Fog

Fog forms when water vapor condenses into tiny droplets that remain suspended in the air. This occurs when air becomes saturated due to one or more of the following conditions:

• Cooling of air: When warm, moist air cools below its dew point (the temperature at which air becomes saturated with moisture).
• Increased humidity: When moist air moves over a cooler surface, such as warm air passing over cold land or water.
• Calm winds: Light or no wind allows moisture to accumulate, preventing dispersion.

As more droplets form, they scatter light, making the fog appear dense and significantly reducing visibility.

Types of fog:

‍There are several different types of fog, each with unique formation processes:

• Radiation fog: Forms when the ground cools rapidly at night, cooling the air just above it and causing condensation.
• Advection fog: Forms when warm, moist air moves over cooler surfaces, like coastal areas.
• Upslope fog: Forms when moist air is forced up a slope, cooling as it rises and condensing.
• Evaporation fog (steam fog): Forms when cold air moves over warmer water, causing water to evaporate and then condense as fog.

Why is fog common in the morning?

‍Fog often forms in the morning due to the cooling of the air overnight. During the night, especially when the skies are clear and there is little wind, the ground loses heat. As the ground cools, it cools the layer of air directly above it. If this air cools enough, water vapor in the air condenses into tiny droplets, creating fog.

This fog tends to persist in the early morning because the sun has not yet risen enough to warm the ground. Without the sun's heat, the droplets remain suspended in the air, causing reduced visibility. Once the sun rises and begins to warm the ground, the air temperature increases, causing the fog to dissipate as the droplets evaporate back into invisible water vapor.

What’s the difference between fog and mist?

‍The difference between fog and mist mainly comes down to density, or how much they reduce visibility.

• Fog is denser and reduces visibility to less than 1,000 meters.
• Mist is lighter, and while it still contains water droplets that can reduce visibility, if you can see more than 1 kilometer ahead but there’s still a noticeable haze, it’s considered mist.

Fog's impact on weather and safety:

‍Fog can significantly affect daily activities, including:

• Driving hazards: Reduced visibility makes fog a major safety concern on roads and highways.
• Air traffic and maritime navigation: Airports and ports often experience delays or adjustments due to reduced visibility caused by fog.
• ‍Temperature moderation: Fog can help moderate temperatures, either by trapping heat at night or by keeping temperatures lower during the day.

Fog and climate

Fog plays an important role in shaping local climates and influencing weather patterns. In coastal regions, where warm, moist air from the ocean meets cooler land or water surfaces, fog is especially common. This creates ideal conditions for its formation, with cities like San Francisco experiencing persistent fog, particularly in the summer. 

Coastal fog can have a cooling effect on local climates, lowering daytime temperatures and increasing humidity, which moderates temperature fluctuations.

In colder regions, fog can trap heat near the ground, preventing rapid cooling overnight and keeping temperatures milder during the winter months. On the other hand, in warmer climates, fog can help lower daytime temperatures, especially in desert or coastal areas where temperature extremes are common.

In colder climates, fog can freeze upon contact with surfaces, forming ice and creating hazardous conditions such as slippery roads and ice accumulation on trees and power lines. This phenomenon, known as freezing fog, can disrupt travel and cause infrastructure damage.

Additionally, in arid regions, fog can provide moisture to plants and animals that lack access to water, especially in coastal deserts where certain species rely on fog for hydration.

Fog is more common during cooler months, particularly in areas with high humidity, and its presence can influence agriculture by creating microclimates that either protect crops from frost or reduce sunlight. Moreover, fog can affect short-term weather patterns by cooling the ground and causing sudden temperature shifts once it dissipates.