A look at last evening's pressure falls here in Ashland, MA...
 

I posted this on Todd's Yahoo Group this morning. Hope you find it
interesting.
----------------------------
Very impressive pressure late yesterday afternoon and evening during the
quick event that produced .77" here. . Take a look at this chart:


5 PM 29.84
6 PM 29.80 -0.04
7 PM 29.72 -0.08 0.07 " hourly rainfall
8 PM 29.64 -0.08 0.09 " hourly rainfall
9 PM 29.59 -0.05 0.08 " hourly rainfall
10 PM 29.50 -0.09 0.21 " hourly rainfall
11 PM 29.41 -0.09 0.21 " hourly rainfall
12 PM 29.36 -0.05 0.07 " hourly rainfall
1 AM 29.29 -0.07
2 AM 29.27 -0.02

The column following the sea level pressure reading is the hourly fall. By
convention, any hourly pressure fall of .06" or more (2 millibars or more)
is considered "falling rapidly".The biggest hourly falls of .09" hourly
occurred at 10 - 11 PM and 11 PM to midnight.

It is interesting, yet not surprising, that the greatest hourly rainfall of
...21" hourly happened during this same two hour span.

"Nature abhors a vacuum". At this 10 PM to midnight span of time, the
column of air atop my VantagePro barometer was loosing airmass (weight?) at
its most rapid rate. Air at the surface rushes into the base of the column
to try to make up this quick loss of airmass within the column, and has no
where to go but up, translating to largest upward vertical velocities (aka
vertical winds) Given ample water vapor to play with, most intense precip
should (and did) occur during that time.

[This is a bit of a simplistic explanation, but nevertheless helps to
explain the numbers in the chart].

Jot - Ashland



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