Hot answers tagged

38

The general rule is: If you read it, it's true. If you hear it, it's magnetic. All charts and textual sources (METAR, TAF, winds aloft, surface analysis charts, etc) use true north as the reference. ATIS/AWOS/ASOS broadcasts, or any information a controller gives you over the radio, is magnetic. Wind direction broadcast over FAA radios is in reference ...


35

Quoting from the METAR decoder: BR Mist (Foggy conditions with visibilities greater than 5/8 statute mile) FG Fog (visibility 5/8 statute mile or less) You may ask "Why 5/8th of a statute mile?" That's because 5/8ths of a mile is 1,006 meters, or about 1 kilometer.


31

A maintenance indicator sign, $, is included when an ASOS/AWSS (Automated Surface Observing System / Automated Weather Sensor System) detects that maintenance is needed on the system. Reference: http://www.faa.gov/documentLibrary/media/Order/JO%207900.5C.pdf Or if you need an easy way to remember it, courtesy of the awesome Ralph J: It means that they ...


26

The main difference between the fog and mist is their density and as a consequence, the visibility. From UK Met office: ... they are two distinct terms for a similar phenomenon. Visibility less than 1,000 metres we call 'fog' and obscurity with visibility greater than 1,000 metres we call 'mist'. This is the same one given in FAA Aeronautical Information ...


25

As far as METARs are considered, the wind direction gives the direction from which the wind is coming. From METAR definitions: Wind Direction. The direction, in tens of degrees, from which the wind is blowing with reference to true north. So, Wind 270 shows that the wind is coming from west. The reporting in ATIS and tower is the same, only ...


18

Wind numbers say where the wind is coming from. Wind 270 means that the wind is coming from the west, and blowing towards the east. If you point west (270), you will have the wind in your face as a headwind. If you point east (270 - 180 = 90), you will have a tailwind, or the wind at your back. Another way to think about this is if you want to take off from ...


18

According to a CFI's lesson plan on the internet, it comes from the French word for mist, brume.


18

The METAR format is defined by the World Meteorological Organization's (WMO) on behalf of ICAO. According to the specification (section 15.6.3), visibility is to be reported in meters. The upper bound for visibility reporting is 9999, which indicates the visibility exceeds 10 kilometers. In North America, the METARs deviate from the standard format by ...


16

METAR is a Meteorological Aerodrome Report which is for a specific place, at a specific time. The first line is a report from Sao Paulo at 22:00 on the 17th. You can use a service such as this one to decode METAR reports. The rest of the lines are more reports for different time and days. The groups are: SBSP Reporting station ICAO code 172220Z ...


13

The definition of "Wind Gust" for METAR reports can be found in the Federal Meteorological Handbook No. 1, chapter 5: 5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and ...


13

The NOTAM format is from the time when people still would call the value of individual bits over a phone line. Ok, that may not have happened, but my point is that data transfer was incredibly slow compared to current day standards. Therefore a compact format saved significant transmission time and cost without losing any information. The format didn't ...


12

I always use the old saying; The North wind doth blow, and we shall have snow... Since this saying originated in England and in England it is colder to the North, we can conclude that a North wind blows from the North. Incidentally, it is precisely the opposite with ocean currents. The Gulf Stream is a North-Easterly current, because if you are floating ...


12

One use case where the compact METAR format is tough to beat is rapidly scanning meteorological trends. For that purpose, METARs are user friendly in the sense that they are expert friendly. For example, a friend invited me to fly with him today on an Angel Flight mission that would have required a landing at Nashville. We had to scrub because the weather ...


11

Complete, unless you want to know the height of base of those TCU's! The bulk of the information you're after can be found in AIS-GEN 3.5 Section 10.17 - AUTO METAR coding In describing the coding used by automated systems, it gives this example NNNhhh/// Cloud detected, but it is unknown whether it is a convective cloud type (ie TCU or CB) And this ...


11

I have a suspicion that you're looking at KLZU's METAR. Basically, they're inputting their manually written METAR into the system, and when they send it out they're adding the Runway in Use(RWY 7) and ATIS code(ATIS G) to their METAR, which they're really not supposed to do. I suspect that LL is the person who recorded it's operating initials at the facility,...


11

It's indeed the layer type and okta, but in the remark the layer type is repeated for each reported cloud (smoke) layer. From your example: FEW037 → FU1 SCT050 → FU2 → 1+2=3 OVC250 → FU5 → 3+5=8 The oktas of the layers are added, that's how OVC is shown as 5 and not 8, by adding the previous oktas it does become 8. FEW = 1 to 2 SCT = 3 to 4 BKN =...


