On 6/27/2011, The National Weather Service recorded relative humidity of 1% at McCarren Airport in Las Vegas, Nevada.

Their announcement: "Las Vegas' largest dew point depression ever recorded was set on June 27, 2011 at 129 degrees. The dew point depression is the difference between the air temperature and dew point temperature. At this time the relative humidity was 1%. The previous record dew point depression was 120 degrees set on July 2, 2007. This makes June 27, 2011 the driest day ever based on the dew point depression!"

More Info: www.accuweather.com