Sometimes a pass of moist air close to the earths surface may cool off rather suddenly. This may happen when a body of cold air moves in or when the moist air drifts over a cooler body of water. When this occurs, condensation takes place, but the cloud which forms is right on the surface of the earth rather than high in the atmosphere. Such a low-hanging cloud is called a fog. In composition the fog is exactly the same as a cloud. The only difference between the two is the place in which each occurs. The cloud is high, the fog forms on the ground.
In certain areas of the world, such as Newfoundland and the city of London, fogs occurs frequently. This is because local conditions being about sudden cooling of moisture-laden masses of air rather often.
Dew and Frost. As the sun warms the earth during the day, evaporation continues to take place. At night, however, the earth cools down as it gives off some of the heat it has received from the sun.
Grass loses its heat especially fast. The air which surrounds the grass is full of water vapor. As the moving water molecules strike the cool blades, they slow down and condensation takes place. Thus, millions of molecules come together on the grass to form the tiny water droplets we call dew. Can you understand why dew forms readily on cool, clear nights? The next time you have an ice cream soda you can watch the process of dew formation. As the cold ice cream lowers the temperature to the glass, water vapor in the surrounding atmosphere condenses on the surface and the glass seems to sweat . This is artificial dew, perhaps, but is from exactly the same reason that dew forms in nature.
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