It can snow even when it is warm because the temperature on the ground is not necessarily the same as at altitude. When warm and humid air meets cold air at altitude, condensation can occur and form snowflakes.
The higher you go in altitude, the colder the air becomes. This can be simply explained by the gradual decrease in atmospheric pressure: as you ascend, there is less air above you, so the air is less compressed and cools accordingly. Generally, the temperature drops by about 6.5°C for every 1000 meters. Therefore, even if it is warm in a valley, it can be quite cold higher up on a mountain where it is more likely to snow than rain. This phenomenon is exactly what explains why you can sometimes see snow on the peaks even when the weather is pleasant down below.
When different air masses meet, it inevitably creates some turbulence. A warm current, generally coming from the south, crosses a cold current often arriving from the north. Being lighter, the warm mass rises above the colder, denser, and heavier mass. As it rises, the warm air cools quickly, causing its moisture to condense. This phenomenon leads to the formation of dense clouds that can result in snowfall, even if the ground temperature still feels relatively mild. It is this energetic meeting between the currents that explains how it can snow while you are almost walking outside in a sweatshirt.
The higher you go in the atmosphere, the colder the air becomes. Up there, the temperature easily drops below zero, even if it’s mild down below! When water droplets encounter these very cold conditions at high altitude, they can quickly turn into ice crystals. And these crystals, if they survive their fall and haven't melted along the way, arrive as snowflakes even when you're strolling around in a sweater down below. This is simply because temperature varies greatly depending on the atmospheric layers the precipitation passes through.
For snow to form, there must be a sufficient amount of water vapor in the air. This humidity is actually the essential fuel for producing snowflakes. The moist air rises, cools up there, and can form tiny ice crystals around fine particles present in the atmosphere when it freezes. These crystals then grow by capturing other droplets or neighboring crystals, thus becoming snowflakes that you find lower down, sometimes even when it is mild near the ground. Without humidity, there is no raw material to make these little flakes, regardless of the temperature.
When a weather front arrives, it's somewhat like the meeting of two groups at very different temperatures that don't get along too well. A warm front crosses a cold front, and there: bam, the conditions mix. At the exact boundary between these air masses, you can end up with mixed precipitation. In simple terms, this means that even if the air at your level seems rather mild, snow is forming much higher up and descends through warmer air layers, sometimes generating wet snow or sleet. So, there's no need to be in the midst of a harsh winter to see a few stray flakes falling from the sky. Yes, the weather is sometimes full of contradictions, and that's because the air masses above your head love to play roller coasters.
Even if the ground temperature is well above zero, it can still snow if the temperature in the upper layers of the atmosphere is cold enough to allow for the formation of ice crystals.
Under certain conditions, it can snow at ground temperatures of up to 8 degrees Celsius, due to the intense cooling of the upper layers of the atmosphere.
Snow can partially melt as it descends to the ground by passing through warmer layers of air, then refreeze into small pellets known as "rolled snow" or "sleet."
A snowfall in the middle of summer can occur in high mountains, sometimes even when the temperature is above 10 degrees Celsius in the valleys below.
Yes, snow varies depending on temperature and humidity. In cold and dry weather, it will be powdery and light, while in temperatures close to freezing and humid conditions, it will become heavy, wet, and sticky.
Yes, in rare cases, ice crystals can form from the moisture present in very cold air, creating what is known as diamond dust. This phenomenon usually occurs at extremely low temperatures and in particularly dry air, without any apparent clouds.
There is no exact temperature, but generally, snow can fall if the upper layers of the atmosphere are cold enough. Even with slightly positive temperatures at ground level (a few degrees above 0°C), snow can reach the surface without completely melting.
When ground temperatures are around zero, it is mainly the temperatures at higher altitudes that determine the type of precipitation. If the air is cold enough at altitude, the precipitation will arrive in the form of snow or sleet. On the other hand, milder temperatures at higher altitudes will favor rain or mixed precipitation.
This phenomenon, called virga, is caused by snowflakes passing through a warmer layer of air near the surface. As a result, the snow melts before reaching the ground, resulting in what is known as virga precipitation.
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