Stratus low fog

Stratus fog: formation, characteristics and impact on visibility. Understand this meteorological phenomenon common in coastal and mountainous regions.

About this subject

Stratus fog, also known as radiation fog, is a type of low cloud that forms when moist air near the ground cools to its dew point, condensing into tiny suspended water droplets. Unlike elevated stratus clouds, stratus fog touches the ground and reduces horizontal visibility to less than 1 km. It frequently occurs in coastal regions of Brazil, such as the Serra do Mar in Paraná, the slopes of the Mantiqueira Range in São Paulo, and the coast of Rio Grande do Sul, especially during autumn and winter when relative humidity is high and nights are long and clear.

Formation of this type of fog requires specific conditions: clear sky, calm wind, and moist air near the surface. At night, the ground loses heat by radiation, cooling the adjacent air. If moisture is sufficient, fog forms and may persist for hours until the sun warms the ground and disperses the droplets. In some cases, stratus fog lifts and transforms into a layer of low stratus clouds, common at airports and highways, causing delays and accidents.

Interestingly, stratus fog plays an important ecological role. In ecosystems like the Atlantic Forest, it provides extra moisture for epiphytic plants such as bromeliads and orchids, which absorb water directly from the droplets. In long exposure photography, moving fog creates smooth, ethereal effects, enhancing textures and depth in coastal or mountainous landscapes.

Frequently Asked Questions

What is stratus fog?

Stratus fog is a type of radiation fog that forms when moist air near the ground cools to its dew point, creating a low cloud layer that touches the ground and reduces visibility.

How does stratus fog form?

It forms on clear, calm nights when the ground loses heat by radiation, cooling the moist air above it. If moisture is sufficient, condensation into tiny droplets occurs.

What is the difference between fog and stratus cloud?

Stratus fog is in contact with the ground (visibility <1 km), whereas stratus clouds are elevated, forming a continuous gray layer without touching the surface.