During the recent cold spell in the northern US, meteorologists issued warnings about exploding trees. In cold conditions, trees also enter a form of hibernation, with changes at a cellular level: cells dehydrate, harden and shrink, increasing their sugar concentration. In severe cold the tree may give way, sometimes with an explosive sound, producing deep vertical gaps known as frost cracks. Frost cracking is especially likely on sunny days when the temperature of the sun-warmed bark drops rapidly as night falls. Frost cracks can harm trees as they may be exploited by insects and fungi, but exploding trees are not dangerous to bystanders.