Water can only enter the air by evaporation. Evaporation means that a substance changes from liquid to gas.
Water evaporates from rivers, lakes and seas, and also from the ground. Water evaporates from hanging wet clothes and also from trees and flowers. Believe it or not, in the hot summer sun, 20 corn trees can evaporate about 1 ton of water a day.
Water also evaporates from our bodies, which is the sweat oozing from our skin; Some water evaporates from our lungs and is excreted with the exhaled air.
As long as there is water, or something containing water, almost all the water is evaporating. So we are always surrounded by water vapor: sometimes there is more water vapor, sometimes there is less water vapor.
If you keep adding a handful of sugar to a glass of clear water for a period of time, the sugar will no longer dissolve, because the dissolved sugar in the water is enough. Water evaporates into the air just as sugar dissolves in water. When there is enough steam in the air, the evaporation of water stops. It was difficult to hang clothes at that time; We are also wet, because the sweat has stopped evaporating. We were very uncomfortable breathing and said, "This weather is really stuffy! 」
We also know that hot water can dissolve more sugar. Similarly, hot air can contain more water vapor. If the hot air containing more water vapor cools down, some water vapor will be squeezed out.
When drinking frozen drinks, I wonder if you have noticed that water droplets will form outside the cup. Someone said, "The glass is sweating." Of course, you understand that these water droplets are not seeping through the glass, but the part of water squeezed out by hot air contacting cold glass. The dew in the morning is also produced in this way. During the day and night, early morning is often the coldest time, so some water vapor in the air will attach to trees and flowers and condense into dew.
In the morning, sometimes it is foggy. It is water vapor that condenses into tiny water droplets attached to countless floating dust. Those water droplets are very small. It takes more than 1 000 to connect them together, so their length is 1cm.
Like fog, most clouds are made up of tiny water droplets. The air near the ground is always hot and contains more water vapor, so it is also "light". Scientifically speaking, the density is relatively small. "Light" gas will rise. But the farther away from the ground, the lower the temperature. When the rising air gets cold, some water vapor it contains condenses into tiny water droplets. This fog in the air is a cloud. But some clouds are not made of water droplets, but of tiny ice flowers. Water vapor condenses in the air, which can become water droplets or ice flowers, depending on whether the temperature at which it condenses is above or below freezing point.
When we look at clouds, they look all kinds: they are big, white and furry; There are also gray ones, all over the sky; Some are like a wisp of smoke, and some are like a mountain. Meteorologists divide clouds into many kinds, each with its own name. We haven't learned it, and it's not easy to distinguish between multiple clouds.
There are three kinds of most common clouds.
The first kind of common clouds, like ponytails and feathers, are called cirrus clouds. Such clouds often appear in the sky after two or three days of clearing up. Cirrus clouds are very high, about 10 km above the ground, and consist of tiny ice flowers. There is a long journey at high altitude, so cirrus clouds fly very fast, almost as fast as the fastest plane. Because of its height, we don't think so from the ground.
The second common cloud, like a mountain, is called cumulus. This is a common cloud in summer afternoon. There are such clouds in the sky, which is often good weather. However, if cumulus clouds pile up and become dark and big, it will become "Lei Yun"-a thunderstorm is coming. Cumulus clouds are much lower than cirrus clouds, and the foot of clouds is generally only 2 kilometers from the ground. It may be seven or eight kilometers thick when it becomes Lei Yun.
The third common type of cloud is called stratocumulus. Stratus clouds often cover the whole sky and turn lead gray. It is sandwiched between two layers of air with different temperatures, so it is not very thick or high, and the peaks are often exposed to stratus clouds. On the day when the sky is covered with stratus clouds, the sky on the top of the mountain is still clear, and stratus clouds are like the undulating sea under your feet.
When the clouds first gather, those water droplets are as fine as fog, so they won't fall. Even if it falls, it has evaporated before it reaches the ground. If those tiny water drops don't grow up gradually, it won't rain. So there are many cloudy days in the sky, and rainy days are always less than cloudy days.
If air containing a lot of water vapor rises to the sky quickly, most of it will condense into larger water droplets and it will start to rain. Falling water droplets may merge with other water droplets on the way and become larger. Raindrops are at least millions of times larger than the water droplets that make up the cloud. Sometimes raindrops are blocked by strong updrafts until they become very big. The diameter may reach half a centimeter, and mud splashes on the ground.
There is more rain and less rain, usually in millimeters. Millimeter refers to how deep the rain falls on the ground, which does not flow, leak or evaporate. A pile of clouds with a thickness of 6 kilometers turned into rain and fell to the ground less than 10 mm. In fact, this amount is not small. It rained 10 mm on the ground of 100 square meters, which is 1 ton in total. Sometimes a rainstorm 1 hour can rain 250 mm Therefore, continuous rainstorm in mountainous areas often leads to flash floods, mudslides and damage to crops, villages, bridges, highways and railways.