Stratus (St) is a low, grey, uniform cloud layer that often covers the sky like a blanket, producing a dull, overcast appearance. It forms at altitudes of 0–2,000 m and is composed of tiny water droplets. When stratus touches the ground, it is classified as fog. The name comes from the Latin stratum (layer), reflecting its sheet-like structure with no significant vertical development.
Stratus forms through several mechanisms: radiative cooling of moist air near the surface (especially on clear nights), advection of warm, moist air over a cool surface (e.g., warm maritime air moving over cold coastal waters), and gentle orographic lifting of stable air against hillsides. It can also form when the base of nimbostratus descends or when fog lifts slightly off the ground as daytime heating begins.
Stratus produces at most drizzle or light snow grains — never heavy rain or hail. However, it creates significant problems for aviation due to low ceilings and reduced visibility, sometimes persisting for days in stable atmospheric conditions. In Spain, stratus is common along the northern coast (Cantabria, Asturias, Galicia) and in river valleys during winter inversions. The persistent stratocumulus decks off the coasts of California, Chile, and Namibia are the marine cousins of stratus, playing a major role in global climate by reflecting sunlight back to space.