Fog is a natural phenomenon appearing as a visible atmospheric layer of tiny water droplets or ice crystals suspended in the air. It occurs near the Earth’s surface, reducing visibility. Common in coastal areas and valleys, fog often prompts questions about its formation.
The Basic Science of How Fog Forms
Fog forms when air near the ground cools to its dew point, causing water vapor to condense into visible droplets. The dew point is the temperature at which air becomes saturated with moisture, meaning it can no longer hold additional water in gaseous form. These droplets are small enough to remain suspended, creating the opaque appearance of fog.
As air cools, its capacity to hold water vapor decreases. If the air contains enough moisture, this cooling can lead to saturation, where the relative humidity reaches approximately 100%. Once saturated, any further cooling or addition of moisture causes the water vapor to condense around microscopic particles in the air, such as dust, pollen, or salt, known as condensation nuclei.
These tiny water droplets collectively scatter light, reducing visibility. The process is similar to how clouds form, but fog occurs at or very close to the Earth’s surface. High humidity is a contributing factor, as it means the air already holds significant moisture, making it easier to reach saturation with less cooling. When the air temperature and the dew point are very close, typically within 2.5 degrees Celsius (4.5 degrees Fahrenheit), fog is highly probable.
Understanding Different Types of Fog
Different atmospheric and environmental conditions lead to various types of fog, each with distinct formation mechanisms.
Radiation Fog
Radiation fog forms on clear, calm nights with light winds. As the ground radiates heat into space after sunset, the air directly above it cools rapidly. This cooling brings the air to its dew point, causing water vapor to condense into fog, often appearing in valleys and low-lying areas where cold air settles. It usually dissipates after sunrise as the ground warms and the air temperature rises.
Advection Fog
Advection fog occurs when warm, moist air moves horizontally over a cooler surface, such as cold land or water. As the warm air passes over the colder surface, it cools from below, leading to condensation. This type of fog is frequently observed as “sea fog” in coastal areas where warm, moist air from the ocean encounters cooler land, like the San Francisco fog. Advection fog can form even with significant wind, unlike radiation fog, and may persist for longer periods.
Evaporation Fog
Evaporation fog, also known as steam fog, forms when cold air moves over warmer water or moist ground. The warmer water evaporates into the colder air, increasing the moisture content directly above the surface. As this warm, moist air mixes with the cooler, drier air, it quickly reaches saturation and condenses into fog, often appearing as wisps rising from the water. This phenomenon is common over lakes or rivers in autumn or early spring when water temperatures are relatively warm compared to the air.
Upslope Fog
Upslope fog develops when moist air is forced to rise along a terrain, such as a hill or mountain, a process known as orographic lift. As the air ascends, it expands and cools due to lower atmospheric pressure. This cooling causes the water vapor in the air to condense, forming fog that blankets elevated areas and can extend downwards into valleys. Upslope fog can persist with higher wind speeds because the continuous upward movement of air sustains the cooling process.
Freezing Fog
Freezing fog forms when liquid fog droplets exist at temperatures below freezing, often between 0°C and -20°C. These water droplets remain in a supercooled liquid state, meaning they are still liquid despite being below their freezing point, until they come into contact with a surface. Upon impact with objects like trees, power lines, or roads, the supercooled droplets instantly freeze, forming a layer of feathery ice crystals called rime. This type of fog can create hazardous conditions, including icy roads and surfaces.
Distinguishing Fog from Similar Phenomena
Fog is often confused with mist and haze, but key distinctions exist based on visibility and composition. Fog is defined internationally as a phenomenon that reduces visibility to less than 1 kilometer (approximately 0.62 miles) at the Earth’s surface. It consists of a dense suspension of tiny water droplets.
Mist is similar to fog, also composed of water droplets, but allows for greater visibility, typically between 1 and 2 kilometers (0.62 to 1.24 miles). Mist is generally less dense and dissipates more quickly than fog. Haze, in contrast, is an atmospheric phenomenon caused by the suspension of extremely small, dry particles like dust, smoke, or pollutants, rather than water droplets. Haze reduces visibility by scattering light, and its presence is not directly tied to the air reaching its dew point, as its particles are solid.