Northwoods Curiosities: Why Trees Explode in Deep Cold

If you are outside during a deep cold snap, when temperatures hit twenty below zero, you might hear loud booms echoing through the woods.

It sounds like a gunshot, but it is just nature. It is the sound of massive sugar maples splitting open under pressure.

Foresters call this a "frost crack." It is a brutal demonstration of physics. Most people think it happens because sap freezes and expands like a busted water pipe. That is wrong. The exact opposite is happening. The tree splits because the wood is shrinking.

The Battle Inside the TrunkWood does not handle extreme cold evenly. The outside of the tree cools down much faster than the inside.

When deep cold settles in, the air temperature drops fast. The outer layers of the trunk (the bark and outer wood) react instantly. They get cold and try to shrink quickly. But the massive inner core of the tree is protected. It stays warmer and keeps its original size.

This creates huge pressure. The shrinking outer skin pulls violently against the solid inner core.

The pressure builds until the wood fibers snap. The failure is instant. The trunk splits up and down the grain with force. That is the sound that carries for miles in the cold air.

The Sun Trigger and the Cost

These cracks almost always happen on the south or southwest side of the tree.

The dark bark soaks up sunshine during the day. This heats the outer wood even when the air is freezing. When the sun sets, that side of the tree cools down fast. That quick temperature swing causes the split.

For a landowner or logger, this matters because of money. A frost crack permanently lowers the value of the timber. The tree usually survives and heals over in spring, but the scar stays in the wood grain forever. It turns high-quality veneer logs into firewood.