No. A high relative humidity reading in the morning doesn't necessarily guarantee a humid, sticky afternoon. Subsequently, a somewhat low relative humidity reading doesn't automatically mean an especially dry day, either. The reason is that relative humidity changes as soon as the temperature changes. In fact, its value can swing wildly as temperatures change, even though the actual moisture in the air hasn't changed at all.
The reason for this is that air's capacity to store moisture changes as its temperature changes. For example, colder air can't store as much water (invisible water vapor) as warm air. So, it makes sense that the percent of saturation (relative humidity) would go up as air cools and gets closer to being unable to hold onto its stored moisture. In this example, the amount of moisture hasn't changed, but the level of saturation has.
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 within meteorology is the dew point.
---David Murphy
