Snow can fall even when the air temperature at ground level is above 32 degrees Fahrenheit, a phenomenon that often surprises those who associate snow strictly with freezing conditions. While water freezes at 32°F (0°C), the atmosphere’s complexity means various factors influence precipitation. Snowfall at 36°F is scientifically explainable, indicating that conditions higher in the atmosphere and other physical processes play a significant role.
The Science of Snow Above Freezing
Snow originates in the upper atmosphere where temperatures are well below freezing, typically between 14°F (-10°C) and -4°F (-20°C). Water vapor in these cold layers freezes directly into ice crystals, which then grow and aggregate into snowflakes.
As these snowflakes descend, they often pass through warmer air layers closer to the Earth’s surface. Even if the air temperature at ground level is above freezing, the snowflakes may not melt instantly. The process of “evaporative cooling” is a significant factor allowing snow to reach the ground in above-freezing conditions. As snowflakes fall through unsaturated air, some of their ice sublimates (turns directly into water vapor) or melts and then evaporates. This phase change requires energy, which is drawn from the surrounding air, causing the air to cool. This localized cooling can lower the air temperature immediately around the snowflakes, helping them remain frozen longer.
This effect is understood through the concept of “wet-bulb temperature,” which accounts for both air temperature (dry-bulb temperature) and humidity. The wet-bulb temperature is the lowest temperature to which air can be cooled by evaporating water into it at a constant pressure. If the wet-bulb temperature is at or below freezing, snow can continue to fall, even if the dry-bulb temperature is slightly above 32°F.
What to Expect from Warmer Snow
Snow that falls when ground temperatures are around 36 degrees Fahrenheit is typically characterized as “wet snow” or “heavy snow.” This type of snow contains more liquid water because it is closer to its melting point. It often appears as large, clumpy flakes that stick together readily. The consistency of wet snow makes it ideal for building snowmen or snowballs due to its stickiness.
Upon impact with surfaces, warmer snow behaves differently than drier, colder snow. It tends to melt more quickly on warmer surfaces like roads and pavements, often creating slushy conditions. However, it can still accumulate on colder surfaces such as grass, untreated sidewalks, or vehicle tops, especially if the ground has been pre-cooled or if the snowfall is heavy. Wet snow is also significantly heavier than dry, powdery snow, sometimes containing two to three times more water per inch. This increased weight can pose risks to tree limbs and power lines, potentially leading to damage or power outages.
Conditions That Favor 36-Degree Snow
Certain atmospheric conditions increase the likelihood of snow falling and accumulating when surface temperatures are slightly above freezing. High humidity plays a significant role in this phenomenon. When the air is humid, the evaporative cooling process, where falling snowflakes cool the surrounding air, becomes more effective. This allows the air to reach or stay closer to freezing, increasing the chance of snow reaching the ground before melting.
Another factor is the rate of precipitation. Heavy precipitation rates can overwhelm the air’s warming effect. This rapid descent of cold snowflakes helps cool the air column more efficiently, allowing snow to survive warmer surface temperatures. Additionally, a cold air mass just above the surface, even if the immediate ground temperature is slightly warmer, can support snow persistence. These conditions create an environment where snowflakes form in cold upper layers and descend through a relatively shallow, slightly above-freezing layer, reaching the ground as snow.