Cienciaes.com: Ice crystals in the atmosphere. We spoke with Luis González MacDowell.

2020-05-31 16:59:33

When we look at the wispy, frayed clouds that often dye our skies semi-transparent white, we are not aware that we are actually looking at a multitude of small ice crystals. These crystals are hexagonal prisms that, when sunlight falls on them, generate halo phenomena in the form of luminous circles that surround the Sun or Moon, false suns, solar columns, etc. We described all this in detail in a previous episode of Ulysses and Science. When the temperature drops enough and the humidity increases, these crystals can grow in a multitude of ways that sometimes fall like beautiful snowflakes.

Scientists have long studied the formation and evolution of ice crystals in the atmosphere, but some of the associated phenomena remain poorly understood. One of these phenomena was discovered in the 1930s by researcher Ukichiro Nakaya. He observed the formation of ice crystals in the atmosphere and realized that they undergo periods of uneven growth as the temperature drops. Water, due to the asymmetry of water molecules, forms hexagonal prisms when frozen. These prisms can be of two fundamental types: some have hexagonal faces very close to each other, they are “flat” crystals, and others form elongated columns with the bases very far apart and the lateral rectangles much longer.

The reason why the growth of crystals in the atmosphere favors one form or another has found an explanation in a study, published in Science Advances, by researchers P. Llombart, EG Noya, LG MacDowell. Luis González MacDowell, our guest today on Talking with Scientists, explains that, when the temperature is between 0ºC and -4ºC, “flat” ice crystals preferentially form, however, when the temperature drops below -4ºC to At -10ºC, columnar crystals form, curiously, below -10ºC to -20ºC, the situation changes again and flat crystals form again and below -20ºC, columnar crystals are again the majority.
This movement of shapes at different temperatures has been studied thanks to simulations with supercomputers, such as Mare Nostrum, where a large number of water molecules are put into play under different conditions of pressure, temperature and humidity.

The simulations reveal that, at low temperature, the ice surface is smooth and soft. Under these conditions, when a molecule of water vapor approaches the surface, it cannot find a place to attach easily and, as a consequence, the crystal grows larger. slowly. When the temperature increases, the surface becomes destabilized, irregularities and steps appear to which the vapor molecules can bind more easily, as a consequence, the crystal grows more quickly. The interesting thing about this case is that when the face of the crystal melts, the surface becomes smooth and the growth slows down again. The transitions occur at different temperatures depending on whether the surface belongs to a hexagonal base or to one of its lateral faces, so they have different growth rates. Thus, when the growth is greater on the lateral faces of the crystal, it widens, favoring the appearance of flat crystals. However, when the bases grow at a faster rate, the crystal stretches, favoring the formation of columns.

These studies help to understand the processes of formation and growth of ice crystals in the atmosphere, a process that could show important variations due to the increase in the average temperature of the planet due to climate change.

I invite you to listen to Luis González MacDowell, professor of the Department of Chemistry – Physics at the Faculty of Chemical Sciences of the Complutense University of Madrid (UCM).

Reference:

Pablo Llombart, Eva G. Noya, Luis G. MacDowell. Surface phase transitions and crystal habits of ice in the atmosphere Science Advances20 May 2020

Wonders of light and water. Halo phenomena. an episode of the podcast Ulysses and Science