Fog

Fog is a visible aerosol made of tiny water droplets or ice crystals. These particles are held in the air near the Earth's surface. You can think of fog as a low-lying cloud. It often looks like a stratus cloud, which is a flat, layered cloud type. Fog is heavily influenced by wind, local land shapes, and nearby water. Because it can make it hard to see, fog affects many human activities. It impacts shipping, travel, and even warfare.

Fog forms through a process called condensation. This occurs when water vapor, which is water in a gas form, turns into liquid. For this to happen, the water vapor molecules must combine into tiny droplets. These droplets usually gather around small particles called condensation nuclei. These nuclei can be bits of dust, ice, or salt. Sea fog is a specific type that forms when vapor condenses on salt particles. Fog usually appears when the relative humidity is near 100 percent.

There are several distinct ways that fog can form. Radiation fog is one common type. It forms after sunset during clear and calm conditions. The Earth's surface loses heat and cools the air touching it through conduction. This creates a temperature inversion, where cool air is trapped under warmer air. This type of fog is most likely to occur in valleys or swampy areas. It is often thickest shortly after sunrise before the sun evaporates the droplets.

Another type is advection fog. This happens when wind moves moist air over a cool surface. This process is called advection. It is very common at sea when warm air meets cooler waters. Along the California coast, a cold front or a pressure gradient can push this marine layer onto land. In the summer, a low-pressure trough can also draw this fog inland. This type of fog can be quite complex depending on the terrain.

Frontal fog forms near a weather front. It works much like a stratus cloud. Raindrops fall from warm air and evaporate into cooler air near the ground. This makes the air saturated, so the vapor condenses into fog. There is also hail fog. This occurs near large amounts of hail. The hail cools the air and adds moisture as it melts or evaporates. This fog is often very dense and appears abruptly.

Extreme cold can create very different kinds of fog. Freezing fog happens when liquid droplets freeze upon hitting a surface. This creates white ice known as rime. In the western United States, people call this pogonip. The name comes from the Shoshone word for cloud. In the Columbia Plateau, these inversions can last for three weeks. In even colder regions like the Arctic, we find ice fog. This is when the droplets are actually tiny ice crystals. This requires temperatures at or below -40 degrees Celsius.

Fog is closely linked to the movement of heat and moisture in our atmosphere. The thickness of a fog layer depends on the altitude of the inversion boundary. This boundary changes based on atmospheric pressure. High pressure can squash a fog bank, while low pressure allows it to expand upward. Fog can also produce drizzle. This happens when droplets coalesce, or join together, into larger drops. By studying fog, we learn how the air, water, and Earth's surface interact.