Why does it rain?

The rain in the sky comes from the clouds in the sky, but it is actually the condensation of water, which comes from the evaporation of water vapor on the ground. When the water droplets in the cloud reach a certain level, that is, they cannot be supported by the rising airflow, the water droplets (possibly ice droplets and snowflakes) will fall to the ground, that is, the rain (hail and snow) we see. The evaporation of water vapor will happen at any time, but it won't rain at any time. In the air, when the temperature is high, the amount of water vapor that can be accommodated is large, and when the temperature is low, the amount of water vapor that can be accommodated in the air is small. When air moves from high temperature to low temperature, water vapor will change from unsaturated to supersaturated and condense into water droplets. When the weight of water drops exceeds the buoyancy of air, it will rain. For example, in the summer afternoon, due to the strong evaporation, rise and condensation of water vapor, showers are formed; Cold air meets southeast hot and humid air to form frontal rain, and so on. As rivers, lakes and oceans are exposed to sunlight, water becomes steam. When the water vapor rises, it becomes small water droplets or ice crystals, and the airflow below constantly transports the water vapor into the clouds. Small water droplets and small ice crystals in the cloud collide with each other in motion, and the volume will increase. The water droplets in the lower layer of the cloud gradually get bigger. After the ice crystals in the upper layer of the cloud increase in volume, they will fall between the water droplets in the lower layer of the cloud, and the water droplets will become larger and larger. When these water droplets are too big for the updraft to drag, it rains. The two conditions that must be met when it snows or rains are 1. It comes from warm and humid air currents in the tropics. There is cold air from high mountain area. The snowfall area in winter (the same as the rainfall area in summer) is distributed near the front, that is, the intersection of cold and warm air masses. This position is not fixed, but is determined by the strength of the two air masses. The cold air mass is weak, the front and precipitation areas have high latitudes, the cold air mass is strong, and the front and precipitation areas have low latitudes. The land cold high that affects China in winter is the Asian high centered on Mongolia and Siberia (also known as the Mongolian Siberian high), which has a very large sphere of influence and has a great impact on the weather in China, especially in the northern region. The cold air affecting our country in the early winter of this year is relatively weak, and the frequency is not much. Even with warm and humid airflow, it is not easy to snow without cold air. However, since 2008, the high pressure in Asia has been very active, and cold air masses have formed southward, affecting China and causing large-scale gale and cooling weather. However, due to the active warm air mass in the south this year, a large number of warm and humid air currents from the Pacific Ocean and the Indian Ocean frequently patronize the southern region. When the powerful cold air mass from Mongolia and Siberia quickly moved south to meet the warm and humid air mass, the cold and warm air just merged into one. Affected by these two air currents, there have been more rainy and snowy weather recently, especially in the Yangtze River basin, and the low temperature weather has lasted for a long time. If there is only a strong cold air mass without a large amount of water vapor provided by warm and humid air mass, there will only be strong winds and cool weather in the south; If there is only a large amount of water vapor provided by warm and humid air masses, there will be no disastrous weather at all without the arrival of cold air masses. When both are ready, disaster comes. (1) water and cloud 1. What is the existing form of water? In nature, water constantly changes its state in circular motion: liquid water, solid ice and gaseous water vapor. Liquid water can be condensed into solid ice and ice particles, can also be evaporated into gaseous water vapor, can be condensed into liquid fog and rain dew or condensed into solid frost and snow hail, while solid frost and snow hail can be dissolved into liquid water or water droplets or sublimated into gaseous water vapor. Evaporation and condensation, sublimation and sublimation, melting and solidification, and the transformation process of water state are endless in nature. 