The Earth's tallest trees, California redwoods, rely on characteristic coastal fog to reach their towering heights. Because fog appears as a cloud that moves off the ocean and sits on the ground, airport monitors of the ceiling height of the clouds were particularly illuminating. Fog drip is water dripping to the ground during fog. It occurs when water droplets from the fog adhere to the needles or leaves of trees or other objects, coalesce into larger drops and then drop to the ground. In the Age of the Dinosaurs, redwood species were dominant over much of the Northern Hemisphere, including what is now the Arctic. The climate was then humid and mild over a much larger region than today. Over millennia climate change reduced redwood habitat. The passage of warm, moist marine air over the cold surface waters of the Pacific creates fog here almost daily in summer. It frequently lasts until afternoon, when it burns off. Another fog bank may move in before sunset. Coast redwoods are distributed along a narrow band of California's northern coast. To obtain sufficient moisture for photosynthesis and growth, redwoods reach into the air with leaves shaped like baseball mitts and capture the fog that rolls in by night and languishes through most mornings. Redwoods are the first trees found to move water in both directions, though others have been identified. Fog is not just a vital element for the redwoods—it's also crucial to the entire redwood forest ecosystem. Some of the moisture drips off the redwood leaves, landing on the forest floor to water the trees and young saplings.