How can it thunder and hail in February?
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 and 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.
How is snow formed?
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 on the way down and rarely falls to the ground.