The melting of snow before it reaches the ground can occur due to the presence of a layer of warm air between the clouds and the ground. This layer of air can melt snowflakes before they reach the ground, sometimes resulting in rain instead of snow.
When snow passes through certain layers of very dry air, it can go directly from a solid state to a gaseous one without even bothering to melt: this is called sublimation. Basically, ice crystals disappear directly into water vapor due to low ambient humidity and a particular pressure in the air. This phenomenon is common in regions where the air is dry, such as in the mountains or in certain cold winter climates. As a result, not a drop of water on the ground while it was snowing ten meters above your head!
When temperatures are warm enough near the ground, snowflakes can melt or evaporate long before reaching solid ground. As they pass through layers of warm air, the snow encounters conditions that quickly transform it. The warmer the air, the more easily these icy crystals lose their solid structure, turning into liquid droplets or even invisible vapor if the heat is particularly high. Without needing to reach extreme temperatures, a sufficiently warm mass of air is enough to ensure that snow does not arrive intact on the ground. This phenomenon explains why you can sometimes see snow falling in the distance but nothing at your feet.
When the ambient humidity is low, the air is dry and can easily absorb additional moisture, particularly that contained in falling snow. As a result, the flakes evaporate along the way before even reaching the ground: this is called virga. At the same time, a lower atmospheric pressure also facilitates evaporation: it lowers the temperature at which water transitions directly from solid to vapor, making sublimation even easier. In short, dry air combined with low pressure means that sometimes, your beautiful snowstorm never really makes it to the ground!
Sometimes, the falling snow passes through a layer of warm air located at high altitude. When a thin layer of warm air is just below the clouds, the snowflakes encounter positive temperatures. They can then melt quickly, even before reaching the ground. This phenomenon creates a liquid precipitation called freezing rain. But if the warm air layer is thick, the snow completely melts into rain, which can then freeze again if it passes through colder air below, causing ice on the ground. This complicated interplay between warm and cold zones is called the thermal profile of the atmosphere: it determines whether you will need to go out in boots or ice skates.
Among the notable phenomena, one often finds virga, a type of precipitation that originates from a cloud but completely disappears before reaching the ground. This is quite common in dry regions such as the southwestern United States or certain desert areas. There, the air is so hot and dry that it absorbs all the moisture before the snowflake even reaches the ground. The same goes for snow events in high mountains: it is sometimes possible to clearly see precipitation falling from a cloud, but it quickly evaporates or sublimates upon contact with warm, dry air layers, leaving just a sort of blurry veil beneath the cloud. This kind of phenomenon is regularly visible in the summer under certain distant thunderstorms.
In some regions, the sublimation of snow into the air can significantly reduce the expected precipitation, thus impacting water resources and local weather forecasting.
The sensation of cold felt when a virga passes overhead is caused by the surrounding air cooling down as it absorbs the energy needed for the evaporation or melting of snow at high altitudes.
Snow can partially melt and then refreeze before reaching the ground, forming sleet or ice pellets that can create slippery and hazardous weather conditions.
Evaporation or sublimation before precipitation reaches the ground plays a crucial role in certain desert climates, sometimes even contributing to soil dryness despite the visible presence of clouds.
Virga refers to precipitation (rain or snow) that is observed falling from a cloud but evaporates or sublimates before reaching the ground, appearing as fine lines suspended beneath the clouds.
When layers of cold air remain at high altitudes, snow crystals can fall without melting immediately despite slightly positive temperatures at the surface. However, in this case, the snow is often wet and quickly melts on the ground.
When the ambient air is very dry, snowflakes can sublimate (go directly from ice to water vapor) before even reaching the ground. In contrast, humid air slows down this sublimation process, making it easier for the flakes to reach the ground.
Melting snow is made up of partially melted ice crystals when they reach the ground. Freezing rain, on the other hand, starts as a liquid and freezes upon contact with a cold surface, resulting in dangerous ice deposits on roads or sidewalks.
This phenomenon commonly occurs in regions with very dry climates and under specific conditions, such as in desert or semi-arid areas, as well as at high altitudes or in places where the humidity level is particularly low.
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