Just how heavy could the rains be­come?

The Guardian - G2 - - Notes And Queries -

On a very wet day we can ex­pect 1-2 inches of rain to fall in the UK, per­haps dou­ble that in ex­treme con­di­tions. But as Har­vey, Irma and other storms have shown, in warmer climes pro­longed rain­fall can be many times heav­ier. So in a warm­ing world, just how heavy could rain be­come?

I think the only safe an­swer here is “lots”. The plan­e­tary weather sys­tem is ex­tremely com­plex, mean­ing that it has many vari­ables, many of which are cur­rently un­known. Sim­ple pro­jec­tions linked to how much wa­ter the at­mos­phere can hold by tem­per­a­ture are a rea­son­able start­ing point, but other vari­ables (such as the ra­tio of sea to land, com­po­si­tion of at­mos­phere, sali­na­tion of sea etc) all act as mod­i­fiers to that pro­jec­tion, mak­ing it in­cred­i­bly dif­fi­cult to say how ac­cu­rate those pro­jec­tions will ul­ti­mately turn out to be.

In short, we know that weather events are go­ing to be more ex­treme and that ex­treme weather events are likely to in­crease in fre­quency, and our best guess at putting a num­ber to this is a 7%-ish (where “ish” rep­re­sents the un­knowns) in­crease in both.


I re­mem­ber stand­ing in the door­way of the Bri­tish Coun­cil build­ing in Dar es Salaam dur­ing a rain­storm, and not­ing that I could not make out any­thing of the build­ing op­po­site, about four yards away. Sev­eral tonnes of wa­ter seemed to fall for 10 or 15 min­utes (and yet the streets were dry a few min­utes later).

I can­not imag­ine how much more rain could fill that air space, un­less one were lit­er­ally un­der wa­ter. My guess would be that more wa­ter in the clouds would sim­ply mean a longer rain shower, rather than denser wa­ter fall. How long such “show­ers” could last be­fore the area be­came un­in­hab­it­able is any­one’s guess.

Fran­cis Blake, Lon­don N17

Storm Des­mond de­posited some 340mm of rain on Hon­is­ter Pass in Cum­bria in 24 hours, on 5 De­cem­ber 2015. The prob­lem wasn’t the in­ten­sity of the rain, as much as the du­ra­tion. Mabl­lechidris

I’m afraid it’s go­ing to get bib­li­cal – as in Noah and his ark.

Jackie Howard, Manch­ester

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