Valley Fog


Early on a cold fall evening, similar to the one pictured to the right, an astronomy student looks out his valley homeís window. He sees that skies are clear and the lake near his home shimmering in the surrounding night light. In a couple of hours he plans to observe the stars in the sky. He comes out of his home a few hours later and sees....


He can't see anything.

He canít see the lake.

He canít see the sky!

Fog has blocked his view.


Many people from all over the U.S., as well as others around the world have witnessed this phenomenon of valley fog formation. Fog is a ground-level cloud that forms when the air temperature lowers to the dew point. Fog is especially known to those who live in valleys. Fog can develop anytime of the year, but valley fog is more prevalent in the fall, followed by winter. It is common for people, who live in valleys, to exit their homes during the night and find fog flowing freely around their homes. It is also common for these same people to drive into patches of fog during early morning rides to work. Fog usually develops during a night in which the sky is clear, the air temperature is cool, and the wind is calm, growing sometimes to a couple hundred feet in height depending on the moisture present in the air. It then dissipates as the sun rises the following day. In deep valleys, fog can persist for several days.

As the night in a valley becomes cool, clear, and calm, the ground radiates (releases) energy gained from the sun during the day in to space. Because the ground radiates energy in to space, it begins to cool. As it cools, the air interacting with the ground begins to warm the ground through conduction, trying to offset the change in temperature of the ground. The ground continues to cool as it still radiates energy, including the energy transferred to it from the air. The air then begins to cool near the ground in and around the valley. This leads to relatively warm air being higher away from the ground and the sides of the valley. Since cold air is denser than warm air, the cold air sinks to the lowest point in the valley, along the valley sides, as shown in the valley fog diagram. As the cold air sinks, warm air aloft replaces that air.  This new warm air transfers more energy to the ground. The ground continues to radiate energy in to space and, through conduction, the air temperature  of the warm air lowers. This air becomes relatively cold, sinks in to the valley and adds more cold air to the lowest part of the valley. This cycle repeats and cold air continues to sink in to the valley. This process is known as Cold air drainage and it is just that; cold air draining in to a valley. The valley begins to fill from the bottom with cold layers of air. If there is sufficient moisture in the air, fog will begin to form in layers as the air temperature approaches the dew point.

In the fall, air temperatures are cool, but temperatures of water bodies such as lakes and streams are still relatively warm. This is due to water taking longer to cool and warm through out the year compared to the air in the atmosphere. In general water bodies warm and cool much slower than air. The maximum and minimum temperatures of air in the atmosphere occur, on average, 6 weeks in to the summer and winter respectively.

River Fog in a Valley

The maximum and minimum temperatures of water bodies occur, on average, 12 weeks in to summer and 12 week in to winter, respectively. For water bodies, since they reach maximum temperature close to the beginning of the fall, it is ideal that fog formation would be at its peak during the fall season. Water bodies are still warm but nights are getting cooler, conditions that can support the formation of fog.

Keeping the above in mind, if there is water body at the bottom of a valley, most of the time it will be relatively warmer than the air in the valley during fall. Molecules from the water body are always evaporating, putting moisture in the air. The warmer the water body, the more evaporation of water and the more moisture present in the air. If a cool, calm, and clear night comes, and the air temperature approaches the dew point temperature, fog will begin to form. The picture above is that of fog developing over a river in a valley.

After fog develops over night, it usually persists until sun rises, afterwards it begins to dissipate. When the sun rises, the energy from the sun begins to warm the ground surrounding the fog, and the top layer of the fog. As the sunís energy begins to warm the ground, the air molecules closest to the ground begin to warm by conduction from the ground.

This is a picture of valley fog in the morning. 

This causes the air molecules at the ground surrounding the fog to become relatively warm. This warming of the air causes the air around the perimeter of the fog to rise.

Energy from the sun is able to warm the top layers of the fog which causes the air temperature in those layers to increase. The air temperature increases away from the dew point temperature, and causes the fog to start evaporating.

  As the sun continues to rise, more of the sunís energy is able to reach deeper and further into the perimeter of the fog. Eventually the fog evaporates.

This picture above shows a valley fog in the morning evaporating as the sun rises.



What the astronomy student observed that night is very familiar to residents of valleys around the world. These residents can come out there homes during the night and find that fog has formed on cool, clear, and calm night. Depending on the amount of moisture in the air, fog that forms can grow very dense. Then as the sun rises, fog usually evaporates.


Written by Marcus Walter.

Clear Valley Night picture came from:

Detailed fog diagram obtained from:

River Fog picture obtained from: