Summertime presents the perfect opportunity to seek shelter on front porches and watch thunderstorms roll through. These storms can leave evidence of their passage when trees are struck by lightning. If a tree is wet right down to its base when struck, the electrical charge from lightning may travel to the ground on the tree’s surface, causing little damage. If the tree is relatively dry, the charge will jump to the wet, sugary sap. Sap runs in narrow channels just under the bark. The charge will try to follow the sap to the ground, but the intense heat of the lightning vaporizes the sap. Having no room to expand, the vapor blows the tree apart, pushing off bark and leaving a long split in the tree.

Cool Links:

Savage Planet: Deadly Skies - How Lightning Forms - based on the PBS series, this site features an excellent animated primer on cloud-to-ground lightning, a lightning quiz, and a video clip showing the creation of an artificial lightning strike.

Thunderstorms and Lightning: the Underrated Killers - A preparedness guide PDF with facts, photos and safety tips. From NOAA and the National Weather Service.

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Tree photo: Erich G. Vallery, USDA Forest Service, www.forestryimages.org