Hail is a type of solid precipitation, that is, ice , very common in midsummer , when it is hotter. How is it possible if ice and heat are a priori incompatible?
It seems a strange phenomenon, that while we are bathing in the pool in August, in a matter of minutes icy balls can fall from the sky, but in reality there is nothing supernatural about it.
Knowing a little physics helps to understand it. For this, Carlos Yagüe, Professor of Meteorology and Director of the Department of Geophysics and Meteorology at the Complutense University of Madrid , explains to Muy Interesante how this curious event takes place.
Hail is formed thanks to a process known in meteorology as accretion . It consists of droplets of water freezing around an ice crystal . This occurs in the upper areas of the atmosphere, where temperatures are colder, and temperatures of -40ºC can be reached inside the clouds. These clouds where hail forms are called cumulonimbus (storm clouds), with instability and vertical currents, key to their formation.
Professor Yagüe clarifies that the accretion process gives off heat, so there comes a time when the temperature of the crystal surface exceeds 0ºC, and therefore it cannot freeze all the water droplets it captures, and this resulting particle is what is known as hail . When the instability is greater and strong vertical currents are produced, these particles will rise to the top of the clouds, where the process will be repeated (since the higher it rises, the lower the temperatures will be) and will re-freeze the water droplets of its around.
At this time is when the phases of dry growth or wet growth take place, common terms among meteorologists. As Professor Yagüe points out, the dry growth phase occurs when there is no liquid water on the surface of the crystal and more ice forms around it; on the contrary, when there are particles of liquid water, it is known as wet growth. When the cloud can no longer bear the weight of the hail, it precipitates.
The fact that it hails more in summer than in winter is due to the fact that “the water droplets begin to freeze at temperatures below 0ºC but up to -40ºC it is possible to find some in a liquid state. For this reason, if the temperatures are excessively cold, we will not find liquid water droplets, which are essential to produce hail ”- clarifies the professor.
In the summer season, the temperature contrast between the ground (hot) and the atmosphere (cold) is greater than in winter, and causes vertical air currents to form. This instability offers the ideal conditions for hail, that the more chaotic the environment within the clouds, the more likely it is to hail and to be of a larger size. This abrupt change is what makes the ice crystals reach the ground without melting. In winter the thermal contrast is lower, so snow falls instead of hail. “Despite this” – he explains – “it does not mean that the generation of hail is not possible in winter , but it is less frequent.”
