Lisa blair, age 10, of richardson, tex., for her question:
where does the dew come from?
If you go outdoors early in the morning, you may find that the ground and the grass arc drenched with pearly dew. This can happen even in the midst of a summery spell of hot, dry weather. In some places where rainfall is scanty, the morning dew seeps into the ground to provide moisture for the thirsting plants.
We say that the dew falls, which suggests that it tumbles down like raindrops onto the ground. But if you wait at night for the morning dew to fall, you will find that this is not true. In the morning, the grass and ground plants may be showery wet, but none of this water fell down from the skies. Nor does the dew rise up from the ground.
The moisture which forms the pearly dew comes right out of the air. It can be compared to the drops of liquid which form on the outside of a glass of cold water. This sweat did not ooze through the sides of the glass. It condensed right out of the warm air in the room.
Ordinary air contains a certain amount of vapor, which is the gaseous form of water. Sometimes therc is more and scanetimes there is less vapor in the air, but the quota of vapor must fo110w very strict rules. Warm air can hold more vapor than cool air. But when air containing its full quota of vapor becomes cooler, it must dump its surplus supply.
When this happens, the surplus vapor in the air may be changed into dew. The same thing happens when the warm, moist air in a room comes in contact with a cold glass of water. As the air is cooled, some of its vapor gas must be changed back into liquid water.
During the daytime, the earth is warmed by the radiant energy of the sun. The air gets its warmth by touching the earth below it. As it gets warmer, the air drinks up more and more water from the land. But after sunset, the heat escapes. The air touching the cool ground and the plants also loses heat. If it had its full quota of vapor, the cooled air now has a small surplus, or it may have twice the amount of vapor it can hold. This surplus of gaseous water is now changed back into liquid to drench the world with showery dew.
On a spring night, the temperature may drop from 61 to 50 degrees. If the warm air had its full quota of vapor, about one quarter of this gas must be changed back into liquid. On a summer night, the temperature may drop from 86 to 68 degrees. If the warm air was saturated with vapor, almost half of the invisible water gas must be dumped as liquid dew.