Take you to know our home-Earth 27. It is raining.

Rain, sometimes gentle and romantic, "wet apricot rain with clothes, blowing cold willows"; Sometimes impatient and tyrannical, "the black wind blows the sea, and the rain passes through Jiang Lai in eastern Zhejiang". Everything happens for a reason. Rain is so changeable because it has many different reasons.

According to the causes, climatologists divide rain into four basic types: convective rain, frontal rain, typhoon rain and topographic precipitation.

Convective rain is the simplest type of rainfall. Under the irradiation of the sun, the water near the ground evaporates and expands and rises. The higher the altitude, the lower the temperature gradually (the average temperature drops by 0.6℃ per 100 m), and the water vapor adheres to the surface of tiny solid dust and condenses into small water droplets. The water vapor on the surface keeps gathering, which makes the water droplets grow up gradually until the buoyancy of the air can no longer support them, and then falls back to the ground in the form of rain. Frequent rainfall in equatorial tropical rain forest area and thunderstorm in summer afternoon in China belong to convective rain.

Frontal rain is a common rainfall type in the monsoon region of eastern China. The front is the interface between cold air mass and warm air mass. When the cold air mass meets the warm air mass, the density of the cold air mass is relatively high, generally under the front and close to the ground; The warm air mass has a low density and will climb slowly along the cold air mass or be forced to rise. In the process of rising, the air flow will gradually cool down and the water vapor will condense, leading to rainy weather. Therefore, the front gradually inclines to the side of the cold air mass from the ground to the sky, and rainfall mainly occurs near the front and in the area controlled by the cold air mass.

If the cold air mass is strong, take the initiative to move to the area controlled by the warm air mass to drive the warm air mass away. This front is called a cold front. Rainstorm in summer, cold wave in winter and sandstorm in spring are all weather phenomena brought by fast moving cold front. The sandstorm weather in spring shows that when the air is dry enough, even if there is a front crossing the border, it will not bring rain, just like holding a dry towel in your hand, no matter how you twist it, it will not drip.

If the warm air mass actively moves to the cold air mass and slowly controls the territory originally belonging to the cold air mass, it is called the warm front. The warm front crossing the border usually brings continuous rainy weather, and the warm air is always so soft and slow. After the warm front crosses the border, the weather will get warmer. "A spring rain and a warmth" describes the influence of a warm front.

If the cold and warm air masses are evenly matched or blocked by high terrain, the front will move slowly or swing back and forth in an area, forming a quasi-static front. Quasi-static front often brings rainy weather lasting longer, which can be imagined as a weak Lin Daiyu, crying all day. "It rains at home in Huangmei season, and frogs are everywhere in the grass pond". Meiyu in the Jianghuai area in June was caused by a slowly moving quasi-static front.

Typhoons are often accompanied by violent storms, which disturb the southeast coastal areas of low latitude land in summer or autumn. From the point of view of weather system, typhoon belongs to a tropical cyclone with strong development and is a low pressure center. Typhoon refers to a strong cyclone that occurs in the Pacific Ocean and mainly affects the east coast of Asia. The super cyclone in the Atlantic Ocean is called hurricane, which mainly affects the east coast of North America.

The cause of cyclone is that a region receives more solar radiation, and the air expands and rises when heated, forming a low-pressure center near the ground. Because the airflow always flows from high pressure to low pressure, the airflow around the low pressure center converges to the center, but the airflow does not completely point to the low pressure center, but is deflected by the deflection force of the earth's rotation, which makes the fluid in the northern hemisphere deflect to the right and the southern hemisphere deflect to the left, and finally forms a vortex that rotates counterclockwise (northern hemisphere) or clockwise (southern hemisphere) around the low pressure center, just like the water in the pool flows into the sewer round by round. Upward airflow prevails in the cyclone center, so the area it controls is rainy and continuous. The anticyclone corresponding to the cyclone is the high-pressure center in the region, which is formed because a certain region is heated less, the temperature is low, and downward flow prevails. The airflow rotates in circles from the high pressure center (clockwise in the northern hemisphere and counterclockwise in the southern hemisphere), and the direction of airflow is opposite to that of cyclone, so it is called anticyclone. Controlled by anticyclone, the weather is fine.

When a cyclone develops strongly, a typhoon is formed. A strong updraft prevails in the center of the typhoon, and the huge pressure difference between the center and the periphery makes it full of wind and rain, destroying trees and knocking down houses. The center of the typhoon, also known as the eye of the typhoon, is a sunny area without wind and rain. There is a movie "Super Typhoon" in disaster movie, which tells the story of rescuing trapped pregnant women during the short clear and windless period when the typhoon eye crosses the border.

There are two very important factors in the formation of typhoon. First, there must be a steady stream of warm updraft to transport water vapor to the sky, which is also a necessary condition for clouds to cause rain. Therefore, typhoons are produced in the vast tropical ocean with high temperature and sufficient water vapor; Second, the earth's rotation must have a certain deflection force, so that when the airflow converges from all directions to the low pressure center, it will deflect and form a vortex. Although the ocean near the equator is warm and the water vapor rises vigorously, cyclones and typhoons cannot be formed, because the geostrophic deflection force is zero in the equatorial region and gradually increases towards the north and south poles, and typhoons can only be formed on the tropical ocean surface five latitudes away from the equator. With a cyclone, it may not necessarily develop into a typhoon. The specific conditions for the formation of typhoons are harsh and complicated, and people still can't fully understand them.

The formation of topographic precipitation is due to the fact that the airflow containing more water vapor is blocked by tall mountains during the movement, and the humid airflow is forced to climb along the hillside. In the process of rising, water vapor condenses and forms precipitation on the windward slope of the mountain. The world rain pole, located in Kilapangqi, northeastern India, has an annual rainfall of over 65,438+00,000mm (the average annual rainfall in China is only about 600mm). It is precisely because the warm and humid airflow from the Indian Ocean is blocked by the Himalayas that a large amount of rainfall is formed on the windward slope of the mountains.

When the airflow crosses the mountain and reaches the leeward slope, the water vapor is basically consumed, and the airflow on the leeward slope mainly descends along the slope, which aggravates the dry heat degree of the airflow. People call this dry and hot airflow "foehn". Strong foehn may cause forest fires, crops will be reduced due to premature maturity due to lack of water, and even avalanches will occur on leeward slopes in mountainous areas, or a large amount of snow will melt, leading to floods. Of course, the weak burning wind can properly accelerate the melting of snow, provide water for pasture and crops, and have a positive impact on human beings.

Convective rain, frontal rain, typhoon rain and topographic precipitation bring a lot of rain to the earth and nourish the growth of all things. Next time it rains, don't just think about the romance of rain, guess its origin!