Cold fronts occur in Mexico. At the same time that the twenty-sixth front of the season moved away from the southeast of the country, number 27 entered the northwest. The combination of cold air masses on the surface with warmer air at medium altitudes brought the threat of a rare phenomenon: “freezing” rain.
The so-called frozen, icy or freezing rain is an atmospheric phenomenon that is caused when precipitation alternates between zones of cold and warm air before reaching a cold surface. Going by its various names, “freezing rain” should not be confused with sleet, although both types of precipitation do have certain similarities.
The freezing rain forms in the atmosphere as snow, and that is how it begins its journey, in an icy state. However, as it falls, it passes through warmer layers of the atmosphere, which causes it to change state, that is, snowflakes turn into water droplets. This is where it differs from sleet, a phenomenon that occurs when this layer is not so warm as to completely melt the flake.
This form of precipitation reaches the surface as rain, with the peculiarity that the temperature at this point drops again below freezing point. This state is called the subfusion state: the water is below 0º C but it is still liquid when landing.
When this rain comes into contact with any surface, the drop of water freezes almost instantly. This implies that wherever this rain falls, instead of creating a layer of snow or cumulus hail, what is created is a layer of solid and transparent ice.
This implies that the surface becomes slippery as soon as the rains arrive, with the consequent risk of falling. As if this were not enough, the mass of frozen water that accumulates in these circumstances is greater than in other types of precipitation, which can imply structural risks in buildings and other elements (such as power lines) where it accumulates.
The idea of liquid water in conditions below its freezing point can be counterintuitive. But the truth is that the phase change of the liquid is not so simple. For the water molecules to amalgamate and the liquid to solidify, a kind of “spark” is needed to trigger the reaction.
In tap water or in a river, small particles of earth or minerals are the ones that trigger the reaction. But in pure water the story is different and the reaction is not so simple. We can experience this ourselves with distilled water. If we manage to keep it at a temperature just below the freezing point, we can solidify it just by putting it in a glass or giving a sudden blow to the container where we have it.
Not so frequent rains.
Fortunately, the risk of suffering this type of rain during the present storm was very limited to areas in the north of the country. During the next few days it will be in the south and southeast where the greatest rainfall will occur, although they will not be under the influence of the cold air mass that remains over the center and east of the country.
The cold front 27 associated with this air mass will continue for several days around the Yucatan peninsula, and in addition to rain, it could leave behind hail and electric shocks. History could repeat itself on Saturday, with a new cold front approaching the north of the state of Chihuahua.
Image | SMN, Conagua