Ticker for April 11, 2003

                
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April 11, 2003     April 11, 2003     April 11, 2003     April 11, 2003

Whoever Named it "Normal" Had a Sense of Humor

The normal April rainfall for Oklahoma's Central climate division (CD5)
is 3.53 inches.  So, before you read any further, ask yourself these
questions:

1. How often has it rained exactly 3.53" in April in CD5?
2. How often is CD5 April rainfall within a tenth of an inch of normal?
3. What about a quarter of an inch?
4. A half inch?
5. An inch?

Before you answer, you should know that the term "normal" doesn't
imply the normal (ugh!) meaning of "normal".  Its roots lie in the
assumption that data represent (or can be made to represent) a normal
statistical distribution ("Ve have vays of making you represent a
normal distribution").

Climatological normals are usually calculated every ten years, based on
30 years of data.  Right now, we're using data from the 1971-2000 period,
and will call these values "normal" until we process the 1981-2010 data.

Okay, on to CD5's "normal" April rainfall.  The following graphic shows
all of the April rainfall totals for CD5 in the modern climate record
(the 108 years from 1895 through 2002).  The "normal" of 3.53" is noted.



By the way, the answers are (out of 108 years):
1. Once.
2. Nine times.
3. 15 times.
4. 28 times.
5. 45 times.

That's right, in April in central Oklahoma, the statistics imply that
the probability of observing normal April, give or take an inch, is
less than half!

Yikes!  This shows the folly of comparing short sets of data to normal -
especially for rainfall, and especially in Oklahoma!  Take heart, though.
As the period of interest becomes longer (toward seasons, for instance),
the significance of normals increases.  Likewise, the significance of
departures from normal increases.

In other words, a 35% departure from normal for one month of precip
is a common event in Oklahoma.  But, for a four-month period, it stands
out a bit in the climate record.

Ticker apologies to the Tulsans and non-CD5 types for using local data
to make today's point.  But it could be worse: imagine a Ticker with
"Norman's normals" splattered all over it!