Visibility and Rain Amounts


The old guys that drink coffee at Hoover's store in Whitehall every morning have a question, and I was elected to ask you for the answer.
When you report on the visibility, how do you measure visibility? They have been talking about this for a couple of weeks.
The only other thing they're talking about is all the rain and not being able to cut their grass. I don't know how I got elected for this, all I do is bring them the newspaper and you know they don't learn anything from reading it. 
So if you give me the answer about the visibility they will think I'm the man.


First, let me clarify the different observation acronyms.  "ASOS" stands for Automated Surface Observation System, the top-shelf platform; "AWOS" stands for Automated Weather Observation Platform.
     Visibility is measured manually or by sensors.  At most major airports, the automated weather systems (ASOS or AWOS) use a device that actually determines the "clarity" of the air (how much dust, moisture & fog, etc.).  The system then converts "clarity" into a visibility equivalent.
If you ever look closely at visibility for Metro Airport, for example, you will see a "maximum visibility" range of 10 miles.  In other words, the system is REALLY saying "visibility of ten miles or more."  I think I remember reading where some of the AWOS systems can estimate out to only 7 miles.
     Manually-estimated visibility is based on the trained observer being able to see "targets" of know distances .. so manual reports can extend well beyond the 10-mile limit of ASOS.   ALL visibility estimates are somewhat hampered by the fact that visibility is rarely constant in every direction.
     Now ... as for the rain ... we have had a string of "wet" days recently, but the daily totals have not been too, too large.  Most of Livingston Parish is running fairly close to normal for the mid-month total ... a lot of "wet" days but not heavy rain days.
     I suspect your local experts at Hoover's will disagree with my appraisal!!  But I am used to being told I am wrong by the best of 'em!  Get me an invite and I'll come visit the morning coffee gang!!

Jay Grymes
Chief Meteorologist
WAFB Storm Team