“Let’s go to the lake today.”
“No, I heard on the radio that there is a thirty percent
chance of rain.”
“Then aren’t the odds seventy percent that it will
not
rain?
The chances are that it won’t rain
at all, so let’s go.”
“No, I’m not going.
I
think it
usually rains whenever there’s a thirty percent chance.
I wonder how they calculate those chances
anyway.”
Who is right here?
Are
the odds really seventy percent that it will
not rain when there is a
thirty percent chance that it will?
Not necessarily.
According
to the
National Weather Service, the probability of precipitation is actually
calculated for a twelve-hour period, ending at either 6am or 6pm local
time.
In other words, the probability is
actually thirty percent that it will be raining
at any given time during
the twelve-hour period.
So, the odds
that it will rain at all during that twelve-hour period seem pretty high.
Now, just because precipitation is likely, doesn’t
necessarily mean that it will actually be horrible.
The National Weather Service considers a
precipitation event to be anything that is at all measurable (so it needs only
to be greater than a hundredth of an inch.)
Also, the probability is that precipitation will occur anywhere within
the forecast area.
Some spots may get
downpours, while others receive nothing at all during the forecast period.
So, here’s the bottom line, according to Dr. Jim Angel,
State Climatologist at the
Illinois State Water Survey:
If there is a thirty percent chance of rain,
then plan for it; pack an umbrella or a raincoat.
If there is a fifty percent or greater chance
or rain, then go to the mall instead of driving all the way to the lake.
