Thundersnowstorm: a physicist from Altai State University Tatyana Andrukhova explained this unusual phenomenon

This winter keeps surprising residents of Altai Krai: sudden temperature changes, hurricanes, and now thundersnowstorms. In recent days, many Barnaul residents have noticed flashes of lightning and rumbles of thunder and are actively sharing their observations on social networks. How unusual is this phenomenon from a scientific point of view? Tatiana Andrukhova, an associate professor of the Department of General and Experimental Physics, Candidate of Physical and Mathematical Sciences at Altai State University, explains.

“The composition of the atmosphere, along with neutral particles (gas molecules, impurities), includes electrically charged particles carrying a positive or negative charge. Compared to the mass of neutral particles, their mass is small, but charged particles generate phenomena in the atmosphere that attract human attention. These are lightning discharges – lightning and accompanying thunder, auroras, luminous discharges from sharp objects, etc.,” notes Tatyana Andrukhova.

This winter keeps surprising residents of Altai Krai: sudden temperature changes, hurricanes, and now thundersnowstorms. In recent days, many Barnaul residents have noticed flashes of lightning and rumbles of thunder and are actively sharing their observations on social networks. How unusual is this phenomenon from a scientific point of view? Tatiana Andrukhova, an associate professor of the Department of General and Experimental Physics, Candidate of Physical and Mathematical Sciences at Altai State University, explains.

“The composition of the atmosphere, along with neutral particles (gas molecules, impurities), includes electrically charged particles carrying a positive or negative charge. Compared to the mass of neutral particles, their mass is small, but charged particles generate phenomena in the atmosphere that attract human attention. These are lightning discharges – lightning and accompanying thunder, auroras, luminous discharges from sharp objects, etc.,” notes Tatyana Andrukhova.

As a rule, thunderstorms occur during the warm season. Their intensity largely depends on the location of the Sun – in mid-latitudes the phenomenon is more often observed in the summer afternoon. The main warning signs of a thunderstorm are cumulonimbus clouds, which usually appear on calm days.

But it turns out that thundersnowstorms occur. A thundersnowstorm is a meteorological phenomenon in which instead of heavy rain, heavy snow, ice rain, or ice pellets fall. The term is used mainly in popular science and foreign literature. There is no such term in professional Russian meteorology: in such cases, a thunderstorm and heavy snow are noted simultaneously.

But why do we not see or hear thundersnowstorms? It's all about the snow – it dampens sound waves, and audibility becomes very poor. But in summer, thunder is clearly audible, even if lightning occurs at a great distance from us. According to scientists, the mechanism of thundersnowstorm is the same as that of a summer thunderstorm.

“Usually, such thunderstorms occur on cold atmospheric fronts of cyclones, where significant contrasts in air temperature are observed in the surface layer and at an altitude of several kilometers. For example, the earth has weak positive temperatures, and at an altitude of 2–5 km the temperature of the air mass can be several tens of degrees lower than at the earth’s surface. Moist and relatively warm air near the Earth's surface begins to rise, where it collides with cold air, causing moisture to condense. In doing so, they form clouds filled with supercooled liquid water (small droplets that remain liquid even at very low temperatures), tiny ice crystals and small “soft” hail called “graul”. This mixture in the cloud can create an electrical charge and lead to lightning,” says the physicist.

Mechanisms of charge generation in clouds:

Electrification of droplets due to ion capture:

1. on the surface of a drop (or crystal) a double electric layer of molecular dipoles is formed, which capture negative ions from the air;

2. large drops and crystals are polarized – the lower part is charged positively, and the upper part is charged negatively. As they fall, these charges interact with the charges of the ions: negative ions are attracted to the bottom of the drop and captured by it, positive ions are repelled. Therefore, the falling drop becomes negatively charged, and positive ions are transported by the upward flow to the upper part of the cloud.

Electrification during phase transitions is the most important mechanism for the formation of electrical charges associated with the freezing process of supercooled water droplets. In each phase (liquid and solid), water molecules dissociate into two ions – positive and negative. And since the concentration of ions in the liquid phase is higher than in the solid phase, a flow of ions occurs through the crystallization front from the liquid phase to the solid phase. Positively charged hydrogen ions are more mobile and penetrate this front faster, creating an excess positive charge in the ice.

“Under the influence of vertical movements, a separation of charge within a cloud occur: positively charged particles accumulate at the top of the cloud while negatively charged particles concentrate themselves at the bottom of the cloud. These charges form an electric field. High values of the intensity of this field, necessary for the occurrence of lightning, are created under the influence of turbulent movements. The inhomogeneities of the electric field generated by macro-turbulence increase the strength of this field to critical values – then discharges begin inside the cloud, between neighboring clouds and discharges to the ground, i.e. lightning formation process. Lightning is one of the manifestations of a thunderstorm. When the voltage in the electric field of a thundercloud reaches a critical value, the process of impact ionization occurs, during which electric charges acquire high speed and move towards the ground,” explains Tatyana Andrukhova.