Archive for category METAR
Understanding Weather Reports
I was standing inside the airport terminal and rain was pouring down outside on the tarmac. I figured this would be a good time for a teachable moment with my student who just got inside from pre-flighting the aircraft when the down pour started. I pulled out my cell phone and called the local AWOS phone number to get an updated weather report.
I could almost repeat what the weather is going to be like, very strong winds with gusts a lowered visibility and very heavy rain. What I got, my to my dismay on the teaching moment, was strong winds with no gusts and only light rain. Both my student and I looked at each other puzzled and he asked me why the AWOS weather was not correct. I said, honestly, I don’t know.
A few minutes later, the storm moved quickly through and the wind calmed down and only light rain was falling. I thought to myself and wondered what the AWOS would say. Winds very strong and gusting with heavy rain. TO say the least, I lost my teaching moment and promised my student I would figure out what this all means. I was hoping the AWOS system was out of tolerance and needed repair but that was not the case. A review of AC 00-45 Aviation Weather Services shed clues on what I was missing and it may be interesting to you as well. I was also able to get a copy of the OFCM Surface Weather Observations and Reports Handbook.
Did you know that most of the weather listed above is averaged over a given period of time? If the weather is changing quickly enough, the newly received weather from the AWOS system may be obsolete as it was in my case above. It begins to make you think about the accuracy of the weather for certain situations, doesn’t it?
For example let us examine the wind component. First of, we know that printed winds are always in respect to TRUE NORTH and spoken winds are always with respect to MAGNETIC NORTH (see note in AIM 7-1-10). Did you know the wind shown above is the average wind over the preceding two minutes? It may not even be accurate anymore but we can be assured the new wind is less than 60° from what it says. The VRB wind is used when the wind over the previous two minutes vary more than 60° and the speed is less than six knots.. A variable wind 060V150 is for speed greater than six knots.
A gust is a little more complicated is the AWOS / ASOS system examines all the wind over the preceding 10 minutes and will report a gust if a rapid fluctuation between low speed and high speed has a difference of 10 knot or more. It is quite possible that no gust exists any longer even though the METAR report is only one minute old.
A wind shift happens when the wind changes by 45° in less than 15 minutes with wind speeds greater than 10 knots.
The visibility group is pretty interesting. The weather report can be automated and / or manual by the tower. If it is automated, there is 10 minutes of sensor data used in the prevailing surface visibility.
The present weather group (in this case -RA) has a time frame associated with it as well. For example, over the previous six minutes, the accumulation rate is observed and reported in the METAR. It makes sense now why I first received a light rain when it was a heavy downpour and a heavy rain when it just stopped a couple minutes before.
There are also distance requirements for the prevailing weather group. If the prevailing weather is at the airport which is defined as with a 5 mile radius there is will nothing with prevailing weather as in our case above (-RA). If the prevailing weather is between 5 miles and 10 miles from the airfield, the code VC is used for “in the vicinity” like this: VCSH for there are showers in the vicinity of the airport. Weather greater than 10 miles from the airport uses DSNT for distant weather and is usually associated with lightening.
What does this all mean to me? I know that the currently reported weather may or may not have changed and I should be ready for anything that happens to show up. I much prefer to have a human observer to an automated station and will trust weather from a military station more than a civilian airport. As always, frequent updates from tower personnel is highly desirable.
