by David Murphy
In some cases, it's not. The biggest problem is that relative humidity changes as the air temperature changes---which makes it a pretty inexact measure. For example, you may have noticed how the relative humidity value I report on Action News in the morning is sometimes a high percentage, perhaps 70 or 80%, but by noon, it's a lot lower. What has happened? Has the air become "drier?" In most cases, the answer is no. The value has changed simply because it's warmer outside.
The reason for this is fairly simple. Cold air can't hold as much moisture as warm air. But as air warms, it's capacity for storing moisture grows. So, it makes sense that the percentage of relative humidity would be lower as we get into the warmer part of the day, because the warmer air has a lot more room for extra moisture than in the cooler morning.
In fact, that's why you see dew on the grass in the morning and not in the afternoon. In the morning, the cool air is more easily saturated, which means it's easier to condense its moisture into dew droplets. In the afternoon, the air is warmer and the moisture that turned into dew hours earlier is now floating around in the air as invisible water vapor.
While relative humidity is still useful in certain applications and the public has grown used to hearing the moisture content of air expressed this way, a more accurate and commonly used measure of atmospheric moisture is the dew point.