Why fogs form

Picture a rainy day. As you watch the raindrops fall, you see in the horizon that fog is forming. Why is that happening? Well, with the evaporation of water from falling raindrops, it can actually raise the dew point in the air beneath the cloud, and if enough vapor is added to the air to saturate it, precipitation fog starts to form.

When cold air passes over a warm body of water, the air above the water which tends to be warmer due to evaporation , will mix with the cold air, creating a layer of fog. So how does fog form?

First it is important to understand that fog is basically a cloud on the ground. This means like clouds it is a collection of tiny water droplets formed when evaporated water is cooled. The way it is cooled determines how fog is formed. The first way that fog is formed is by infrared cooling. Infrared cooling happens due to the change of seasons from summer to fall and winter. During the summer the ground absorbs solar radiation.

As air passes over it is made warm and moist. Fog is a cloud that touches the ground. Fog can be thin or thick, meaning people have difficulty seeing through it. In some conditions, fog can be so thick that it makes passing cars. Fog shows up when water vapor , or water in its gas eous form, condense s. During condensation , molecule s of water vapor combine to make tiny liquid water droplets that hang in the air.

You can see fog because of these tiny water droplets. Water vapor, a gas, is invisible. There has to be a lot of water vapor in the air for fog to form. In order for fog to form, dust or some kind of air pollution needs to be in the air. Water vapor condenses around these microscopic solid particles. Sea fog , which shows up near bodies of salty water, is formed as water vapor condenses around bits of salt.

Depending on the humidity and temperature, fog can form very suddenly and then disappear just as quickly. This is called flash fog.

Fog is not the same thing as mist. Fog is dense r than mist. This means fog is more massive and thicker than mist. There are more water molecules in the same amount of space in a fog. Mist can reduce visibility to between one and two kilometers. Types of Fog There are several different types of fog, including radiation fog, advection fog, valley fog, and freezing fog. As heat is transferred from the ground to the air, water droplets form. Ground fog does not reach as high as any of the clouds overhead.

It usually forms at night. Advection fog forms when warm, moist air passes over a cool surface. This process is called advection , a scientific name describing the movement of fluid.

In the atmosphere, the fluid is wind. When the moist , warm air makes contact with the cooler surface air, water vapor condenses to create fog. Advection fog shows up mostly in places where warm, tropical air meets cooler ocean water. The Pacific coast of the United States, from Washington to California, is often covered in advection fog. Earth rotates under these satellites as they make the trip from pole to pole.

NOAA is developing a new generation of geostationary and polar satellites. These satellites will be able to take very high-resolution photos of clouds and fogs. This information can tell pilots or drivers where to expect fog, and can help save lives.

Satellite data can be used to predict the likelihood of fog forming. Current geostationary and polar satellites, however, are capable only of producing a low-resolution picture, like the image on the left.

The next generation of geostationary and polar satellites, the GOES-R series and JPSS, will be able to produce a much more detailed and accurate image, like the image on the right.

The Short Answer:. Clouds and fog both form when water vapor condenses or freezes to form tiny droplets or crystals in the air, but clouds can form at many different altitudes while fog only forms near the ground.