Fog and clouds are both accumulations of tiny droplets of water that have been condensed from the air. These water droplets are very small, on the order of 0.02 to 0.1 mm in diameter, and a very slight upward movement of the air will keep them from falling. If they do fall they usually evaporate. Fog is sometimes described as a cloud that forms at or near the surface. A fog, as a cloud, forms because air containing water vapor and condensation nuclei has been cooled to the dew point. Some types of fog form under the same “C” night conditions favorable for dew or frost to form, that is, on clear, cool, and calm nights when the relative humidity is high. Sometimes this type of fog forms only in valleys and low-lying areas where cool air accumulates. This type of fog is typical of inland fogs, those that form away from bodies of water. Other types of fog may form somewhere else, such as in the humid air over an ocean, and then move inland. Many fogs that occur along coastal regions were formed over the ocean and then carried inland by breezes. A third type of fog looks much like steam rising from melting snow on a street, steam rising over a body of water into cold air, or steam rising over streets after a summer rain shower. These are examples of a temporary fog that forms as a lot of water vapor is added to cool air. This is a cool fog, like other fogs, and is not hot as the steam like appearance may lead you to believe.
Sometimes a news report states something about the sun “burning off” a fog. A fog does not burn, of course, because it is made up of droplets of water. What the reporter really means is that the sun’s radiation will increase the temperature, which increases the air capacity to hold water vapor. With an increased capacity to hold water the relative humidity drops, and the fog simply evaporates back to the state of invisible water vapor molecules.
Clouds, like fogs, are made up of tiny droplets of water that have been condensed from the air. Like Howard, an English weather observer, made one of the first cloud classification schemes in 1803. He used the Latin terms cirrus (curly), cumulus (piled up), and stratus (spread out) to identify the basic shapes of clouds. The clouds usually do not occur just in these basic cloud shapes but in combinations of the different shapes. Later, Howard’s system was modified by expanding the different shapes of clouds into ten classes by using the basic cloud shapes and altitude as criteria. Clouds give practical hints about the approaching weather.
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