Does weather watch lore stand up to scru­tiny?


WEATHER lore tells us that we’re go­ing to have a dry sum­mer this year. That’s def­i­nite. The ev­i­dence and cer­tainty for this long-range fore­cast lies in the fact that oak trees opened their leaves this spring be­fore ash trees did.

The old rhyme comes in a num­ber of vari­a­tions; one ver­sion is: ‘Oak be­fore ash we’ll just have a splash; ash be­fore oak we’re in for a soak’. If the oak leafs be­fore the ash, we are set to have just a splash of rain over the sum­mer but if the leaf buds of the ash burst first then a soak­ing wet sum­mer is guar­an­teed.

Peo­ple noted in the past that in some years the oak leafed be­fore the ash while in other years the op­po­site hap­pened. The an­nual com­pe­ti­tion be­tween the two tree species was keenly watched. What it meant was un­known in the past but many felt it had to mean some­thing; a sign, per­haps?

Over time, weather watch­ers mar­ried the sum­mer weather they ex­pe­ri­enced to the tree bud burst they noted in spring and came to the con­clu­sion that dry sum­mers fol­lowed in years when the oak leafed be­fore the ash. Hence the old rhyme: ‘Oak be­fore ash we’ll just have a splash; ash be­fore oak we’re in for a soak’.

So there you have it: we are go­ing to have a dry sum­mer. Or are we? Does the lore of weather watch­ers stand up to sci­en­tific scru­tiny?

Un­for­tu­nately, the old rhyme does is not in keep­ing with the facts as we know them. There is no known con­nec­tion be­tween what hap­pens in spring­time when leaves ap­pear on the trees and what the weather is go­ing to be like dur­ing the fol­low­ing sum­mer.

Leaf burst is trig­gered by day length. Leaf open­ing has evolved to co­in­cide with the day length that oc­curs be­tween late March and early May. Fur­ther­more, oak is more sen­si­tive to tem­per­a­ture than ash so if the spring is mild oak will re­spond faster than ash to in­creas­ing day length.

April was a cold month this year due to pretty per­sis­tent north-east­erly wind but March was rel­a­tively mild so oak got off to a fly­ing start. In a year when both March and April are cold, oak suf­fers a set back and ash leafs ear­lier.

So, whether oak leafs be­fore ash is a mat­ter of spring tem­per­a­ture and has noth­ing to do with whether or not we are in for a splash or a soak dur­ing the sum­mer.

Ash trees were late com­ing into leaf this year - a sign of a dry sum­mer to come?

Newspapers in English

Newspapers from Ireland

© PressReader. All rights reserved.