Posted by: Rob Viens | May 8, 2012

Torrents of Rain in Rio

On May 8th Darwin was back in the field collecting arachnids in the rain, noting:

“Torrents of rain. — I am at present chiefly collecting spiders. In the course of a few hours 26 rain fell.” (May 8)

I was baffled by the strange way he reports rainfall – “26 rain fell”.  Assuming it was a typo, I looked at other copies of the diary, and to the best of my knowledge can only assume that Darwin handwriting was unclear.  Needless to say this led to a minor obsession by me to figure out what he meant…

Most of his other discussions of rainfall use inches (which makes sense for the time period).  So I think it is safe to assume that is the units. But if we assume it is inches, the entry would imply 26 inches of rain fell in a few hours. Now the world record for one hour (actually 42 minutes) is 12 inches (in Missouri in 1947) and for 24 hours it is 72 inches (on Reunion Island –  during a cyclone in 1966).  Considering that Rio was heading onto the dry season in May, I think this interpretation is highly unlikely.

That leaves us with two other options that suggest that there is a decimal point missing  – either 0.26 inches or 2.6 inches. Both of these are within the realm of possibilities, but since the climate graph below suggests that average rainfall in May is 3-4 inches (for the month) – the lower number might make more sense. (But Darwin also calls it “torrents of rain”, so who knows?)

Rio De Janeiro Climate graph contributed by


Supporting the lower number is also an entry in Darwin’s  Zoological Notebook from June which describes the rainfall a little later in the month (but does not mention the 8th at all):

“From May 10th . . . to June 8th inches 3.75 fell.— On May 17 it rained very heavily, between 9 AM & 3 PM 1.60.— out of which 1.06 fell in three hours.— During 6 minutes 0.38.” (Zoological Notebook)

Precipitation totaling 0.38 inches (about 1 cm) in 6 minutes is pretty impressive (though nowhere close to the record).  The 1 minute record for “falls” to Guadeloupe where they measured 1.54 inches (almost 4 cm) in 1 minute. Zoinks!

One of the most impressive records was just set in 2007, when the island or Reunion received almost 13 feet of rain in 3 days! Can you imagine – that is several feet deeper than most in-ground swimming pools.

Now I have not forgotten those spiders that Darwin was collecting in the rain, but we’ll have to come back to them another day.

Okay, I can’t resist – here is one of the spider species that Darwin collected in Rio. Something to look forward to in the coming days – a Brazilian crab spider – Epicadus heterogaster (by Henrique Cafundo via animals, animals, animals blog).

Brazilian crab spider

Stay dry and pleasant dreams! (RJV)


  1. […] hear the sound of the rain on the trees that Darwin captures so well. For more on rain in Rio, see Torrents of Rain in Rio, but for now the sounds (and Darwin’s “jumping”) remind me of another beetle – […]

  2. […] de Janeiro, Brazil – Torrents of Rain in Rio Click for weather […]

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