Fog: What is it, how does it form, and where is it most common?
This past weekend, you may have noticed fog reducing your visibility as you drove to your destination. Fog can occur due to many reasons, and can have a big impact on commutes on a daily basis.
Simply put, fog is tiny visible water droplets in the atmosphere that can often be thought of as a cloud resting at or near the Earth's surface. This is most common on calm and clear nights where there are high levels of moisture in the atmosphere.
One of the most common ways that fog forms is known as radiation fog. This process takes place overnight as daytime heat begins to leave the ground during clear conditions with light winds. In this setup, there is a thin layer of moisture at the surface underneath drier air. The rising air from the heat leaving the ground overnight quickly passes through the moist layer at the surface before reaching drier air. The moist layer does not absorb the rising heat as well, so the air near the surface cools quicker than the air above it. This means that the temperature at the surface reaches the level of "saturation" at a faster rate. So as this moisture continues to rise, it becomes water vapor, and fog forms. This is most commonly observed in the morning before the heating of the Earth's surface from the sun begins.
This is a similar process that occurs with river valley fog. Clear and calm conditions are once again needed; the main difference is that this occurs over warm bodies of water, like a river. The cold, dry air over the warm, moist air at the surface of the river is allowed to sink. This forces the warm, moist air to rise and saturate the air surrounding it. This rising motion once again leads to the formation of water vapor, and steam-like fog forms over the river valley.
These two types of fog are the most common in Central Missouri and in the Midwest, with dense fog occurring 10-20 days on average each year. In the east, areas like eastern Kentucky, West Virginia, and eastern Tennessee see 30-40+ days on average each year. This is due to the mountainous and valley-filled terrain. This, paired with the moisture that is often present in those areas, leads to ideal conditions for more frequent fog formation.
Even a little formation of fog can lead to large impacts on commutes. It only takes hazy conditions to reduce visibility from 6 miles to 1-2 miles, with dense fog reducing visibility to <1/4 of a mile. When driving in fog, it is important to turn the headlights on and use low beams. It is also important to slow down, leaving plenty of room between you and the traffic in front of you. Finally, keep an eye on the right lane, and use windshield wipers with the defroster to keep your windshield clear.
