Cirrostratus clouds are high-altitude formations that often cover the entire sky in a single, continuous layer. They exist far above the layers where most weather occurs. These clouds are characterized by a delicate, expansive appearance that gives the sky a subtle, milky sheen.
Defining Characteristics and Altitude
Cirrostratus clouds are high-level clouds, typically forming in the upper troposphere above 20,000 feet (6,000 meters) in temperate regions. They share this formation zone with cirrus and cirrocumulus clouds, but are distinguished by their sheet-like structure. Visually, they appear as a thin, whitish, veil-like layer that can cover the entire sky.
The cloud layer is often transparent enough that the sun or moon remains clearly visible, though the light is diffused, resembling a view through frosted glass. This semi-transparency distinguishes them from lower, thicker cloud types like altostratus. Due to their altitude and thin nature, cirrostratus clouds rarely produce precipitation that reaches the ground.
Composition of Ice Crystals
The extremely low temperatures above 20,000 feet dictate the physical makeup of cirrostratus clouds. They are composed almost exclusively of minute ice crystals, not the liquid water droplets found in lower-level clouds. Temperatures in this part of the atmosphere are consistently below freezing, often dropping to \(-40^\circ \text{C}\) or colder, which prevents liquid water formation.
These frozen particles form when water vapor deposits directly onto an ice nucleus, bypassing the liquid phase. The ice crystals frequently take the shape of hexagonal prisms, including plates and columns. Their average size is small, often around 10 micrometers, but this uniform structure gives the cloud its unique properties.
The presence of these hexagonal ice crystals is directly linked to the cloud’s distinctive optical effects. The specific six-sided structure of the frozen particles acts like a prism when light passes through. This architecture allows the cloud to persist for long periods while maintaining its uniform, sheet-like appearance.
Optical Effects and Weather Significance
Cirrostratus clouds are famous for creating the 22-degree halo, a striking optical phenomenon seen around the sun or moon. This luminous ring forms due to light refraction as sunlight or moonlight enters and exits the cloud’s hexagonal ice crystals. The 60-degree angle between the faces of the prisms bends the light at a minimum deviation of approximately 22 degrees, resulting in the visible circle.
The clouds and the accompanying halo hold value in short-term weather forecasting. Cirrostratus clouds are often the first visible signs of an approaching warm front, a large-scale weather system where warmer air rises over colder air. As the warm front advances, the cloud layer tends to thicken and lower, signaling that precipitation is likely to follow within the next 12 to 24 hours.
As the front draws nearer, the ice crystals increase in density, causing the cloud to become less transparent. This thickening eventually obscures the sun or moon, leading to the disappearance of the halo before the main storm system arrives. Recognizing the shift from a transparent, halo-producing cirrostratus to an opaque layer indicates impending change in local weather conditions.