Saturday, April 25, 2015

Exploding Trees and Lightning

If you needed another reason to avoid standing under a tree during a lightning storm, here is another: the tree might explode and shred everything in its environment.

Such an event happened in the University of Washington Arboretum on March 31st and I visited the site this week during a run.....and I was amazed.   Here I am in front of the tree... this tall fir blew out form the center, with spokes of the trunk projecting out on the ground likes spokes of a bicycle wheel (picture taken by my colleague Lyatt Jaegle).

Another perspective:

 Branches and tree shrapnel was obvious HUNDREDS OF FEET from the tree.   Some of the wood was ejected so forcefully that it hit the ground and was deeply embedded--- over 100 feet away!   So deep I could not pull it out.

Here is a branch that pushed deep into the ground about 75 feet away.

A view from the air (courtesy of KOMO TV) and on the ground right after it happened (courtesy of the UW arboretum)  are shown below.  The original tree was over 100 ft tall.

Why did it explode?  When lightning hits the tree, it ran down the moist inner sapwood, since that portion of the tree conducts current better.   The huge current produced rapid heating and the water turned to steam, which in turn produced huge pressures that caused explosive expansion.

Can you imagine if you were standing near that tree?  You would have been torn apart or speared by flying debris.   Of course, standing under a tree during a thunderstorm is a bad idea for other reasons, such as getting electrocuted by the lightning current.   When I was a student at Cornell, a bunch of student were sheltering under a tree during a storm.  Lightning hit.  Several were seriously injured with some never recovering.

Want to see a video of lightning-caused tree explosion.   Check this out and look at the debris ejected over the parking lot.  This was a much smaller trees that the one shown above.

It turns out that cold can also cause trees to explode.  Water expands as it freezes (something I learned well when I put a full water bottle in the freezer), producing huge force.   So under very cold conditions the moist inner sapwood can freeze putting such a large force on the outer bark that the tree explodes.  The sound of such exploding trees supposedly sound like gunshots.   Native Americans such as the Sioux and the Cree sometimes called the first moon of the new year the "Moon of the Cold-Exploding Trees".    Don't worry, such cold does not occur here in western Washington.

And burning trees can exploded during wildfires when the sap is flammable, something that has been observed with eucalyptus trees in Australia.

Here in the Northwest, the big danger from trees is not the ones that explode, but rather the ones that fall during strong winds.  Big trees hugely increase the risks during windstorms and many people have been injured or killed in our region from falling trees and branches.  Big trees are a force multiplier for Northwest windstorms.

Don't get me wrong... I love trees.   But on rare occasions, danger lurks.

PS:  If you want to visit the exploded UW tree, it on on the north side of the UW arboretum, a few hundred feet northwest of the visitor center.


Sysiphus said...

The video is amazing. By my calcs, the first piece of shrapnel coming toward the camera is moving around 1,500 ft per second (about 50 ft per video frame, assuming 30 frames per second. That is about Mach 2. Could be faster, but it hard to judge distances.

Old Navy said...

Had a simliar tree explosion in Chimacum WA on the Olympic Peninsula two years ago, with very similiar results. Good post - thank you!

Suki said...

Are you so sure about trees not exploding from cold in western WA? In my years living on Waldron Island, some winters the Northeasters coming down from the Frazier River valley bring pretty intense cold, and you can hear the trees breaking like gunshots in the woods, even when the wind is still. My folks always told me it was the trees exploding from freezing.

Dan Kingsbury said...

I was just starting my bike ride home from the Microsoft Bike Shuttle dropoff at the Montlake Blvd Market when I saw the lightning flash and just about immediately heard the "crack!" sound. I noticed that the sound had more of a rifle-shot audio spectrum than I was used to hearing, thunder is often just a rumble when you're miles from the strike.

Jarv said...

My friend lived on 202 south of Redmond on five acres which had tall cedars. Lightning struck the medium height cedar next to his corral. It split the tree, arced to the fence which had a hot line on it and ran down the line and arced to the horse which was standing near the fence. The bolt continued down the hot line to the booster and blew it up. It ran down the cord to the outlet and then inside the house. The family was watching a ballgame on their TV and the set imploded as they sat there. No one hurt but they did find their horse dead on it's back with it's all four feet sticking straight up in the air.