2. How does water become steam? Liquid water and solid ice exposed to nature evaporate water into the atmosphere all the time, forming water vapor. The speed of water evaporation is related to temperature, the nature and shape of evaporation surface, the temperature in the air, wind speed, air pressure and other factors. Under normal circumstances, the average kinetic energy of water molecules on the evaporation surface increases at high temperature, and it is easy to jump out of the water and break into the atmosphere, and the evaporation speed is faster; When the temperature is low, the average kinetic energy of water molecules on the evaporation surface decreases, and it is not easy to jump out of the water and break into the atmosphere. Water molecules on the steaming earth and rivers, lakes and seas on the land surface are constantly evaporated, rising and mixed into the atmosphere by sunlight. After the water vapor rises into the sky with the warm updraft, the temperature becomes lower and lower, and the water vapor reaches a supersaturated state, so the water vapor forms small water droplets under the action of condensation nuclei. Countless small water droplets gather together to form a cloud. 4. What are the types of clouds? Clouds are classified according to their heights, and can usually be divided into four types: high clouds, medium clouds, low clouds and direct clouds. The height of high clouds is more than 6 kilometers, which are usually divided into cirrus, cirrostratus and cirrocumulus. The height of the cloud bottom is between 2500 meters and 6000 meters, which is generally divided into high-level clouds and cumulus clouds; The height of low cloud base is less than 2500m, which can be divided into stratocumulus, stratocumulus and nimbostratus. The height of the cloud bottom in the vertical exhibition is less than 2500 meters, which can be divided into cumulonimbus clouds and cumulonimbus clouds. 5. How did the halo come into being? When the light from the sun and the moon passes through the cirrostratus, the refraction or reflection of ice crystals forms a halo. When light enters the ice crystals in cirrostratus, it is refracted twice and dispersed into light of various colors in different directions. In fact, when there is cirrostratus, there are countless ice crystals floating in the sky, but the same circle of ice crystals around the sun can refract the same color light into our eyes, forming a halo of internal infrared purple. In the cirrostratus composed of ice crystals in the sky, there will be one or more colored halos of internal infrared violet centered on the sun and the moon, and sometimes there will be many colored or white light spots and arcs. These halos, spots and arcs are collectively called halos. 6. Where did the fog come from? Clouds and fog are composed of tiny water droplets or ice crystals suspended in the air. Clouds are fog in the air and fog on the ground. There is no essential difference between the two. The only difference is that the water vapor in the air is supersaturated near the ground, thus forming small water droplets that are visible to the naked eye but difficult to see. Shortly after sunrise, due to the sunlight, the fog began to evaporate, and was often lifted into a gray cloud, and then rose to the sky and became a cloud. 7. There are several types of fog. According to the specific situation of air supersaturation, fog can be divided into radiation fog, advection fog fog, frontal fog fog, uphill fog and steam fog. (2) Dew and frost 1. How is dew formed? In sunny mornings in summer, due to the low temperature, the surface heat radiation is faster, and the temperature near the surface layer drops faster. When the actual temperature is lower than the dew point temperature, the water vapor in the air will condense into small drops and become dew when it meets the surface of cold flowers or leaves. 2. How is the frost formed? In autumn and winter, affected by cold wave or cold air, the temperature drops obviously and the ground heat dissipates quickly. From the night when the blue sky is slightly windy to the early morning, when the temperature of objects or crops drops below zero degrees Celsius, the water vapor in the air directly condenses on the ground objects or crops like ice cream. 3. What is the difference between frost and frost? Frost is a kind of white crystal like ice crumbs that is condensed by water vapor in the air directly on the ground when the temperature of an object or crop drops below zero degrees Celsius. Frost refers to the temperature at which the surface temperature of crops drops rapidly to the point of danger, which is called "dark frost" or "black frost". 