It can snow even when the temperature is above zero degrees Celsius because snow forms in clouds where the temperature can be below zero degrees, and when it falls to the ground, it can pass through a warmer layer of air before reaching the ground.
Atmospheric humidity is essentially the amount of water vapor present in the air around us, a completely invisible vapor. The warmer the air, the more humidity it can hold, like an invisible sponge. Conversely, when the air cools, it reaches a point where it can no longer hold all that water: this is called the dew point. From that moment on, water vapor becomes visible in the form of fog, dew, or even ice crystals. It is precisely this phenomenon that leads to the eventual formation of snow, even when the temperature near the ground seems mild.
Clouds form when moist air rises into the atmosphere and cools at high altitudes. The higher the air rises, the lower the temperature drops, even if it remains mild at ground level. Up there, when the temperature falls below zero degrees Celsius, the water contained in the clouds can become ice crystals, even though everything appears calm and mild below. These tiny crystals gradually capture more moisture, grow larger, and form snowflakes. These snowflakes then fall to the ground. And they can continue to fall as snow, without completely melting, even if the temperature increases as they pass through warmer layers closer to the ground.
In the clouds, tiny water droplets and ice crystals float quietly. When the crystals catch onto other crystals or droplets, they gradually grow larger. Once heavy enough, they fall under the effect of gravity. During their descent, these crystals can grow even larger by capturing additional droplets. Depending on the temperature encountered during the journey to the ground, these crystals become either rain or snow. Even if it is slightly above freezing near the ground, the flakes formed higher up can reach the earth in the form of snow, if they haven't had time to completely melt before their arrival.
The temperature varies greatly from one layer of air to another: even if it is mild at ground level, it can freeze just a few hundred meters higher. Snow forms at high altitudes, where it is generally very cold, often well below zero. If this snow falls quickly to the ground, it doesn't always have time to melt completely before reaching the bottom. As a result, you can easily observe snowflakes reaching the ground, even though your thermometer clearly indicates a positive temperature. This temperature difference in the atmospheric layers explains why it can still snow with several degrees above zero at human height.
Sometimes, when the flakes fall in milder air (above freezing), they start to partially evaporate. And evaporation is something that sucks a lot of heat from the surroundings. As a result, the air just around the flakes cools down very quickly, like a mini air conditioning in action. Consequently, even if it's a little above freezing down below, the flakes remain quite cold and do not immediately melt while passing through this slightly warmer air. It's like when you get out of the water and your skin is wet: evaporation cools you down and you feel a bit chilly, even if the weather is nice.
There is a weather phenomenon called 'wet snow' or 'melting snow' that occurs when snow crystals begin to slightly melt while passing through a warm layer but remain solid upon reaching the ground.
The temperature displayed on your thermometers generally measures the temperature at ground level. However, snow forms at higher altitudes, in areas that are often colder than the temperature you observe at the surface.
Snow can fall at ground temperatures of up to around +5°C, especially if the air higher up in the atmosphere is cold and dry enough to preserve the snowflakes during their descent.
Evaporative cooling can even cause a sudden drop in ambient temperature when it snows, creating the surprising impression of colder weather after the onset of snowfall.
The texture of snow is greatly influenced by the temperature and humidity of the air. Snow falling at temperatures close to zero degrees Celsius is often wet, sticky, and compact because the flakes easily stick together due to the high humidity. In contrast, when temperatures are significantly below zero, the flakes remain dry, light, and powdery, as they do not easily bond with each other.
Snow is made up of ice crystals that form directly in the clouds, while sleet occurs when snowflakes partially melt as they fall toward the ground and then refreeze upon contact with a colder layer of air. This difference in processes leads to a clear distinction in texture, with snow being lighter and fluffier, whereas sleet appears as harder, more compact grains.
This is mainly due to the phenomenon known as melting/evaporation cooling. When snow falls into air slightly above freezing, some of the flakes melt or evaporate. These processes absorb heat from the surrounding environment (latent energy), resulting in a rapid decrease in ambient temperature.
Snow contains a lot of air trapped between its ice crystals. This air acts as a natural thermal insulator, slowing down heat transfer. As a result, snow generally melts more slowly than ice cubes or crushed ice, which are more compact and contain much less insulating air.
Yes, it can indeed snow even with a positive temperature at ground level. This is possible when the upper layers of air are cold enough to allow ice crystals to form and fall as snow without immediately melting. If the descent to the ground is rapid and/or the humidity and wind conditions are favorable, the snowflakes will reach the ground without completely liquefying.
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