Dew Point Map The Current Dew Point map shows the dew point, contoured every 10 degree F, for the most recent hour. The dew point (or dewpoint) is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dewpoint is a saturation point. When the dew point temperature falls below freezing it is called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. If the relative humidity is 100%, the dew point is equal to the current temperature. Given a constant dewpoint, an increase in temperature will lead to a decrease in relative humidity.

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The Fog Forecast (FOGcast) Map The FOGcast map shows where areas of widespread fog are expected for the current day. Fog is a cloud in contact with the ground. Stratus clouds are usually the only clouds that touch the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km. Fog forms when the difference between temperature and dewpoint is (5 °F) 3 °C, or less. Fog begins to form when water vapor (a colorless gas) condenses into tiny liquid water droplets in the air. Since water vapor is colorless, it is actually the small liquid water droplets that are visible in the form of fog or any other type of cloud.

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