How is the rain, snow, hail and dew formed?

The formation and disappearance of dew

In the early morning of warm season, the dew that people often see on the grass, leaves and crops on the roadside does not fall from the sky. The reason and process of dewing are the same as frost, but the temperature is above 0℃.

Above 0℃, the temperature at which air is saturated with water vapor due to cooling is called "dew point temperature". In warm seasons, when the ground objects are cooled by intense radiation at night, the temperature of the air in contact with the surface of the objects drops, and after it drops to the "dew point", excess water vapor is precipitated. Because the temperature is above 0℃ at this time, these excess water vapor condenses into water droplets and adheres to ground objects, which is dew.

Dew, like frost, mostly appears on a clear night without wind or breeze. At the same time, objects that are prone to condensation are often objects with large surface area, rough surface and poor thermal conductivity. Sometimes, dew forms in the first half of the night, and the temperature continues to decrease in the second half of the night, which makes the dew on the object freeze. It's called frozen dew. Some people classify it as frost, but its formation process is different from frost.

Dew usually forms at night. After sunrise, the temperature rises and the dew evaporates.

Dew often appears in the growing season of crops. This is beneficial to agricultural production. In summer in northern China, evaporation is very fast. When there is little rain and drought, the leaves of crops are sometimes dried by the sun during the day, but when there is dew at night, the leaves will return to their original state. People often say "rain and dew", which is the reason.

The formation and disappearance of frost

In the early morning of cold season, grass leaves and clods are often covered with a layer of frost crystals. They shine in the rising sun and melt when the sun rises. People usually call this phenomenon "frosting". Looking through the calendar, every year from 10, there is always the solar term "first frost". We have seen snow and rain, but no one has seen frost. In fact, frost does not fall from the sky, but is formed in the air near the ground.

Frost is a kind of white ice crystal, which mostly forms at night. In a few cases, it can also begin to form before sunset when the sun is tilted. Usually, the frost will melt soon after sunrise. But when the weather is cold or in a cool place, the frost will last all day.

Frost itself is neither harmful nor harmful to plants. What people usually call "freezing injury" is actually the "freezing injury" when frost forms.

The formation of frost is not only related to the weather conditions at that time, but also related to the nature of attached objects. When the surface temperature of the object is very low, but the air temperature near the surface of the object is relatively high, there is a temperature difference between the air and the surface of the object. If the temperature difference between the surface of an object and the air is mainly caused by the radiation cooling of the surface of the object, when the warmer air contacts the surface of the object, the air will be cooled, and when the water vapor is supersaturated, the excess water vapor will be precipitated. If the temperature is lower than 0℃, the excess water vapor condenses into ice crystals on the surface of the object, which is frost. Therefore, frost is always formed under weather conditions that are conducive to radiation cooling on the surface of objects.

In addition, clouds hinder the radiation cooling of ground objects at night, and clouds in the sky are not conducive to the formation of frost. Therefore, frost often appears on clear nights, that is, when the ground radiation cools down strongly.

In addition, the wind also has an influence on the formation of frost. When there is a breeze, air flows slowly over the surface of supercooled objects, constantly supplying water vapor, which is beneficial to the formation of frost. But when the wind is strong, because the air flows quickly, the time to contact the surface of cold objects is too short. At the same time, when the wind is strong, the air in the upper and lower layers is easy to mix with each other, which is not conducive to cooling down and will also hinder the formation of frost. Generally speaking, when the wind speed reaches level 3 or above, it is not easy to frost.

Therefore, frost usually forms in sunny, breezy or calm nights in cold seasons.

The formation of frost is not only related to the above weather conditions, but also related to the properties of ground objects. Frost is formed on the surface of an object cooled by radiation, so the easier it is to radiate heat and the faster it cools, the easier it is to frost on it. Similar objects, under the same conditions, have the same mass and heat. If they radiate and dissipate heat at the same time at night, objects with large surface area will dissipate more heat, and at the same time, the temperature will drop faster, and it will be easier to frost on them. That is to say, if the surface area of an object is relatively large relative to its mass, it is easy to form frost on it. Grass leaves are very light, but they have a large surface area and are easy to frost on. In addition, the rough surface of an object is more conducive to heat dissipation than the smooth surface, so the rough surface is more prone to frost, such as clods.

Frost disappears in two ways: one is to sublimate into steam, and the other is to melt into water. The most common thing is that it melts and disappears after sunrise due to the rising temperature. Water melted by frost is good for crops.

How is the hail formed?

Hail falls from the clouds like rain and snow. But the clouds that hail are very strong cumulonimbus clouds, and only very strong cumulonimbus clouds can hail.

