The air of late bites shrewdly, and has sufficient nip in it to make us understand what William Cowper meant when he spoke of "our severest winter, commonly called the spring". Of course the cold snap and the snow may have happened just by chance, or it could be a nasty greenhouse side-effect, but there are those who would say the chill bears out the prophesies of Alexander Buchan.
Buchan was a Scottish meteorologist who carried out a detailed statistical study of weather in his native Scotland. He found that certain parts of the year were significantly colder or warmer than they ought to be. He identified six cold periods and three unseasonably warm ones. The warm spells were July 12th-15th, August 12th-15th, and December 3rd-14th; and the cold spells were February 7th-14th, April 11th-14th, May 9th-14th, June 29th to July 4th, August 6th-11th, and November 6th-13th. As can be seen, the current wintry conditions coincide closely with Buchan's second allegedly cold period near the middle of April.
As it happens, meteorologists nowadays treat "Buchan's Spells", as they are called, with scepticism, since more sophisticated statistical analysis over longer periods does not support their regular existence - not on those dates, at any rate. But it's a common experience that very cold conditions come along unexpectedly at some time during the late spring, and popular explanations have evolved in many cultures to explain them.
Irish tradition, for example, identifies a regular cold snap in late April or early May called "Scairbhin na gCuac". Some weeks later on mainland Europe comes the period of the "Ice Men", or the "Frost Saints" as they are sometimes known. These are the four saints Mamertus, Pancras, Servatius and Boniface, whose feast-days fall on May 11th, 12th, 13th, and 14th respectively, a four-day period during which a sharp and injurious frost was thought to be inevitable.
Or if the cold spell be even further delayed, we can blame St Dunstan. The story goes that early in his career before he took to piety, Dunstan set himself up in business as a brewer. To gain a competitive edge, he cleverly bartered his soul to the devil in return for an annual spring frost severe enough to blast the apple crop; this, Dunstan reckoned, would stop the local cider-makers in their tracks - and force everyone to drink his beer. Presumably Dunstan managed by some means to release himself from his side of the bargain, but the devil has kept his word ever since, giving a regular blast of severe frost in the three days leading up to St Dunstan's feast-day on May 19th.