cloud that touches the earth's surface and is so thick that visibility
is less than 62 mile (1 km) is called fog. If visibility is greater
than .62 mile (1 km), the ground-based cloud is called mist.
Fog develops when air becomes chilled to the dew point. This usually
occurs when warm, moist air flows over cold water. Fog often forms
along the northeastern coast of the United States and Canada. The
Gulf Stream-a warm ocean current that flows in a northeasterly pattern
from the Gulf of Mexico toward Newfoundland, Canada-brings a steady
stream of warm air over the colder North Atlantic Ocean, frequently
causing heavy fog to develop. Sometimes the ground temperature helps
to cause fog. When warm, moist air passes over colder or snow-covered
ground, the cold temperature of the ground lowers the temperature
of the air to the dew point, and fog forms.
1875 Paul jean Coulier, a Frenchman, experimented with air and fog.
He sealed moist air in a glass container and applied pressure to
the air. Fog appeared as the air was squeezed. After repeating the
experiment a number of times, fog no longer appeared. Coulier wondered
why. Finally, he decided to add new air to the container. He wanted
to see if the new air would make a difference. It did. Fog reappeared
when he applied pressure to the new air. Water vapor was condensing
around something in the new air that was too small to see. Coulier
believed that dust particles were attracting water vapor. Dust in
the air, much finer than the dust you may find on the furniture
in your home, most likely consists of salt particles from ocean
spray, volcanic dust, and even molecules of certain gases that have
condensed in the air. Coulier concluded that fog appears only when
water vapor is able to condense around dust particles that are present
in the air. Applying pressure forces vapor to attach itself to dust
particles. Once all the dust particles have been used during condensation,
no additional fog can appear. The discovery of the role of dust
particles in the atmosphere was an important step toward understanding
how the hydrologic cycle works.