A Glimpse at METAR Reports

By Alexis George, Weather Observer & Meteorologist

METAR observations are submitted every hour of every day at Mount Washington Observatory. METAR is a format for reporting weather information that gets disseminated through the Aviation Weather Center. While METARs are primarily used by aircraft pilots, you can actually view METAR data for any station across the country. It might seem difficult to decipher these weather reports at first, but I’ll dive into reading the basics of METAR to help readers learn something new!

On Saturday, November 4th, I submitted an hourly observation to the National Weather Service at 6:57 p.m. The observation reads as follows:
KMWN 042257Z 27020KT 0SM FZFG DRSN VV000 M03/M03 RMK VRY LGT ICG

Let’s break down the elements of this report:

KMWN- Station identifier. KMWN is Mount Washington’s designated METAR station identifier code.

042257Z- Date and time of report. The report was submitted on the 4th of this month at 2257 UTC which translates to ‘042257Z’.

27020KT- Wind group. The wind direction is always reported first with the speed after. So, 270 is the wind direction and 20 is the wind speed reported in knots. So this means that winds were from the west at 270° with a wind speed of 20 knots (which is 23 mph).

0SM- Visibility group. Visibility is the measure of how far you can see in the atmosphere. In this case, freezing fog was limiting visibility to 0 statute miles at the time of this observation. When fog is obscuring visibility at the station, observers can utilize the resource below to identify short range visibility marks to determine visibility at the time of observation. For the 6:57 p.m. observation on November 4th, fog was limiting visibility so greatly that I could not even see to the end of the deck!

However, when the summit is clear and visibility is higher, it is possible to see up to 130 miles from the summit. On crystal clear days, you can see into five states from Mount Washington, including New Hampshire, Maine, Vermont, Massachusetts, and New York—even into Canada!

As a night observer, I use many of the same natural features plus the lights of known locations to determine visibility. My favorite visibility marker is Portland, Maine, because on a perfectly clear night, you can actually see the ocean right behind and next to the city!

FZFG DRSN- Present weather group. The weather occurring at the 6:57 p.m. observation was freezing fog and low drifting snow. Low drifting snow is snow that is being transported vertically by the wind to a height of less than 6 feet above the ground. Low drifting snow and blowing snow (snow transported by the wind to a height of more than 6 feet above the ground) will start occurring at Mount Washington much more often during the winter season and can greatly reduce visibility for days on end at the summit.

VV000- Sky condition group. This group reports how much of the sky is covered by clouds and includes the height of each cloud layer. However, since the lowest cloud layer was obscured by a surface-based obscuration (freezing fog) at this observation, the height of the base of the lowest cloud layer is indeterminable. When this occurs, an observation of the vertical visibility in hundreds of feet into the clouds is taken. So in this case, the vertical visibility into the cloud was 0 because fog was reducing vertical visibility to 0 feet. If I had been able to see 100 feet up into the cloud, then I would have reported sky condition as VV001 (then VV002 for 200 feet and so forth).

M03/M03- Temperature and dew point group. Temperature and dew point are measured at every hourly observation by using the sling psychrometer.

This old-school device consists of two thermometers, one with a wet bulb and the other with a dry bulb. This device is spun in the open air to determine relative humidity. Spinning the psychrometer can take a few minutes to up to 15 minutes (the longer sessions occur when the ambient air temperature is close to the freezing mark) to get the dry and wet bulb readings. The dry and wet bulb readings are then converted into degrees Celsius. So, at this observation, the wet bulb and dry bulb values were both 27°F (since there was fog at the station, the relative humidity was 100%). The 27°F is then converted to Celsius and followed by M (meaning minus) since the values were below the freezing mark.

RMK VRY LGT ICG- The remarks. This section can consist of plain-language meteorological information that elaborates on any of the data in the body of the report. So the remarks section could include things like when precipitation began or ended, a thunderstorm, or lightning visible from the station, and so forth. In this case, the below-freezing temperatures and a 100% relative humidity from fog present at the station was resulting in very light rime ice developing at the time of this observation.