Orographic fog, also known as mountain mist, is a type of fog that forms when humid air is forced to rise up a mountain slope. This upward movement causes the air to cool and expand, leading to saturation and the condensation of water vapor into tiny droplets, creating fog. The fog often envelops the mountain summit.
How Orographic Fog Forms: A Step-by-Step Explanation
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Orographic Lift: Air masses encounter a mountain range or hill, forcing them to rise. This is known as orographic lift. (https://en.wikipedia.org/wiki/Orographic_lift)
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Adiabatic Cooling: As the air rises, it expands and cools adiabatically (without heat exchange with the surroundings). (https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/fog/types-of-fog)
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Condensation: If the air is sufficiently humid, the cooling process eventually reduces the air temperature to its dew point. At this point, the water vapor in the air condenses into tiny water droplets, forming fog. (https://content.meteoblue.com/en/research-education/educational-resources/meteoscool/weather/clouds/fog/)
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Fog Formation: The accumulation of these water droplets creates the characteristic orographic fog, often blanketing the mountain slopes and peaks. The fog can be particularly dense and persistent, especially if the wind is relatively calm and the air is moist. (http://rotormind.com/blog/2013/Orographic-Fog)
Examples and Observations
- Orographic fog is commonly observed on the windward slopes of mountains.
- Strong winds can create banner clouds downwind of the upper slopes of steep mountains. (https://en.wikipedia.org/wiki/Orographic_lift)
- The presence of low-level clouds at sea level indicates that the fog is not solely orographic in origin. (https://www.reddit.com/r/WeatherGifs/comments/9mg3wb/orographic_fog_on_gran_canaria/)
Distinguishing Orographic Fog
It's important to distinguish orographic fog from other types of fog, such as radiation fog or advection fog, which form through different mechanisms. (https://www.sciencedirect.com/science/article/pii/S0169809502000972) Orographic fog is specifically linked to the rising motion of air over elevated terrain.
