WHEN DOES FOG CASCADE LIKE A WATERFALL?

The island of La Palma is one of the most volcanically active in Spain’s Canary Islands, with seven eruptions recorded since the 15th century. One of the island’s currently dormant volcanoes is the Cumbre Vieja, seen here with fog descending from its slopes. Driven by easterly winds, clouds pile up against the summit and then conform to the shape of the terrain as they cascade down the western side before being dissipated by the warmer air below. This image should dispel any illusions that mountain fog simply consists of low-hanging clouds. The water droplets in clouds, mist, and fog are very small. Although they range markedly in diameter—between 1 and 100 microns—the mean size is about 10 to 15 microns. Most of the water in fog is actually in the form of water vapor that surrounds the droplets. Clouds look like they’re opaque and substantial because they contain a large number of droplets. But if enough fog to fill an Olympic-size swimming pool were to be condensed, it would yield only about a quart of water.