Meteorologists classify condensed water vapor in the atmosphere based on appearance and altitude. This system provides a standardized way to describe the sky and forecast weather. The term “low clouds” describes those that form closest to the Earth’s surface. These clouds are the most observable and significantly impact daily weather.
Defining Cloud Altitude Layers
Meteorologists organize the troposphere into three altitude classifications: high, middle, and low. The low-level layer, or low-étage, extends from the Earth’s surface up to approximately 6,500 feet (2,000 meters).
Clouds in this bottom layer are predominantly composed of liquid water droplets, but they can contain ice crystals during cold winter conditions. The middle layer, indicated by the prefix “alto-,” ranges from 6,500 to 20,000 feet. The high layer, using the prefix “cirro-,” occupies the space above 20,000 feet.
Characteristics of the Low-Level Cloud Genera
Within the low-level layer are three primary cloud genera: Stratus, Stratocumulus, and Nimbostratus. Stratus (St) clouds are uniform, flat, and featureless layers, often appearing dark gray to nearly white. They are the lowest layered clouds, sometimes forming as low as the surface up to 2,000 meters. These clouds typically produce light precipitation, such as drizzle or snow grains, creating the classic overcast sky.
Stratocumulus (Sc) clouds are hybrids, combining the layered structure of Stratus with lumpy, cellular elements. These clouds appear as patches, rolls, or clumps, often with thick and thin areas. They commonly occur between 1,000 and 4,000 feet, sometimes producing light drizzle. Nimbostratus (Ns) clouds are thick, dark gray layers defined by their continuous precipitation, which can be steady rain or snow. While Nimbostratus bases usually reside in the low layer, the cloud’s vertical extent can be multi-level.
Fog: The Absolute Lowest Cloud
The absolute lowest manifestation of a cloud is fog, which is a Stratus cloud with its base resting directly on the ground. If the cloud base is a few hundred meters above the surface, it is Stratus; if it descends to ground level, it is classified as fog.
Fog forms when moist air near the ground cools to its dew point, causing condensation that reduces visibility below 1,000 meters. Two common mechanisms are radiation fog, which forms overnight as the ground rapidly cools, and advection fog, which occurs when warm, moist air moves over a cooler surface. Fog dramatically impacts ground-level visibility, making it a significant low-level weather phenomenon for travel.