Rainfall – May 2013


IMG_0175A magnificent rainbow and a distinct secondary bow seen over Fratta Todina on Sunday 26th May.  Some features were more pronounced to the naked eye than in the photo above, so below I have selected a portion of the primary rainbow and enhanced the contrast and saturation to emphasise the dark area known as Alexander’s band above the primary rainbow and the supernumerary rainbows on the inner side of the primary bow.

IMG_0179The supernumerary rainbows are slightly detached and have pastel coloured green and violet bands that do not fit the usual pattern.  Supernumerary rainbows are created by the interference of light waves. Their presence cannot be explained using particle theories.  The first correct explanation of their formation was provided by Thomas Young in 1804 (see here).


“With hey, ho, the wind and the rain                                                                                                      …the rain it raineth every day.”  King Lear III,2

Well not actually every day although with rain on 20 days it seemed like it. The weather remained cool and unsettled throughout the month.  Even on the wettest day the amount of rain that fell was modest compared to previous years but the cumulative effect was a total rainfall well above average. That was sufficient to push the 12 month cumulative rainfall up to 1293mm, just below the record level reported in March.

Thunderstorms rumbled round quite frequently but we only had hail on one occasion and fortunately it was not very violent so the damage was slight.  More memorable was to look out one morning in late May to see fresh snow not only on the peaks of the distant Apennines but down to 900m on nearby Massa Martana.


137mm of rain fell in 20 days during May. This is how it was distributed day by day:

Rainfall day by day - May

The maximum rainfall in a day was the 15mm on the 25th, far less than the previous maximum for the month of 65mm in 2010. Here is how this May compares with previous ones and other months over the past eight years:

Max rainfall in a day - May

(See footnote for changes in definition of max and min in all plots this month.)

It fact it rained on 20 days, more than in any any other of the eight Mays that I have recorded.  A total exceeded only once, in December 2008 with 21 days of rain.

Monthly raindays - May

The total rainfall for the month was 137mm. Here is how that compares with previous years and other months:

Monthly rainfall - May

The twelve month total rainfall now stands at 1292mm spread over 114 rain-days.  Over the past seven years the twelve month figures have fluctuated in this manner:

Previous 12 months aug 06 thru may 13


The detailed historical statistics for previous Junes are shown on the above plots by a uniform blue column.

Last year June was exceptionally dry both in terms of the amount of rainfall and the number of rain days.  Hopefully this year will emulate that or at least be more normal with under half the rainfall and half the rain days we have endured in May.  But we may have to wait a while as the professional forecasters are saying that the current cool, wet, unsettled weather is expected to continue thru the first half of the month.

With a little help from my friends I will be back at the start of next month with a report on all the rain that falls in Monte Castello di Vibio in June.


My thanks to Chris for covering the days I was away from MCdV,.

Footnote on plots

More observant readers will have noticed a subtle but important change in the plots showing Maximum rainfall in day, Rain-days per month, and Rainfall per month. The monthly range from minimum to maximum shown by the black arrow has been changed to show the range measured over all previous years, ie it excludes the most recent data for each month.  I have made the change to make it clearer when a datum for the most recent month equals or exceeds the previous extreme.


About PeterCham

Lifelong engineer with a passion for ships and the sea. A former company director. Now enjoying retirement in rural Italy - vines and wine, olive trees and virgin olive oil, green valleys and timeless hilltop towns, great food and magnificent opera, and, above all, dear friends.
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