Coast redwoods are the tallest trees in the world. Some grow more than 300 feet high. A tree that tall has a problem. Water is heavy, and pulling it from the roots all the way to the top takes a great deal of work.

Along the northern California coast, thick fog rolls in from the ocean on summer mornings. The fog drifts through the redwood branches and collects on the needles. Drops form, join together, and fall to the forest floor like light rain. Scientists have measured this. In some forests, fog drip supplies as much as a third of the water the trees receive in a year.

The trees also drink from above. Redwood needles can take in water straight from the fog, so the top of the tree does not have to wait for water sent up from the roots.

The fog helps other living things too. Ferns, banana slugs and young redwood seedlings all depend on the damp ground the fog leaves behind. Drier summers mean less fog, and less fog means a thirstier forest.