Fog AlertDate Posted: Wednesday, November 9th, 2011
Dense fog is causing delays this morning. Please be extra cautious when driving. Use of headlights and or fog lights are recommended for vehicles.
What is fog?
Fog is a cloud near the ground. The same processes that produce clouds thousands of meters above the ground can produce clouds near the surface. Both fog and clouds form when the atmosphere can no longer hold all of the moisture it contains. This happens when 1) the temperature of air drops to its dew point, which is the temperature at which air is holding as much moisture as it can, or 2) the amount of moisture in the air increases until the air reaches its dew point. Once air has reached its dew point, the water in it condenses, forming tiny water droplets that we see as fog. Fog is a hazard mostly for one very important reason: reduced visibility. Airport delays, automobile accidents, ship wrecks, plane crashes, and many other transportation problems are frequently caused by fog.
Fog can occur almost anywhere in the world. Fog is classified based on how it forms, which is often related to where it forms. The following are the most common types of fog:
* Coastal Fog: When warm, moist air blows over a cold surface, the surface can lower the temperature of the air to its dew point.
* Evaporation (or “Steam”) Fog: Water is always evaporating from the surface of streams, lakes, and oceans.
* Radiation (or “Ground”) Fog: Common on clear nights with little or no wind, this type of fog is formed from the rapid cooling of the Earth’s surface, usually on cloudless nights.
* Valley Fog: Cool air is denser than warm air, which causes cool air to sink and warm air to rise.
* Upslope Fog: As air rises over obstacles, it cools. If air is blown over hills or mountains, it may cool enough to reach its dew point.