64 Fog-Low

Fog FOCUS Question Why does fog form?

Clouds Near the Ground High in the mountains of Costa Rica is a beautiful cloud forest. The mountains are covered with trees, mosses, and other plants. They are also often covered with something else—cloud! In this forest, clouds stay near the ground. Clouds that are close to the ground are called fog. Let's find out why some clouds stay near the ground instead of rising into the sky.

Making Clouds Clouds are made of droplets of water floating in the air. The water begins as liquid on the ground. Sunlight warms the water until it turns into a gas called water vapor. The water vapor rises into the air. This process is called evaporation (ee-vap-uh-RAY-shun). The air higher in the sky is cold. As water vapor rises, it cools down. The water vapor turns back into droplets of water. This process is called condensation (con-den-SAY-shun). The water droplets join together to form clouds.

Making Fog Sometimes the air is cold closer to the ground. This cold air cools the water vapor before it rises very high. The water vapor condenses near the ground. The low cloud that forms is fog. Whether fog forms depends on two things: moisture and air temperature. Water evaporates into the air. Another word for moisture in the air is humidity. The more water vapor the air has, the higher the humidity. The air near the ground must be very humid for fog to form. Science in Your World Fog makes it hard to see. Drivers must drive slowly. Sailors look for lighthouses to know if they are close to shore.

Air temperature is also important. When warm, humid air gets cooler, water vapor condenses. The temperature at which water vapor in the air condenses is called the dew point. The dew point affects whether or not fog will form. In places with dry air, the dew point is low. The air would have to get much colder for water vapor to condense into fog. In places with moist air, the dew point is high. The air doesn't have to cool down as much for water vapor to condense. Fog can form when the air temperature near the ground is close to the dew point. Water vapor in the moist air can easily condense and turn into a low cloud—fog!

Different Types of Fog Not all fog is the same. Fog can be thick or wispy. It can rise up a slope or sink into a valley. The type of fog depends on how and where it forms. Look at the diagrams on these two pages to learn how different types of fog form. Radiation Fog (or Ground Fog) After the Sun goes down the ground and the surrounding air get cooler. When the air temperature reaches the dew point, the water vapor condenses into fog. Advection Fog Warm moist wind blows over chilly water or ground. The moist wind gets colder and condenses into fog. Word Wise Have you heard the phrase "in a fog"? This can mean a person is confused dazed, or not thinking clearly. Can you explain why?