It can rain and be foggy at the same time. This coexistence is a common meteorological event that occurs under specific atmospheric conditions. While both rain and fog are composed of water droplets, their distinct formation processes allow them to overlap without canceling each other out.
Defining the Meteorological Components
Fog is essentially a cloud that forms at or very near the Earth’s surface. It consists of extremely tiny, suspended water droplets that are so light they remain airborne. Fog forms when the air near the ground cools to its dew point, the temperature at which the air becomes saturated and water vapor condenses around microscopic particles like dust or salt.
Rain is precipitation made up of much larger water droplets that fall from higher-altitude cloud systems, such as nimbostratus or cumulonimbus clouds. These droplets have coalesced enough to overcome atmospheric resistance and gravity, causing them to descend. The difference between fog and rain is fundamentally about the size of the water droplets and the altitude at which they form.
How External Rain Coexists with Fog
One of the most common ways rain and fog occur together is when rain falls from a cloud layer situated above a layer of ground-level fog. Raindrops originating from the higher cloud simply pass through the lower fog bank on their way to the surface. The fog persists because the air mass remains saturated, typically at 100% relative humidity.
The falling rain can help maintain or even thicken the fog layer. As the rain falls, some of the water evaporates in the air below the cloud, adding moisture to the lowest level of the atmosphere. This added moisture helps keep the air saturated, ensuring the tiny fog droplets do not evaporate. This combination is often observed during the passage of a warm front, where warm rain falls into cooler air near the ground, sometimes referred to as “frontal fog.”
When Fog Itself Produces Precipitation
Fog is capable of producing its own form of precipitation, which is typically classified as drizzle or mist. Drizzle is distinct from rain because it consists of very fine, uniform droplets, each less than 0.5 millimeters in diameter. These droplets are too small to be considered true rain, which requires a more extensive growth process at higher altitudes.
This precipitation occurs through the process of collision-coalescence within the fog layer. The tiny water droplets that make up the fog bump into each other and merge, slowly growing in size. When a droplet grows large enough to overcome atmospheric resistance, it begins its slow descent toward the ground as drizzle.