During the summer the ground absorbs solar radiation. As air passes over it is made warm and moist. When the seasons change this mass of warm moist air collides with the cooler that is now prevalent. This cause is the water vapor in the air mass to condense quickly and fog is formed.
This fog is often called radiation fog due to the way it forms. This kind is the most common type of fog. It also happens when an unseasonable day of warm weather combined with high humidity is followed by dropping temperatures. The next way that fog forms is through advection. Advection is wind driven fog formation.
In this case warm air is pushed by winds across a cool surface where it condenses into fog. In weather reports, we often hear that the fog will "burn off" by a certain time. That terminology is actually a misnomer, however.
Fog doesn't burn off, according to The Weather Channel. Instead, as solar energy — heat from the sun — heats up the ground near the edge of the fog, drier air enters the foggy area, causing the fog to evaporate. Ground fog usually first begins to evaporate at the edges, where it is thinnest. Then the evaporation works inward, toward the center of the fog, where it is thickest. Some places, like San Francisco, are known for fog. The California city is noted for its foggy summers, with photos of the landmark Golden Gate Bridge shrouded in fog as a popular scene on postcards and framed art.
So why is the city so foggy in the summer? Essentially, it's because air is moving from a colder place over the ocean to a warmer place over land, according to KQED, the public radio and television affiliate in San Francisco.
As the air moves, fog forms and then moves over land. This type of fog is called advection fog. As foggy as San Francisco is, it's not as foggy as it used to be. Summer fog in the city has decreased by about 33 percent over the past years because of climate change and warming ocean temperatures, KQED reports. If those trends continue, the city will likely continue to experience less fog than in the past. It is foggy for the same reason as San Francisco, cold air from the north combines with warm air to create thick fog nearly every day.
So why are they two different things? Clouds can form at many different altitudes. They can be as high as 12 miles above sea level or as low as the ground. Fog is a kind of cloud that touches the ground. Fog forms when the air near the ground cools enough to turn its water vapor into liquid water or ice. There are many different types of fog, too. Ice fog forms when the air near the ground is cold enough to turn the water in fog into ice crystals.
Ice fog forms only at extremely cold temperatures. Ice fog is common in parts of Alaska and Canada. Another kind of fog is freezing fog. Sometimes it is cold enough, but the air does not have any particles. When it comes into contact with cold surfaces such as roads and sidewalks, it instantly forms a dangerous icy layer. The smoke provides particles for the water vapor to condense around.
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