Why does hail only fall in summer? How to predict hail?

　　Why does hail only happen in summer?

　　The weather is hot in summer, and the sun scorches the earth, which easily produces a large amount of hot and humid air near the ground. The hot and humid air rises rapidly and the temperature drops sharply, sometimes as low as -30°C. The water vapor in the hot air condenses into water droplets when cooled, and quickly freezes to form small ice beads. Small ice droplets roll up and down in the clouds, constantly adhering to the surrounding water droplets and condensing into ice, becoming heavier and heavier, and finally falling from the sky. This is hail. It can be seen that hail can only be produced when hot and humid air currents rise strongly. It is estimated that the airflow rising speed must exceed 20 meters per second. Therefore, hail mostly occurs in summer. In winter, the temperature near the ground is very low, and it is impossible to generate strong and rapid updrafts, so hail cannot form. Hail is very harmful. It destroys crops, damages houses, and harms people and animals. Now, people have developed effective means of artificial rainfall before hail appears to prevent hail from forming, thereby eliminating the disasters caused by hail.

　　How to predict hail?

　　Feeling hot or cold

　　If the morning is cool in the hail season, the humidity is high, and the solar radiation is strong at noon, causing strong air convection, it is easy to develop into cumulonimbus clouds and form hail. Therefore, there are sayings: "It's cool in the morning, but it's freezing in the afternoon" and "There's heavy dew in the morning, but there's heavy hail later."

　　Discern the direction of the wind

　　Hail is often preceded by strong winds and the wind direction changes violently. Farmers' proverbs include "evil clouds grow when the wind blows, and hailstones fall with the wind", "wind twists the clouds, and hailstones fall". In addition, if the south wind blows continuously, the wind direction turns to the northwest or north, and when the wind strength increases, hail is often accompanied. Therefore, there is a saying that "if the east wind does not blow, it will not rain, and if the south wind does not blow, there will be no hail."

　　Observing the clouds

　　There are many proverbs from various places that describe the precursors of hail based on the color of the clouds. For example, "We are not afraid of black clouds, but we are afraid of black clouds mixed with red. We are most afraid of white worms growing under red and yellow clouds." Other sayings that the cloud shape is a precursor to hail include "dark clouds rolled into a ball in the afternoon, wind, rain and hail came", "the sky was yellow and muggy, dark clouds rolled in, and the Milky Way roared to prevent hail", etc., indicating that the air convection was extremely strong at that time, and the clouds developed rapidly, like thick smoke rushing straight upward, and the clouds rolled up and down, back and forth. This kind of cloud is very prone to hail.

　　listen to thunder

　　The thunder was dull. Continuously, the people called this kind of thunder "Ramo Thunder". Therefore, there is a saying that "nothing will happen if there is thunder, but eggs will be laid by dull thunder". This is because horizontal flashes in hail clouds have a higher frequency and wider range than vertical flashes. The thunder and echoes emitted by various parts of the lightning are mixed together and sound continuous.

　　Recognize lightning

　　Most of the lightning in hail clouds is lightning between clouds, that is, "horizontal flashes", which shows that the process of hail formation in the clouds is very advanced. Therefore, there is a saying that "vertical flashes can prevent hail disasters, and horizontal flashes can prevent hail disasters."

　　See things

　　There is a lot of experience in detecting hail by looking at physical phenomena. For example, in Guizhou there are proverbs such as "Swan geese fly low, and hail comes quickly", "Willow leaves turn, and hail falls." Shanxi has proverbs such as "Cattle and sheep do not lie down on beams at noon, so beware of hail in the afternoon", and "White beads appear in the grass, and hail falls steadily." It should be noted that the above experiences should generally not be judged based on any one item, but should be analyzed and applied comprehensively.