Guess how destructive an egg-sized hailstone is.

Since April, severe convective weather has been very frequent in many parts of the south, with thunderstorms, gale and hail occurring from time to time. For example, at around 15:30 on April 27, hail as big as eggs fell in Zengcheng District, Guangzhou.

Photo/ From the afternoon to the evening of April 27, hail fell in some places in Guangxi and Nanning. Some hailstones were larger than 5 cm in diameter, larger than eggs.

Hail is hard, spherical, cone-shaped or irregularly shaped solid precipitation that falls from thunderclouds.

When we hear the word "ice", we will think of the cold winter. So when does hail usually appear? In winter or in summer?

In fact, hail mostly occurs in late spring and early summer, and sometimes occurs in strong and long-lasting thunderstorms in midsummer.

Why don't these ice lumps falling from the sky appear like snowflakes when it's cold? This has to do with the principle of hail formation: hail is originally a small water drop in a cumulonimbus cloud. Cumulonimbus clouds are very tall, up to 10 kilometers in height. The higher the cloud top, the lower the temperature. The temperature at the top of a cumulonimbus cloud is generally below -20℃, and there is a lot of water vapor in the cloud, with strong convection up and down in the cloud.

The small water droplets in the cumulonimbus cloud rise rapidly with the air flow, and the temperature decreases as the altitude increases. When the temperature reaches below 0°C, the water droplets condense into ice crystals. Therefore, the upper layer of the cumulonimbus cloud is ice crystals, and the lower layer is water droplets.

As the air flow rises, some water droplets and ice crystals collide with the surrounding ice crystals and supercooled water droplets and grow larger, freezing on the ice crystals to form an opaque ice core, which forms the core of the hail.

Hail grows as it rises. The rising air current is weak at high altitudes. When it can no longer support the growing hail, the hail falls in the rising air current. If it falls into another stronger rising air current area, the hail will rise again and grow larger again. The air currents in cumulonimbus clouds rise and fall violently, so the hail rolls up and down in the air again and again. More and more ice crystals and supercooled water stick together, like a snowball, and it gets bigger and bigger. Once the hail is so heavy that the rising air current in the cloud can't support it at all, it falls from the sky and becomes the hail we often see.

Therefore, an important factor in the formation of hail is the strong updraft and humid air. The humid and hot air rises strongly, which makes it easy to form cumulonimbus clouds . This is why hail often occurs in late spring and early summer, or in midsummer when there are strong thunderstorms, and more than 70% of them occur in the afternoon or evening.

In winter, it is cold near the ground and there is not enough updraft. When small water droplets encounter cold air at high altitudes, they condense and turn into snowflakes directly, without rolling back and forth in the air. Of course, hail will not form. According to the climate data statistics in the figure below, we can also see that hail mostly occurs in southern cities such as Wuhan, Changsha, Chongqing, and Guiyang from February to May. In the north, hail generally occurs in May and June.

Sometimes hailstones are as big as soybeans or mung beans, but sometimes they can be as big as chestnuts or even eggs. This kind of hailstone is very powerful. When an egg-sized hailstone falls from the sky, its power is amazing:

These hailstones are like bombs carefully crafted by nature. They can easily penetrate car glass and pose serious threats to agriculture, construction, communications, electricity and transportation.

During seasons when severe convective weather occurs frequently, everyone must pay attention to forecast and warning information in advance, make preparations early, and minimize losses.