How an ice storm uproots a tree

Atlanta's recent ice storm was so severe that some meteorologists were predicting that the city could lose more than a quarter of its trees in a single day. So, just how does ice take down a tree?

Top image: Gerd Altmann

Well, one way, it turns out, is by becoming so heavy that the tree is snapped at or pulled up by the roots. Today, commenter (and physicist) FlowerGirlPhysicist answered this physics question from commenter Ifailedcalculus: "So because it's so icy out and i have no power. How much extra weight can ice put on trees to pull them up from the roots?! Also the sound. How cold does it have to be for you to hear trees cracking a mile off?"

FlowerGirlPhysicist

Depends on how big the tree is and how deep the root system goes. I don't know the numbers exactly. But ice is a good way to do it — a decent layer of ice can apply as much force as heavy machinery.

one cubic meter of ice (a box that's like 3 feet on each side) is 920 kg, or 2000 lbs. So literally a ton. But a ton of ANYTHING on your tree, it's going to feel that.

And what about the second question, on why the sound of the cracking trees seems to carry in the cold?

FlowerGirlPhysicist

Second question first: Cold enough to cause a temperature inversion. Normally the air gets colder the higher up you go. With a temperature inversion (on a cold day), the colder air is near the ground and the warmer air is higher up.

Sound waves travel faster in warm air than cold air. Sound waves move from a cold air region to a hot air region more easily than the other direction. If you try to send a sound wave from a cold region to a hot region, it might very well reflect off the boundary between the hot and cold air and stay in the cold air region.

On a warm day, the sound waves go up into the colder air and dissipate instead of getting to your ears, so you don't hear as much

How an ice storm uproots a tree

On a cold day, the sound sticks around in the cold air and bounces around until it gets to you.

How an ice storm uproots a tree