What causes fog and why does it appear to be very thick at times, limiting our visibility?
Tristan Lindsay, 5 years old
Pre-schooler at Lee-Scott Academy in Mrs.Carl’s class
Helping Aubie this week is Dr. Luke Marzen, Assistant Professor of Geography, in AU’s College of Sciences and Mathematics.
If you have ever wondered what it would look like inside of a puffy cloud high above in the sky, then walking or driving through fog provides the most similar experience while on the earth’s surface. Basically fog is a cloud at the ground level.
A cloud forms through the cooling of air as it rises and expands in the atmosphere. As air cools, it loses the ability to hold water as a gas and when air cools to a certain temperature it will undergo condensation and change into water droplets that form on small particles in the air such as smoke, dust or sea salt. These water droplets exist in a form that is much larger than water vapor and will scatter light at all wavelengths that your eyes can see. The scattering of this light results in clouds and fog that appear as a white color.
The major difference between the formation of water droplets in a cloud or in fog has to do with how the air cools. Unlike clouds, which form from air cooling as it rises high in the atmosphere, fog forms from cooling at the surface of the earth.
Almost all fog is formed at temperatures that are above freezing through one of two main ways. This causes two different types of fog. The first is called advection fog and this occurs when warm moist air moves over cooler oceans, lakes, or snow-covered land. Advection fog also occurs when air rises and cools as it climbs up a hill or mountain slope.
The other type of fog is known as radiation fog. Radiation fog mainly occurs when air near the surface cools overnight. As the sun sets, the earth’s surface begins to give away heat from the surface causing it to cool and reach saturated conditions which causes fog. The thickness of the fog depends on many things including factors such as moisture availability and the amounts of condensation nuclei in the air.
Thanks for your question,
Aubie and Dr. Marzen
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