Well, it’s still winter out there.  It was 18^o F at dawn in Pittsburgh but by Monday it will be back to 41^o.

These yo-yo temperatures can wreak havoc on roads and bridges and our landslide-prone hillsides.  If the temperature drops fast and far enough it even hurts living things.  At super low temperatures the trees explode.

I had never heard of this phenomenon until a conversation in Maine last fall when I asked Ann Sweet, who runs the Harbourside Inn, how cold it gets in winter at Acadia National Park.  Ann said the ocean keeps the island warmer than interior Maine but every once in a while it gets so cold that the trees explode.

Wow! And why?

Tree sap contains water and water expands when it freezes.  The expansion increases pressure under the bark and in extreme cases causes the bark to explode.  This doesn’t happen all the time because trees draw down sap into their roots in autumn, leaving room under the bark for expansion.  If they didn’t do this they wouldn’t live through the winter.

The danger for cold-explosion comes when the trees haven’t had time to draw down their sap or when the temperature falls extremely low.  Both occurred in north-central Washington in December 1968 when temperatures fell to -47^oF.  The fruit trees in Wally and Shirley Loudon’s orchard exploded.

Native Americans were well aware of this phenomenon.  According to Wikipedia, the Sioux and Cree called the first full moon of January “The moon of cold-exploding trees.”

When the moon was full on January 9, Pittsburgh’s average temperature was 10 degrees above normal.  I don’t think we’re in any danger of exploding trees.

(photo of tree exploded by lightning in Central Park, New York by David Shankbone.  Click on the image to see the original on Wikimedia Commons)

p.s. It is much more common for trees to explode when hit by lightning.