Relative humidity (RH) is the amount of moisture in the air as a percentage of the amount the air can actually hold. Warmer air can hold more moisture than cooler air, which means that for a given amount of atmospheric moisture, RH will be lower if air is warm than it would be if the air is cool. This can be seen by comparing the daily 9am maps (higher RH values) with the daily 3pm maps (lower RH values) for any month of the year.
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© Commonwealth of Australia 2007, Bureau of Meteorology