The relative humidity (RH) of surface air (2 m height) tends to decrease from morning into early afternoon, especially when the sun is shining and the weather is quiet and stable. As evening approaches, RH usually rises again. Explain the physical basis for this behavior (not discussed in text).

The Relative Humidity tend to decrease from morning to afternoon and as evening approaches increases and this as a result of the relationship between RH and increase in temperature

Explanation:

The relative humidity is the amount of water vapor the air is holding right now as a percentage of what it would be holding if it were saturated. If you increase the temperature, however, the amount of water vapor the air can hold increases, so the relative humidity decreases and vice versa for decrease in Temperature. This explains the reason for high RH in the morning decreasing into the afternoon and rising again in the evening.