Tree leaves flip up when it's going to rain?

Question

I read this in a fictional novel and I'm wondering if there's any truth to it, or if perhaps it's only true of trees in Iowa or only true of certain types of trees.

Anyway, the character said that she knew it was going to rain because the leaves on the trees had started flipping over, so the more stemmy sides were facing up.

It's raining today and all the tree leaves I can see look just as usual.

Is my character a crack-smoker or what?

Answer 1

No, quite a few trees do that, alot of them are in the Family Fabaceae,

Subfamily Mimosoideae.

A good very common example (at least here in SC) is the Persian Silk Tree, Albizia julibrissin.

Answer 2

This is true to a degree.  It comes from the tendency of plants to produce their leaves for maximum photosynthesis efficiency - and they get the most sunlight in fair weather.  Combine that with the fact that fair weather is associated with high pressure systems which spin clockwise and air masses descend toward the earth.  So a plants leaves show their "top" side for when the prevailing winds blow in these conditions, so they don't "miss out on" any sunlight.  When it's about to rain, air pressure changes from high to low, which spins counter-clockwise and rise up into the atmosphere.  So now the air is coming from a different direction, and can flip the leaves over.

This doesn't always happen, because it depends on the degree of difference in air pressure from the high and low systems (a big difference = stronger wind) and the size and degree of "wetness" if the leaves were wet (the water adds weight to the leaf, so it may not blow around as much).  The best time to see this effect is in the hours before it actually rains.  Leaves will turn over, though, anytime the wind shifts from the "typical" direction during the growing season - you can sometimes see this without any rain forecast. It can also happen more frequently in areas around large bodies of water, because water changes temperature more slowly than land, and this can create air movement between these areas.

Answer 3

No, I see that a lot, not with all kinds of trees though.  It's weird, I dunno if it's the way the wind start to catch them or what, but yeah, I know just what she meant.