By Chris Spears
DENVER (CBS4) – We received a great question this morning from CBS4 viewer Fletcher.
“It is 13 degrees outside and the humidity is 88%. Why isn’t it snowing? The atmosphere is pretty well saturated but why isn’t the moisture freezing and falling?”
Relative humidity is one of the most misunderstood weather variables out there. It has one meaning, and one meaning only. It tells us how close the air is to being saturated. That’s it!
In meteorology the word saturation doesn’t mean wet, as in rain or snow, it means “full” … as in the air is holding all of the water vapor that it can for the current air temperature and pressure.
A relative humidity of 88% just means that the air can hold 12% more water vapor before it’s full, or saturated.
Relative humidity can literally change by the minute. That’s because it’s directly related to air temperature. It can also change if the amount of water vapor in the air changes. We measure the actual water vapor in the air by using the dew point temperature.
To get rain or snow, you need something else happening in the atmosphere. Specifically, inside the cloud.
It’s called the Bergeron process and it plays a role in helping to create snow that will eventually fall out of the cloud, and hopefully, reach the ground below.
If this topic has caught your interest then here’s a little more information about the air’s ability to hold water, which is directly connected to temperature.
For every one degree you warm, the “tank” to hold water vapor gets bigger and bigger. For every degree you cool it gets gets smaller and smaller. It’s an exponential relationship.
Because of this, 13 degree air is extremely dry, meaning there isn’t a lot of water available for snow, despite the fact that in Fletcher’s case, the air is almost saturated.
While high relative humidity can be helpful to predict extensive coverage of low clouds, and things like fog, dew or frost, it doesn’t necessarily mean that you are going to receive rain or snow.
Meteorologist Chris Spears travels weekly in the CBS4 Mobile Weather Lab reporting about Colorado’s weather and climate. Check out his bio, connect with him on Facebook or follow him on Twitter @ChrisCBS4.