Ans: During the day, the sun warms it up, and the warm air absorbs moisture from the grass, the earth, and any bodies of water readily. If the weather warms up, the humidity will drop. Even if the humidity remains constant, the amount of moisture in the air will increase. When things cool down at night, the cooling air can/will hit the saturation point. The humidity level is now likely to be close to 100%. If it reaches 100% and attempts to go higher, fog will form. This effect is also responsible for rain and snow.