From Cumulus to Cirrus: Exploring the Diversity of the Sky

8. Altostratus: The Sun-Dimming Veil

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Usually existing between 6,500 and 20,000 feet, altostratus clouds occupy the middle levels of the troposphere. Covering a great swath of the sky, these clouds show up as a gray or bluish-gray sheet or layer. Though as a watery or dimly visible disk, altostratus are thin enough to show the position of the sun, unlike the lower stratus clouds, so producing a unique filtered light effect that can dramatically change the look of the landscape below. Important markers of shifting weather patterns, these clouds often develop ahead of warm fronts and major storms. Usually giving way to altostratus as a warm front approaches, cirrostratus clouds may subsequently thicken into nimbostratus, hence generating precipitation. For meteorologists and weather buffs, this sequence of cloud forms can offer insightful data about the oncoming system. Water droplets make up altostratus clouds; at higher altitudes or in colder climates, ice crystals form. Their arrangement lets them produce fascinating optical effects. Sunlight passing through these clouds can create a phenomena known as a "corona," or colored rings encircling the sun or moon. Diffraction of light by water droplets or ice crystals in the cloud generates these coronas. Local weather conditions can be much influenced by the presence of altostratus clouds. Their capacity to cover vast stretches of sky indicates that they can influence temperatures all around. Altostratus clouds reflect some incoming solar radiation back into space during the day, which lowers surface temperatures. But at night they can function as an insulating layer, catching heat close to the Earth's surface and so preventing temperatures from declining as much as they would under clear sky. Although not as striking as some other cloud forms, altostratus clouds are quite important for our daily weather experience since they usually indicate the change from fair to more turbulent conditions. Professional meteorologists and amateur weather enthusiasts should pay close attention to their appearance and development since they can offer useful hints about atmospheric conditions and approaching changes in temperature.