Lillian Pawettiere, age 10, of Flushing, , N.Y., for her question:
Why are the deserts so cool at night?
The desert lands of North America slide down the slopes of the Rockies to meet the Great Plains and, towards the southwest, spread their dusty carpets wide over the land. The desert roads take us straight across the lonely prairies where we glimpse maybe a ranch, a mine, a busy cowboy and his cattle. In the distance, purple‑grey hills and rosy‑brown mesas reach up their fingers to touch the vivid blue of the sky. Here the stars at night shine their brightest and the cloudless days sparkle with sunshine. The secret of the beautiful deserts is in the clean, clear sky overhead.
Thirsty winds and breezes blow this way and that way all around the world. They soak up moisture from the face of the sea, sip up the dew and dry out the soil and the damp forests. The moisture they steal turns to water vapor, an invisible gas which mingles with the air. When the air cools, some of its vapor is turned back again into water. It becomes a puff of misty droplets .» and a cloud i s born,
Sooner or later, the clouds bring rain to replace the moisture which was stolen by the thirsty winds. But some places, of course, get more rain than others, In some places the rain does not bring enough water to replace the moisture stolen by the drying winds. These parched regions are the deserts, But sooner or later a little rain falls everywhere and showers fall even on the driest desert.
Rain comes from clouds and clouds form from vapor in the air. And the sparkling air above a desert is almost always very short of water vapor. This is why clouds are rare and the scenery glitters with dazzling sunshine. The stars blink their brightest because there are no misty clouds to dim them.
But clouds also act like blankets. During the day, the sun warms the earth and the seas and after sunset, the clouds keep the warmth of the day from escaping.
A desert, then, is short of blankets to pull overhead at night, p summer day may be parched and scorching but the heat escapes from the ground soon after the sun stops shining: The night cools fast, even after the hottest day. The dry air above the desert lets the sunbeams come down to warm the ground during the day. But it has no cozy clouds to blanket the ground at night, no way to save some of the daytime heat after the sun has set.
When rain comes to the desert, it usually comes with a deluge. Ragged grey clouds swarm over the sky and a downpour may last for several days and nights, If this happens during the warm season, the desert will get a few hot, humid nights. For the rain clouds will stop the daytime heat from escaping, just as they do everywhere else.