Cirrus Clouds

Cirrus clouds are the thin and wisp-like clouds seen at high altitudes (higher than 20,000 to 26,000 ft., or 6,000 to 8,000 m). The name cirrus comes from the Latin word for “curl.” They are composed predominantly of tiny ice crystals, because they form in the cold region of the troposphere. If cirrus clouds drop their ice crystals, these crystals evaporate before they arrive at the ground.

Cirrus clouds can take on a variety of formations, including a more tuft-like characteristic called cirrocumulus, which also include supercooled water droplets. Some cirrus clouds are called cirrostratus; this type of cloud formation occurs when the thin strands of clouds are so dense that they cannot be deciphered. Other formations include cirrus aviaticus (also called contrails, the artificial cirrus clouds generated by aircraft), cirrus castellanus (castle-like, with towers rising from a base), cirrus duplicatus (multiple sheets), cirrus fibratus (fibrous, and resembling a horse’s tail), cirrus floccus (rounded above), cirrus intortus (tangled), cirrus Kelvin-Helmholtz (slender and signaling atmospheric turbulence), cirrus radiatus (with horizontal banding), cirrus spissatus (thick and gray in color when in front of the sun), cirrus uncinus (hooked, and reminiscent of cirrus fibratus but with more curling at the ends), cirrus vertebratus (rib-like horizontal strips of clouds), and cirrus mammatus (rounded underneath).

Cirrus clouds are generally seen in fair weather, sometimes following a thunderstorm, and their wisps typically point in the direction of the highaltitude wind flow. In the case of cirrus Kelvin-Helmholtz, the clouds lie in the turbulent atmospheric region. Cirrus clouds usually form in the summer and winter, when opposing weather fronts meet, such as warm, dry air and cool, dry air. Some meteorologists use cirrus clouds to predict rain.

Because of the high altitude of these clouds, their albedo effect is often overridden by their greenhouse effect. This imbalance is because of the fact that lower clouds are weaker at conserving solar heat, but are very good at reflecting it back into the atmosphere. In contrast, the high cirrus clouds can both conserve heat and reflect it, but are often better at conserving it. Studies are currently being conducted at the U.S. National Aeronautics and Space Administration (NASA) to determine the role of cirrus clouds in global warming and the Earth’s climate. The modeling of these clouds and their effects is difficult, because of the irregular nature of the sizes and shapes of their ice crystals.

Clouds can be found in the atmospheric layer called the troposphere. The troposphere is the lowest atmospheric region and is where all weather takes place. At the equator, it reaches up to 11 mi. (18 km) from the Earth’s surface. The next atmospheric layer is the stratosphere, extending to 31 mi. (50 km) from the Earth’s surface. A cloud forms when water vapor reaches its dewpoint and condenses to form a water droplet. These droplets condense around cloud condensation nuclei (CCN), which are often particles of aerosol providing a scaffold around which the cloud can form. Each CCN is approximately one one-hundredth the size of the cloud droplet, which is approximately one one-hundredth the size of a rain droplet (usually about 0.08 in. or 2 mm in diameter). The nature of the clouds allows them to reflect sunlight away from the Earth, known as the albedo effect, but also to trap infrared light beneath them on the Earth’s surface. This latter phenomenon adds to the greenhouse effect.

In 2007, investigations at the University of Alabama at Huntsville found that global warming might paradoxically be leading to a thinning of these greenhouse-inducing cirrus clouds.