But the morning was also marked by another phenomenon -less unusual, but still rare: a strong acqua alta. The sea rose to 1.15 meters above its average level, and flooded calli and campi with up to ten inches of water.
Now, the two phenomena are obviously correlated, since they are both driven by atmospheric conditions. The fact is, however, that one might suspect that the correlation is an inverse one. High levels of water are usually due to a strong wind from the south-east, which pushes waters of the Adriatic basin toward the Venice lagoon. This wind usually brings warmer air in, so in normal conditions it should twart the cold weather needed for sustained snow falls.
Indeed, I must say I have never seen the two phenomena in coincidence in my life. If we assign the probability of 4" of snow to be one in a thousand, and the probability of a 1.15m acqua alta to be one in five hundred, their product would make the coincidence a "twice in a million" one, or something that happens by chance only once in fifteen hundred years!
In reality, we need to apply some corrections to that back-of-the-envelope estimate. The phenomenon of acqua alta is most typical in the winter months - so there is at least a factor of four to discount because of that. Also, there are two maxima of the water level per day. This leaves us with an estimate of once every two hundred years.
I could continue to try and correct the estimate further, but it is quite meaningless: suffices to say that it is the first time in 40 years that I see such a phenomenon. So now let me just paste here a few pictures of the event, taken from a mobile phone (which begs your excuse for the poor quality of the material).
Rio del Malcanton invades the walkway. Notice the falling flakes.
Icy waters with rubbish floating around.
Lakes in Campo Santa Margherita.
Stores try to fight the water invasion with pumps, but it is an uphill struggle.
A floating rubbish bag with a cap of snow.