Clouds in the sky aren't always a sign of an impending storm. Fair-weather cumulus clouds, those puffy white picturesque ones, are the sign of a nice day. High wispy cirrus clouds (mare's tails) form from ice crystals in the upper atmosphere, but don't necessarily bring any bad weather. Thick blankets of low, gray stratus clouds often linger as a marine layer in coastal areas and do nothing more than moderate the temperature. These clouds are a result of warm moisture-laden air cooling over cold ocean currents. Fog is a ground-hugging cloud formed when temperatures descend below the dew point.
More often clouds are associated with stormy weather and change. Clouds form along the edges of air masses where air of different temperatures is forced to mix. These fronts usually signify wind or rain and bring major temperature differences over a short period of time. Clouds also swirl in large spirals of low pressure thanks to the effects of the Earth's rotation. These low-pressure systems may develop into hurricanes (in areas with abundant warm ocean water) or cyclones (in higher latitudes) and the rain they deliver is significant. Warm-weather thunderstorms are the more formidable version of fair-weather cumulus clouds. Enough heat and latent water vapor in the air can create violent updrafts and cause enormous anvil-shaped thunderheads to grow in the sky. Lightening, thunder and heavy rain can result. The most severe of these storms spawn large hail stones, tornadoes and water spouts that are dangerous to people.
Ever wonder why cloudy nights are generally warmer than clear nights? The Earth absorbs heat from the sun all day long and stores it. At night, it is re-radiated outwards into the atmosphere and beyond. On clear nights this heat escapes and has no effect on temperature. Cloudy nights are a different story. Clouds act as insulation, trapping some of this escaping heat and holding it closer to the Earth's surface. It is this simple effect that determines many of the nighttime low temperatures. If a consistent cloud cover keeps temperatures from falling below freezing at night, this has important weather implications for agriculture, such as crop selection and planting dates.
Viewed from space, clouds appear as white blobs obscuring the Earth. White clouds have a higher albedo than does the darker surface of the Earth, meaning that they turn away and reflect back more of the incoming solar radiation. More intense weather resulting in more clouds (such as an active hurricane season) can have long-term impacts on climate. Even as clouds trap heat escaping the Earth's surface at night, they keep many of the sun's rays from reaching the Earth to warm it in the first place. When a cloud cover is persistent over a large area, this can lead to a general cooling. This is especially noticeable after major volcanic eruptions. The particles ejected by volcanoes into the atmosphere provide more nuclei for water molecules to collect around and, thus, cloud formation. Also, the ash clouds themselves filter and reflect sunlight. Famines and crop failures occurred for several years around the world after Krakatoa erupted in Indonesia in 1883, one of the most direct examples of clouds affecting climate.
