Snow is typically associated with colder weather but just how cold does it have to be to snow? You might be thinking that it has to be below freezing (32°F/0°C) but that’s incorrect. It actually can be well above freezing. Snow has been recorded at 52° on 10/27/2015 in Minneapolis/St Paul and at 49° at JFK Airport in New York City. The theoretical limit is 52°/11°C but it takes a lot of different circumstances to line up. Generally though, the temperature at the surface needs to be below 41°F/5°C for it to snow.
How can snow form above freezing?
You’re probably asking yourself, how can snow form above freezing? Doesn’t ice form at 32°F or 0°C? It does but remember that snow is a tiny ice crystal formed way up in the atmosphere. While the temperature near the ground may say one thing, it’s a different reading the higher up you go. If the temperature in the clouds above you, the snow crystals can form and still fall through the air producing a snowfall when the temperature near the ground is above freezing.
Also, the air contains different amounts of moisture in different places. If a place is dry enough and you introduce moisture to it, that moisture will start to evaporate. That evaporation produces a cooling effect on the surface below it. That’s why we sweat – so when the moisture evaporates we feel cooler! Have you ever noticed frost on a windshield but nowhere else? The moisture evaporated off the surface and cooled it enough until it was below freezing and it froze. Eventually it becomes too warm to sustain ice crystals and melts but that ice formed while the air temperature was above freezing!
Combine those two factors and we have a large portion of the puzzle as to why snow can form at temperatures above freezing. In order to understand that, let’s look at how snow forms for a clue as to how cold does it have to be to snow.
How does snow form?
When moisture is introduced into the atmosphere and the temperature is below freezing, it starts to crystallize. These crystals start combining and then fall to the ground as snowflakes. These flakes in turn fall to the ground as snowfall.
What happens if the temperature is above freezing? That moisture stays as a liquid but it is subject to the conditions around it. If the air is dry, some of that moisture is going to evaporate until the air is equally moist. That evaporation will cause the surface of the water droplet to cool. If it cools to below freezing, it too will freeze and form a snowflake. Get enough of them and they will fall producing snowfall at temperatures above freezing.
Can you predict snow above freezing?
Sure you can! There’s a lot of math that goes into it, including fancy calculations that can factor in the air temperature and the amount of moisture in the air and tell you if snow can form or not. However, there’s a nice calculator that will do this for you:
Another way is to pay close attention to whats called the Wet Bulb temperature. You can do this with a sling psychrometer. It basically has two thermometers – one for the air temperature and the other has a fabric around it to measure the temperature if the air was saturated with moisture. As long as the wet bulb temperature is below freezing, it can snow as long it’s also below freezing in the clouds above you.
So, how cold does it have to be to snow? As we’ve learned, there’s a lot of different factors that come into play but generally it has to be cooler than 41°F/5°C to snow but under some special circumstances it can snow up to 52°F/11°C.