A common question we hear with wintertime rain is how much snow would we get if we weren’t getting rain? But this question overlooks one major variable, temperature. Asking how much snow we’d get when it’s raining on a 60 degree day in January is like asking “what would this orange taste like if it wasn’t an apple?”
The amount of snow or rain we get from a particular storm system depends on how much moisture the air can hold. This amount changes drastically as temps warm up. A general rule of thumb is that the air can hold TWICE the amount of moisture for every 20 degrees we warm up. This is why your hair dryer blows hot air. It is more efficient at evaporating water. A good way to measure the amount of moisture or water in the air is with something called a saturated mixing ratio. This is simply the amount of water by weight that can be evaporated into a given amount of dry air. Here is an example, on January 29th we warmed up to 60 degrees. A kilogram of dry air (yes air does have weight) at 60 degrees can hold about 11.2 grams of water (assuming normal air pressure near the surface of about 1000 mb). At 40 degrees the air can only hold 5.3 grams. At 20 degrees, 2.3 grams. So it becomes clear the “what if?” game becomes much more complicated.
This also shows us why our heaviest snowfalls tend to occur when temps are in the upper 20s and low 30s. This sweet spot for snow is just cold enough to keep it frozen while maximizing how much water the air can use to make snow. On the other hand this gives a little credit to the old “it’s too cold for snow” saying that you may have heard. While the claim itself is false, it can be too dry for anything more than light snow. At -10 degrees the air can only hold 0.6 grams of water. Even a dry/fluffy snow wouldn’t amount to much given the lack of moisture the system would have to work with at that temperature.
If you’d like to find the saturation mixing ratio of other temperatures (see how much water is in the air on an 80 degree day with 100% humidity) check out this calculator by clicking here.