Fog consists of water vapor molecules that float as tiny water droplets in the air but remain near the ground. Essentially, fog is a cloud touching the Earth’s surface, forming similar to clouds. High humidity plays a significant role in fog formation, and depending on its rate and temperature, fog can appear and vanish suddenly.

Water in its vapor state is transparent and invisible. The higher the air’s temperature, the greater its kinetic energy, allowing more water molecules to exist as vapor.

When warm, moisture-rich air cools abruptly, the water molecules slow down too much to maintain their vapor form and combine into small liquid droplets. These droplets, while still small enough to float in air currents, appear opaque as light reflects off the air-water interface.

radiation fog

Radiation fog forms on the ground during calm, clear nights when heat absorbed by the Earth’s surface during the day radiates into the air. As the heat rises, the air near the surface cools until it becomes saturated.

Cold air holds less water vapor than warm air, causing the water vapor to condense into fog. Radiation fog typically dissipates as the ground warms up again, but it can persist all day in the winter.

Radiation fog is also known as shallow fog or ground fog when it occurs in a narrow layer below average eye level on land and below about 10 meters at sea.

valley fog

Valley fog develops at the bottom of valleys as cold, dense air settles and condenses to form fog. It is restricted to local terrains like hills and mountains and can persist for several days.

advection fog

Advection fog forms when horizontal winds push warm, moist air onto cold surfaces, leading to fog formation through condensation. This phenomenon is common at sea, where warm tropical air interacts with cold water. Advection fog can cover large areas, with the Golden Gate Bridge in San Francisco Bay often obscured by it.

Sea fog, a type of advective fog, occurs when warm, moist air descends from land into chilly oceans, or when warm fronts clash with cold ocean currents. The northeast coast of the UK is particularly prone to sea fog due to the cold waters of the North Sea.

uphill fog

Uphill fog, a type of hill fog, occurs when moist air is pushed up a slope, hill, or mountain by wind, cooling and condensing to form fog as it descends down the slope.

evaporation fog

Evaporative fog is akin to advection fog, forming when cold air passes over moist land or warm water. When warm water evaporates into the lower atmosphere, it warms the air, causing it to rise. This upward movement of warm, moist air mixes with cooler air until reaching 100% humidity, resulting in fog formation. Evaporative fog is commonly observed at lakes, ponds, and outdoor pools.

Why does altitude affect temperature?

Consider the atmosphere as consisting of air masses. The higher an air parcel, the less compressed it is due to the weight of the atmosphere above, allowing for greater volume. This expansion requires energy, leading to sacrifice of thermal energy and a decrease in temperature.

Does sound travel further on foggy days?

“Sound propagates through the air as pressure waves move air molecules back and forth. In fog, water droplets scatter more sound energy, attenuating the sound and reducing the distance it can travel,” explains physicist Robert Matthews.

However, the complete impact of fog formation conditions on sound travel is not fully resolved. “On warm, highly humid days, smaller droplets have minimal effect on sound waves,” he adds.

“Moist air, being denser than dry air, allows sound waves to travel more effectively and be heard over greater distances,” Matthews further explains.

Submitted by: Rich French, London

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