It turns out that you don’t need temperatures below freezing for snow to fall. In fact, snow can fall at temperatures as high as 50 degrees.
Most residents of the northern United States have probably seen 40-degree snowfalls before, but snow at temperatures greater than 45 degrees is hard to come by.
Snow is a form of ice crystal, and, although it can fall through a layer of air that is above freezing, it does require temperatures below 32 degrees to form in the sky. It’s always below freezing somewhere in the atmosphere; in the summer, that level may be several miles above the ground, whereas freezing levels descend during winter.
When moisture overlaps with below-freezing temperatures at cloud level, snowflakes can form. It’s not uncommon for snowflakes and ice crystals to form aloft in tall thunderclouds even during July! Of course, they melt before hitting the ground.
In the winter, those snowflakes form closer to the surface, and they have a much easier time reaching the ground — especially when ground temperatures are at or below freezing.
But certain circumstances may allow them to survive their descent to the ground through pockets of air that are above 32 degrees. There are several factors that determine their odds of making it to the ground.
Because most snowflakes only fall at 3 to 5 mph, they have a long residence time surfing through the atmosphere before colliding with the ground. If air temperatures are a smidgen above freezing, gradual melting will take place. The warmer the ambient temperature, the more quickly the snowflake will melt.
But here’s where things get interesting. As the snowflake melts, the change of state from solid ice to liquid water requires an input of latent heat, or heat energy to support the phase change. That extra nudge of heat is sucked in from the surrounding environment and from the remainder of the snowflake itself. In other words, the actual process of melting cools the rest of the snowflake and the air around it.
It’s the same process that makes sweating work. After exercising, or spending more than 3.8 seconds outdoors in Florida, a layer of liquid sweat will be present around your body. As that evaporates, the change of state draws heat from your body, cooling you down.
In the case of snowflakes, much of that liquid water also evaporates into the surrounding air, cooling the snowflake even further. The drier the air, the more quickly said water evaporates.
That means that, under the right conditions, a snowflake can be in balance — melting at the exact rate that heat is sucked out of it by evaporative cooling. Assuming the snowflake falls quickly enough and is sufficiently large, the remaining chilled snowflake may be intact when it hits the ground.
In dry environments, that can happen at temperatures well into the 40s. It’s most common with snow squalls, which feature briefly heavy snowfall rates and plenty of snowflakes. They also form along the interface of opposing air masses, usually cold fronts, when temperatures quickly plummet.
Under the right — though rare — circumstances, snow could even fall at a temperature of 50 degrees. You would need an extremely dry, desert-like environment with single-digit relative humidity, with a moist air mass running over top of it.
Other types of frozen precipitation can occur at even higher temperatures. When it comes to sleet or graupel, or soft snow pellets rimed with ice, it’s much less challenging to bring them to the ground without melting. Because they have a lesser surface-area-to-volume ratio than snowflakes, there is less exposed surface for the air to eat away at, and the frigid core of the ice pellet has a better shot at surviving the descent.
So the next time it snows in your neck of the woods, check the thermometer. If it’s above freezing, you’ll understand how it’s happening.