How do you get a negative dew point?

MIKE MOSS SAYS:        Virginia,     The dew point is a reflection of how much water vapor is present in the air, and represents the temperature at which the air would become saturated if it was cooled sufficiently. A negative dew point simply means the air is dry enough that it would have to be cooled to that very low temperature, in order to cause moisture to begin to condense out as ice crystals or supercooled water droplets. There isn't any special physical significance of a negative temperature or dew point on the Fahrenheit scale. Just as a temperature of -2 F is simply four degrees colder than a temperature of +2 F, a dew point of -2 simply means the air is a little drier than air with a dew point of +2.