Hail, commonly known as hailstone, which is most common in summer or at the end of spring and the beginning of summer. It is very little, like green beans, soybean and it can also be big, like chestnuts, eggs, grains of ice; the heavy hail is bigger than a grapefruit. In our country, many places will suffer from hail disaster with different degree except Guangdong, Hunan, Hubei, Fujian and Jiangxi provinces.
Especially in the northern mountainous and hilly region, the terrain is complex and the weather is changeable, so people suffered from great hail disaster and it also does great harm to agriculture. Heavy hail destroyed crops, damaged homes and people and livestock were often killed by crushing. Therefore, hail disaster is one of the serious disasters in our country. Hail and rain, snow all falls from the clouds, but cloud that forms hail is cumulonimbus cloud with strong development. And only cumulonimbus cloud that develops thrivingly may form hail.
Cumulonimbus clouds, like various clouds, are formed by the rising of air near the ground. Air rises from the ground and in the process of rising, air pressure reduces and volume expands. If there is no heat exchange with surrounding air, due to the expansion energy consumption, air temperature will drop. This temperature change is called the adiabatic cooling.
According to calculations, in the atmosphere every 100 meters of air rise, the temperature will drop about 1 degree because of adiabatic change. We know that under a certain temperature, there is a limit of holding water vapor. If this limit is reached, we call it "saturation". When the temperature lowers, the air will accommodate less water vapor. Therefore, the original air that is not saturated may reach sturation due to the adiabatic cooling in the process of cooling. After the air becoming saturated, excess air will be attached on the condensation nucleus that floats in the air, forming water drop.
When the temperature is below zero, excess water vapor will condense into tiny ice crystals. These droplets and ice crystals gather together and float in the air and become clouds.
There are various forms of air movement in the atmosphere, forming clouds of different forms. The clouds formed by convective motion are cumulus humilis, cumulus congestus and cumulonimbus clouds. People call them cumuliform clouds. They are pieces of isolated cloud mass that develops upward. In convection motion, there is upward movement and downward movement and cloud mass often forms in the updraft area, and gaps forms in the downdraft area. Sometimes the sky can be found. The cumulus clouds form a variety of clouds due to convection strength, whose cloud size varies greatly. If cloud convection movement is very weak, updraft can’t reach condensation level so it won't form clouds and there is only dry convection. If convection is strong, it can form cumulus ongestus, whose top is like broccoli, making up of many bubble clouds that have clear outline. The cloud can be up to 4 ~ 5 kilometers thick. If convection movement is very fierce, it can form cumulonimbus cloud. The bottom is very dark cloud but the top develops high, which can reach about 10 km. The cloud top edge blurs and the top of the cloud often spreads, forming the anvil shape. General cumulonimbus clouds can form thunder shower and only particularly strong cumulonimbus clouds, which is very tall and have strong updraft with abundant water vapor can form hail. This kind of cloud is usually called as the hail cloud. The hail cloud consists of water droplets, ice crystals and snowflakes. It is usually divided into three layers: the bottom layer of temperature is above 0℃, which consists of water droplets；The intermediate temperature is -20℃, which consists of cooling water droplets, ice crystals and snowflakes. The top layer of temperature is below -20℃， which is basically composed of ice crystals and snowflakes. The flow is very strong in the hail cloud, usually in the direction of the cloud, and there is a very powerful updraft coming in from the bottom of the cloud and out of the upper part of the cloud. There is also a sinking air flow coming in from behind the clouds and out of the clouds.
This is the precipitation area where the hail usually falls. These two groups are connected with the downdraft and updraft and environmental air flow, so the structure of the strong air cloud is relatively constant. The strong updrafts not only provide adequate water vapor for the hail cloud, but also support the hailstones particles to stay in the cloud, making it quite large when it lands.
And how is the hail grows the hail cloud? In the hail cloud, strong updraft carries many large and small water droplets and ice crystals moving and some of the water droplets and ice crystals merge and freeze into larger grains of ice. These particles and supercooled water drops are translated to water flow accumulation area by the updraft and then, it becomes the center of the hail. The core of the initial growth of the hail has a good growth conditions in the accumulation zone.
Hail embryo comes into the habitat under the carrying of updrafts, which encounters supercooled water droplets and grows into a transparent ice layer and then comes into the low temperature area with a small number of water in the area that water is abundant and temperature is not very low. The low temperature zone is mainly composed of ice crystals, snow and a small amount of supercooled water droplets and the hail embryo stick with them, then an opaque ice layer forms.
The hail has grown up at this moment, and the updraft there is weak. When it can’t rest the hail that grows up, the hail in updrafts will drop. In the process of falling, the hail constantly merges with ice crystals, snow and water droplets and continues to grow. When it fall to high temperature zone, the supercooled water droplets will form a transparent ice layer.
If it falls to another stronger updraft, then the hailstorm will rise again and repeat the process. So the hailstones grow opaque layers again and again; Due to the difference of time, water content and other conditions, the thickness and other characteristics of each layer are different. And finally, when the updraft can't hold up the hail, it falls out of the cloud and becomes the hail we see.
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