Ken Jackson of Baltimore said that when he was preparing to fly to Utah he heard forecasters say the relative humidity there was 15 percent. "What is the lowest relative humidity recorded?" Lower than that, Ken. Relative humidity is the water vapor content relative to the atmosphere's capacity at a given temperature. Warmer air can hold more water, so the number falls when dry air heats up. Chile's Atacama Desert averages 15 percent. In June, McCarran International Airport in Las Vegas reported 2 percent relative humidity.
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