About spring and summer, sometimes in the early morning, the weather is still very good, at noon or afternoon suddenly hail. Although the air temperature was very high at that time, there were falling rocks with spheres, cones, or all sorts of other shapes. In winter, though the air temperature is very low, there is no hail. What is the cause?
To clarify this issue, we must first know how hail is formed. The reason that hail and thunderstorms have the same form, they are from the falling clouds; just through the hail accumulating cloud, the convection is very intense, so this cloud accumulating water is also called hail clouds.
Hail clouds mostly appear in the warm season, very rarely in the cold season. Hail clouds are created by strong convection and are the product of unstable air. Under the heat of the sun in warm and humid seasons, unstable air is easily generated. There was a lot of steam in the air; the lower air layer was more susceptible to the hot sun shining, forming a hot air column, generating intense convection, and gradually developing evolve into ice accumulating clouds. Furthermore, the airflow in this cloud rises and falls very strong enough to support the cloud’s rocks, causing the rocks in the cloud to be lifted in the air. Constantly combining with tiny water droplets and the snowflakes above the road drifted, forming transparent or opaque rocks intermingled. When they reach a certain extent, the rising airflow that cannot support females will fall, turning into hail. That is the principle that hail only appears in the warm season.
In winter, the sun shines on the ground. Very little ground heat is absorbed, not enough to cause strong convection currents. Moreover, the air is dry, so even if there is convection, it is not easy to form a heavy cloud accumulating water. Even if they form, the convection air currents in clouds cannot be strong enough to keep them afloat in the sky. So there is no hail in the cold season.
Perhaps someone will ask that the environment for the formation of hail is that the air temperature must be very low. In the summer, it is very hot, so how can ice be formed? This is because in summer, even though the above-ground temperature can be above 30 ° C, the rising air temperature decreases with altitude. In the upper cloud, it is only about 20 ° C. The cloud was about 4 km from the ground, and the temperature was below 0 ° C. The top of a hail cloud can grow about 10 km from the ground, so the low-temperature area has enough air thickness to form a freezing point.