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Local Weather History: December (& Other Months) Sunshine & Precipitation Climatology

December may be the cloudiest on average, but it does not have the biggest rains.

Posted: Dec 5, 2019 10:38 PM
Updated: Dec 5, 2019 11:04 PM

At West Lafayette, December is (on average) the cloudiest month of the year with us only receiving 35% of the available sunshine.

November is second at 39%.  January & February are tied for third at 44%.

The month of maximum sunshine?  August with 70%, followed by July at 67%.

What is interesting is how precipitation frequency & intensity climatology does not follow the clouds doctrine.  This, in turn, explains much about our weather patterns in the year.

For example, December again..................

So, it is the cloudiest month & for the number of days with at least 0.01" of melted snow or ice or just plain rain, we average the 4th greatest number of days of any month in the year.  35% or nearly 11 days see at least 0.01".  So it is only beaten by the wet months of April, May & June when it comes to daily measurable precipitation.

However, when we look at amounts greater than 0.50" December drops way down to 10th place for occurrences in the month.

We take 1.00" or greater in a day & December is 12th place for the least number of days.

This tells us that December sees a lot of cloudy days with light precipitation during the month, while other months, like July that are one of the sunniest, rank among one of the wettest with quick bursts of high-intensity, convective rainfall.

July ranks second with percentage of possible sunshine, but it ties for second with June & August for greatest number of +1" rainfall events with 1.0 even.

May averages the greatest number of days were year with at least 0.01" for the month (12 days).  It also average the greatest number of days with +1.00" of rainfall with 1.2.  So, not only is may usually persistently wet, but it rains heavily.

Interestly, July is the month with the greatest number of days with daily rainfall exceeding 0.50" with 3.2.  May is second with 3.0 & June third with 2.9.

However, all of these months average quite high for sunshine percentage overall.

This shows that much of this rainfall is highly convective & associated with thunderstorms often.

December is associated more with just persistent lighter precipitation that lingers on.

On a side note, November rank just behind the summer months for number of days with +0.50" & +1.00" rainfall as our second severe season & strong storm systems tend to cross the area during that time.

Parts of the Lower Mississippi Valley have November as their wettest month of the year, close to March.

Novembers also tend to be the wettest months of the year in the Pacific Northwest.

A brilliant climatologist whose work I really like, Brent Brettschneider (University of Alaska-Anchorage), created this map of the average wettest month of the year (based on total precipitation for each month):

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