Cirrostratus clouds are a high-altitude formation composed entirely of minute ice crystals. They appear as thin, expansive sheets high in the troposphere. While they are not associated with immediate severe weather, they serve as reliable atmospheric signals. Recognizing these formations is a practical way to predict significant shifts in upcoming weather patterns.
Defining Cirrostratus Clouds
These high-level clouds are typically found at altitudes exceeding 20,000 feet. Their composition consists exclusively of ice crystals due to the sub-zero temperatures. Cirrostratus often appear as a translucent, whitish veil that can cover the entire sky, giving the sun or moon a distinctive milky appearance. They are classified into two main species: fibratus, which has a fibrous or hair-like texture, and nebulosus, which is a more uniform, featureless sheet.
The defining characteristic of these clouds is their ability to produce a large, bright halo around the sun or moon. This optical phenomenon occurs when light refracts through the hexagonal ice crystals suspended within the cloud layer. Because the cloud is so thin, shadows can still be cast on the ground, helping to distinguish cirrostratus from lower, thicker cloud types. The presence of a halo is often the only way to confirm a nearly invisible layer of cirrostratus is overhead.
Current Weather While They Are Overhead
The immediate conditions on the ground when cirrostratus clouds are present are usually stable and fair. The clouds do not produce precipitation that reaches the surface, as any falling ice crystals, known as virga, evaporate long before descending. This high, thin moisture is a sign of a relatively calm atmosphere at the surface, with little vertical air movement.
The sky often appears subtly washed out or whitish, a condition sometimes described as “milky sunshine.” Despite the widespread cloud cover, the air remains generally dry and calm at the observer’s level. The appearance of these clouds is a sign of high-level moisture advection, indicating that a stable period of weather is beginning to give way to an influx of atmospheric water vapor. This momentary calm is the first visible sign of a much larger, slow-moving weather system approaching.
Forecasting Weather Changes
The meteorological significance of cirrostratus clouds lies in their role as a predictor of future weather changes. Their appearance almost always signals the approach of a warm front or, less commonly, an occluded front. These clouds are typically the second stage in a cloud progression, following the higher and patchier cirrus clouds. They demonstrate that a large amount of moisture is beginning to saturate the upper layers of the troposphere.
As the warm front continues its advance, the cirrostratus layer will gradually thicken and descend to lower altitudes. This process marks the transition from high-level cirrostratus to mid-level altostratus clouds. As the cloud deck thickens, the sun or moon may become completely obscured, and the halo effect will disappear. This loss of the halo is a traditional sign that a significant weather change is imminent.
The deepening cloud cover eventually lowers further into nimbostratus, which are the low-level, dark, featureless clouds associated with prolonged precipitation. This entire sequence, from the initial cirrostratus to the onset of rain or snow, generally occurs within a timeframe of 12 to 24 hours. The presence of cirrostratus, particularly the uniform nebulosus species, is a reliable indicator that the current fair weather will soon be replaced by widespread, continuous precipitation.