On 2013-05-08 13:54:41 +0000, Weatherlawyer said:

Why does the metric for 30 inches mean about 1016 millibars?
I was looking at a tape measure for the first time in (an interesting
career????) and noticed that 1016 millimeters is 40 inches. Is it time
I bought a new tape?

I can't say I have ever noticed that 760 mm appears at the centre of
even the worst storms on the charts I use.

Whilst I am on the subject how high is thickness calculated for 500
(is it) DAM.

I think that you may be confusing many separate subjects here.

The usual usage of the word thickness is for the height difference
between two atmospheric levels, such as the 1000mb and 500mb.

Cold air will be denser than warm, and therefore the difference in
height between the two pressure levels will be less. It is much easier
to attach a pressure sensor to a balloon which can be filled with a
precise amount of gas so that it rises through the atmosphere at a
steady rate. By this method the time taken to reach various pressure
levels, 850mb, 700mb, etc. can be read from a stopwatch, and the
heights of those levels calculated. Subtract one height from another,
and you have the 'thickness'.