Cumulonimbus clouds, like all kinds of clouds, are formed by the rising and condensation of air near the ground. When air rises from the ground, the air pressure decreases and the volume expands. If there is no heat exchange between the rising air and its surroundings, the air temperature will decrease, because expansion consumes energy. This temperature change is called adiabatic cooling. According to the calculation, every time the air in the atmosphere rises 100 m, the temperature will decrease by about 1 degree due to the adiabatic change. We know that at a given temperature, there is a limit to the amount of water vapor contained in the air, and reaching this limit is called "saturation". When the temperature is lowered, the amount of water vapor that may be contained in the air will decrease. Therefore, the originally unsaturated air may reach saturation due to adiabatic cooling in the upward movement. After the air reaches saturation, excess water vapor will attach to the floating condensation nuclei in the air and form water droplets. When the temperature is below zero degrees Celsius, excess water vapor will condense into fine ice crystals. These water droplets and ice crystals gather together and float in the air to become clouds.

There are different forms of air movement in the atmosphere, forming different forms of clouds. Clouds formed by convection include light cumulus, thick cumulus and cumulonimbus. People call it cumulus clouds. They are isolated upward developing clouds, because there are upward and downward movements in convection, which often form clouds in the updraft area, become gaps in the downdraft area, and sometimes you can see the blue sky.

Cumulative clouds are different because of different convective intensities, and the sizes of clouds are also very different. If the convective movement in the cloud is weak and the updraft can't reach the condensation height, there will be no cloud, only dry convection. If the convection is strong, it can develop into cumulus clouds. The top of cumulus clouds, like cauliflower, consists of many well-defined convex clouds, and the thickness of clouds can reach 4-5 kilometers. If the convection is intense, a cumulonimbus cloud can be formed, the cloud bottom is dark, the cloud top develops very high, reaching about 10 km, the edge of the cloud top becomes blurred, and the cloud top often expands to form an anvil. Generally, cumulonimbus clouds may produce thunderstorms, but only when they are particularly strong, the clouds are very high, there is a strong rising gas in the clouds, and there is a lot of water in the clouds, will hail be produced. This kind of cloud is usually called hail cloud.

Hail clouds consist of water droplets, ice crystals and snowflakes. Generally, there are three layers: the bottom layer is above 0℃ and consists of water droplets; The middle layer has a temperature of 0℃ to -20℃ and consists of supercooled water droplets, ice crystals and snowflakes. The temperature of the top layer is lower than -20℃, which is basically composed of ice crystals and snowflakes.

In the hail cloud, the airflow is very strong. Usually in the direction of the cloud, a strong updraft enters from the bottom of the cloud and flows out from the top of the cloud. There is also a downward airflow flowing in from the middle behind the cloud and out from the bottom of the cloud. This is also the precipitation area where hail usually occurs. These two organized updrafts are connected with the airflow between the downdraft and the environment, so the airflow structure in the heavy hail cloud is relatively continuous. The strong updraft not only delivers enough water vapor to the hail cloud, but also supports the hail particles to stay in the cloud and grow to a considerable size before landing.

How does hail grow in hail clouds? In the hail cloud, the strong updraft carries many large and small water droplets and ice crystals, some of which fuse with ice crystals and freeze into larger ice particles. These particles and supercooled water droplets are transported by the updraft to the water content accumulation area, which can become the hail core. The initial growth core of these hailstones has good growth conditions in the water content accumulation area. After the hail A enters the growth area with the updraft, it collides with supercooled water droplets in the area with large water volume and low temperature, and grows into a transparent ice layer, and then enters the low temperature area with small water volume, where it is mainly composed of ice crystals, snowflakes and a small amount of supercooled water droplets, and the hail adheres to them and freezes to form an opaque ice layer. At this time, the hail has grown up and the updraft there is weak. When it can't support the growing hail, the hail will fall in the updraft and continue to grow through the fusion of ice crystals, snowflakes and water droplets in autumn. When it falls to a higher temperature area, the supercooled water droplets that hit it will form a transparent ice layer. At this time, if it falls into another stronger updraft area, the hail will rise again and repeat the above growth process. In this way, hail grows in transparent layer and opaque layer; Due to the differences in growth time and water content, the thickness and other characteristics of each layer are also different. Finally, when the updraft couldn't support the hail, it fell from the clouds and became the hail we saw.

The cause of fog

Fog and clouds are water vapor condensates composed of small water droplets or ice crystals floating in the air, but fog is generated in the near layer of the atmosphere, while clouds are generated in the higher layer of the atmosphere. Since fog is water vapor condensation, we should look for its reason from the conditions that cause water vapor condensation. There are two reasons for water vapor saturation in the atmosphere: first, evaporation increases water vapor in the atmosphere; The other is the cooling of the air itself. Cooling is more important for fog. When there are condensation nuclei in the air, if water vapor continues to increase or melt, condensation will occur in saturated air. Fog is formed when condensed water drops reduce the horizontal visibility to less than 1 km.