11

It indicates ice accretion (buildup). This seems to be very obscure, I finally found it in some NOAA release notes for ASOS software v3.07: (S01126) ADD ICE ACCRETION REMARK TO METAR/SPECI REPORTS Discussion: The amount of ice accreting on a flat surface (planar icing) as estimated by the data from the freezing rain sensor, will be included ...


10

Geographically METARS are for a single specific location (the station identified in the report). An automated METAR with no human supervision covers the conditions within approximately 6 inches of the weather station reporting it (for example, a badly positioned AWOS/ASOS station may be shielded from wind in some directions which affects what it reports). ...


10

First of all: I have found out that an ISA atmosphere is: 15 degC temperature 05 degC dewpoint 1013.25 pressure Not entirely correct. The temperature and pressure are ISA values at mean sea level. These alter with altitude. Dewpoint is not defined in ISA, as ISA does not contain dust, moisture and water vapor. ISA-30 atmosphere means temperature ...


9

Colors At present1, they mean (image source): For more details, please see this description. Fill-ins These are called sky/cloud coverage. Image Source 1 From SkyVector: The color is controversial because the FAA is making the NWS swap the colors for VFR and MVFR. When the new aviationweather.gov becomes the current aviationweather.gov, we'...


9

RVR often has a letter at the end to indicate the visibility trend, so you can tell if it's going to improve (and maybe you should circle overhead for a while) or worsen (and you should consider going to your alternate). U means an "upward" trend: visibility is improving. D means a "downward" trend: visibility is worsening. N means no change.


9

ISA+X is the ISA atmosphere with X°C of variation. It´s used to determine how diferent (in terms of Temperature) of the Standard the actual condictions are and adjust the calculations. ISA as you said is: +15°C / 0ft / 1013.2 hpa / LAT 45° / Term. Grad. -2°C/1000ft So, ISA +5 is: +20°C / 0ft / 1013.2 hpa / LAT 45° / Term. Grad. -2°C/1000ft The ...


8

Not necessarily. Automated stations have a number of sensors for detecting, Wind speed and direction Visibility Obscurations to vision Cloud coverage and ceiling Temperature and dew point Barometric pressure and altimeter setting Precipitation Freezing rain (Icing) Lightning (thunderstorms) It is likely that some of these are not functioning properly, ...


7

If it ain't broke... For what its worth many new aviation apps like ForeFlight and the such display the information in user friendly formats. (source) To be honest I have not decoded a METAR in some time. You are correct that the system comes from a time when transmission bandwidth was not what it is today. Keep in mind things in aviation are slow to ...


7

My questions are: Is it really normal to see reports without viz? Yes! Well, maybe not "normal", but it does happen. Automated weather stations are like another piece of equipment and components fail from time to time, including the components which measure the visibility. The other components also fail from time to time (I've see ASOS/AWOS reports ...


7

The format follows a standard, although this one is different because of the R at the front... The format is the "Runway State Group": First two digits are the Runway Number, 88 means all runways, 99 means a "repetition of the last message as no new information received". The third digit is the runway deposits, it tells you what is on the runway: 0 = ...


6

The reason its reported as "greater than 6 statue miles" is because 6 statute miles is the equivalent of 10 kilometers. ICAO reports METAR visibility in meters, and reserves 4 places in the coded METAR for the number of meters. With only 4 places, 9999 is the largest value, and this corresponds to "10km or more." Reference: Aerodrome forecast - TAF decode


5

We might speculate why the METAR is giving innaccurate data, but the question asks, "What should one do..." I will take the liberty of inferring the question to be "what should one do if one is unable to obtain an altimeter setting for the airport from which he or she is preparing to depart?" Note that this question could be asked due to an airport with a ...


5

This work sheet may be of use and generally the system of Okta is used to denote coverage in a Metar, but I would say L1 Clouds: Cumulus (Cu) with little vertical extent but it could also be L5 Clouds: Stratocumulus (Sc) not from spreading out of Cumulus (Cu) But I will agree with Ron on this one, its BKN (5-6 Okta based on the picture in my ...


5

The instrument they are using is not certified, hence the ESTMD. When they get a certified one, the ESTMD will drop off the METAR. Technically, the primary certified instruments are inop or unavailable, so backup equipment, believed to be reliable, but not certified, is being used.


Only top voted, non community-wiki answers of a minimum length are eligible