4. What harm does frost do to crops? Crops consist of many cells. When the temperature drops below zero degrees Celsius, the water between crop cells begins to freeze. According to physics, when an object freezes, its volume will expand. Therefore, when the ice particles between cells increase, the cells will be compressed and the water in the cells will be forced to seep out. When a cell loses too much water, its original gelatinous substance gradually solidifies. Especially after the severe cold and frost, the temperature rises suddenly, and the water permeated by crops will soon become steam and be lost. The water lost by cells cannot be recovered, and crops will die. (3) Rain and snow 1. Why does the cloud rain? In the cloud, with the continuous supplement of water vapor in the air, the supersaturated water vapor condenses on the cloud droplets, which makes the cloud droplets increase. When it increases to a certain extent, the cloud drops begin to fall due to gravity. In the process of falling, the big cloud drops fall fast and the small cloud drops fall slowly, so the big cloud drops will catch up with the small cloud drops and merge into larger cloud drops. In this way, the cloud drops will gather bigger and bigger like snowballs, and finally fall to the ground and become raindrops. 2. How did the thunderstorm come about? On a sunny day in summer, when there is warm and humid air flow somewhere, convective movement will occur. Warm and humid airflow rises from the ground and condenses into clouds due to supersaturation of adiabatic cooling. In the area controlled by downward flow, the air heats up adiabatically, and the relative humidity of the air is small, so it can't generate clouds, so it forms a light cumulus cloud with a convex top and a flat bottom like steamed bread. If the convection continues to develop, because the middle part of the updraft is stronger than the surrounding area, it will form a thick cumulonimbus cloud like Xiangshan or pagoda, or even a larger cumulonimbus cloud like towering clouds. In cumulonimbus clouds, ice crystals are broken and separated due to collision, friction and heating, so the hot end is negatively charged and accumulates at the top of the cloud, and the cold end is positively charged and moves to the bottom of the cloud. The positive charge area at the bottom of the cloud will induce the negative charge area on the ground. The stronger the charge separation in the cloud, the greater the potential difference between the cloud bottom and the ground. When it is large enough, the discharge phenomenon that breaks through the air will occur, causing lightning. 3. Why does the air discharge in the thunderstorm cloud cause lightning? At the moment of air discharge, the high voltage between the cloud bottom and the ground will reach more than kilovolts per centimeter, and the air temperature will suddenly reach 10,000 degrees Celsius or even 20,000 degrees Celsius. At such a high temperature, not only the air itself will expand rapidly, but also the cloud drops and rain will evaporate rapidly, so we will see lightning. At the same time, on the path of lightning, the air will rapidly cool and contract, and the expansion and contraction speed can reach 1000 meters. 4. Why does thunder always appear after lightning? In thunderstorm clouds, lightning and thunder occur at the same time. However, due to their great differences in propagation speed in the atmosphere, people always see lightning first and then hear thunder. Light travels 300 thousand kilometers per second, while sound can only travel 340 meters, so lightning reaches our eyes almost at the moment of air discharge, while thunder can only reach our ears after a period of time. The longer the lightning appears, the farther away the thunderstorm area is from us. 5. Why do hailstones sometimes appear in thunderstorm clouds? Hail is a small ice hockey or ice cube falling from a cumulonimbus cloud. Cumulonimbus clouds that can hail are called hail clouds. In hail clouds, like cumulonimbus clouds under thunderstorms, they are composed of water droplets, snowflakes and ice crystals at first. The updraft in the clouds is relatively strong. It sends the growing water droplets at the bottom of the cloud to the middle and upper part of the cloud to become supercooled water droplets. They either meet with ice crystals and snowflakes or naturally freeze to form hail embryos. When encountering cold water droplets, an opaque layer of ice will form on the surface of the embryo. When the updraft weakens and the hail embryo drops below the 0℃ line, part of its surface will melt into water again, and some water droplets will stick to it. When it is brought above the 0℃ line by the enhanced updraft again, the water on the embryo surface freezes again, forming a relatively transparent ice shell. Because the updraft in cumulonimbus clouds is sometimes strong and sometimes weak, the hail embryo churns in the air again and again, and is constantly wrapped by layers of opaque and transparent ice coats, until the updraft can no longer stand, it falls straight down and forms hail.