In addition, excessive wind speed and strong disturbance are not conducive to the formation of fog.

Therefore, in the area that is conducive to the cooling of the lower air layer, if the water vapor is sufficient, the wind is mild, the atmospheric stratification is stable, and there are a large number of condensation nuclei, it is most likely to produce fog. Generally speaking, there are more opportunities to form fog in industrial areas and urban centers, because there are abundant condensation nuclei there.

Rime, rime

In nature, ice crystals and water droplets formed on ground objects are not all frost and dew. Some phenomena look like frost and dew, but they are caused by other meteorological conditions.

For example, the original temperature in a certain area is low, and the temperature of various ground objects is also low. When the weather suddenly warms up (for example, the temperature rises sharply 10℃), some large and heavy objects cannot become as warm as the surrounding air at once, thus forming a relatively large temperature difference between the air and these objects. If the temperature is lower than 0℃, ice crystals will form on the object, which is called "hard". If the temperature is above 0℃, it will condense into water droplets on the surface of the object, which is called "water". The formation of "window frost" and "water" on glass windows in winter is similar to this.

Hard and water, frost and dew are all caused by the temperature difference between air and ground objects. The temperature difference between hard glaze and water glaze is caused by the warming weather, and the temperature difference between frost and dew is caused by the radiation cooling of ground objects. Therefore, they reflect different weather conditions and attach different objects. They are different weather phenomena.

In early winter or late winter, sometimes there is a strange phenomenon: when the liquid rain falling from the air falls on branches, wires or other objects, it will suddenly freeze into a layer of smooth and glittering ice, which is called "Song Yu". What is this phenomenon of dripping water into ice? In fact, the raindrops here are not ordinary raindrops, but supercooled raindrops. This kind of situation is not common, only when cold and warm air meet and warm air is strong. This is a lower temperature (slightly lower than zero degrees Celsius) layer near the ground, with an air layer or cloud layer above zero degrees Celsius, and then a cloud layer below zero degrees Celsius. Snowflakes falling from here melt into raindrops when passing through the warm layer, and then enter the cold air layer near the ground, and the raindrops quickly cool down. Because the diameter of these raindrops is very small, although the temperature drops below zero degrees Celsius, they fall before freezing and get cold when they touch the ground.

In addition, when there is supercooled fog, it is especially beneficial to the growth of ice crystals on ground objects. At this time, white ice flowers are formed on wires and branches, which is called "rime". When there is fog and the temperature is higher than 0℃, fog drops stick to leaves or other objects, which is called "fog condensation" and is most common in forests.

They are not frost and dew, because they are formed for different reasons.

Formation and types of snow

As we all know, clouds are composed of many small water droplets and small ice crystals, and raindrops and snowflakes are composed of these small water droplets and small ice crystals. So, how is snow formed?

In the water cloud, all water droplets are small water droplets. They grow into raindrops mainly through continuous condensation and collision.

Ice cloud is made up of tiny ice crystals. When these small ice crystals collide with each other, the surface of the ice crystals will heat up and melt, and they will stick together and freeze again. Repeat this for many times, and ice crystals will increase. In addition, there is water vapor in the cloud, so ice crystals can continue to grow through condensation. However, where the ice cloud is generally high but not thick, and there is not much water vapor, the condensation growth is slow, and there are not many opportunities to collide with each other, so it cannot grow to a great extent to form precipitation. Even if it causes precipitation, it often evaporates in the process of falling and rarely falls to the ground.

The most favorable cloud droplet growth is the mixed cloud. The mixed cloud consists of small ice crystals and supercooled water droplets. When a mass of air is saturated with ice crystals, it is unsaturated with water droplets. At this time, the water vapor in the cloud condenses on the surface of ice crystals, while the supercooled water droplets are evaporating, which produces the phenomenon that ice crystals "adsorb" water vapor from supercooled water droplets. In this case, ice crystals will grow rapidly. Besides, supercooled water is very unstable. If you touch it, it will freeze. Therefore, when supercooled water droplets collide with ice crystals in mixed clouds, they will freeze and adhere to the surface of ice crystals, making them grow rapidly. When small ice crystals grow up, they overcome the resistance and buoyancy of air and fall to the ground. This is snow.

In early spring and late autumn, the air near the ground is above 0℃, but this layer of air is not thick and the temperature is not very high, which will make the snow fall to the ground and melt completely. This is the so-called "wet snow", or "both rain and snow". This phenomenon is called "sleet" in meteorology