision tool for myself in preparation for the PfCO assessment!

One of the most important aspects of flying anywhere, and especially in polar regions, is up to date and accurate information about the weather. Airports and many weather stations report current and forecast weather conditions in a condensed format known as METAR, or the slightly more in-depth TAF. METAR stands for Meteorological Aerodrome Report, and TAF stands for Terminal Aerodrome Forecast. As well as being a standard aeronautical system, I think using METAR symbology would make an excellent way to log detailed meteorological observations in metadata for field scientists.

Below is a METAR forecast for 27th March 2019 for the airport in Longyearbyen, Svalbard:

ENSB 270850Z 13020KT 9999 FEW025 SCT080 M05/M12 Q0994 NOSIG RMK WIND 1400FT 12017KT

 

The METAR starts with a four letter location code: ENSB is the code for the Longyearbyen airport. Then the date and time that the forecast was posted, starting with the day of the month and the time in HHMM format followed by ‘Z’. The Z indicates that the forecast is in Greenwich Mean Time or simply “Zulu”, so this forecast was posted on 27th march at 0850 GMT. As pointed out by @arwynedwards on Twitter, this is a deviation from the NATO standard format (270850Z MAR19) by omitting the month and year information. I suspect this is because the high frequenc of METAR updates makes this information largely redundant. For recording field meteorology it might be more useful to use the NATO standard – while the month and year of the field work may usually be obvious on an individual project basis, it could be crucial info when collating data from a range of sources.

The third block of characters describes the wind speed, with the first three numbers showing the direction the wind is coming from (in this case 130 degrees, or approximately South East). The last two digits are the speed (20) and KT shows that the speed is measured in knots.

The next set of four digits shows the visibility in statute miles or kilometers. In this case the visibility is greater than the maximum shown on a METAR so it is recorded as 9999, which can be interpreted as greater than 10km.

Next is cloud conditions. This is achieved using a set of three letter codes offering a qualitative description of the cloud cover. In this case FEW means there are a few clouds and their height in feet/100 is shown. Here the few clouds sit at 2500ft (025*100). there are also scattered clouds sitting at 8000 ft (080*100).

The temperature and dew point are described using M for minus, so here the temperature is -5 C an the dew point is -12 C. When these values are similar (say, within 3 C) we need to worry about mist, fog or precipitation.

QO994 shows that the pressure is 994 hPa. NOSIG is a flag to suggest that no significant changes to these conditions are expected over the forecast period.

The rest of the information in the METAR is classified as “remarks” as signified by the code RMK. In this case the remark is that the winds aloft at 1400 ft are much stronger and coming from a different direction to the winds at surface. In this case, 17 knots coming from 120 degrees.

Overall, this looks like a good day to fly in terms of high visibility and low chance of precipitation, but the low temperatures will reduce the battery life and risk icing, and the wind speed is just outside of the safe flight envelope for many small UASs including the DJI Mavic and Phantom series. For those reasons I’d call a NO-GO for a small quadcopter flight.

The power of the METAR is that all that information can be conveyed in a simple string of unambiguous characters. They are frequently updated to reflect changing conditions and forecasts. This METAR was sourced from allmetsat.com who provide up to date METAR for over 4000 stations.

 

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