Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 3:52 PM GMT on January 13, 2013
While watching the local evening news last week, I winced when I heard expectations for a rapidly disappearing snowpack in central Pennsylvania. That's because these medium-range forecasts were based "solely" on sunshine and temperatures in the 40s during the first half of this week (January 7-10). There wasn't any mention of dew points, which, as you will soon learn, play a pivotal role in the melting of snow.
To get a sense for weather conditions in State College during the first half of the week, check out (below) the meteogram at University Park's Airport, which shows plenty of sunshine on Tuesday, the 8th, and the daytime start on Wednesday, the 9th. In fairness, there was some melting, but our snowpack here in State College hung pretty darn tough through Thursday (January 10).
Early Friday morning (January 11), in anticipation of the snowpack rapidly melting, I took a number of measurements in my side and back yards just before light rain started to fall. The snow depth in my yard varied from two to four inches, so I decided that three inches was reasonably representative of the snowpack in my neighborhood.
The meteogram at the University Park Airport, PA, from 11Z on January 8, 2013, to 12Z on January 9, 2013. Courtesy of the University of Wyoming.
The snowpack in State College went pretty quickly this weekend (see the Web Cam at Penn State's Arborteum farther down on the page). So why didn't sunshine and temperatures in the 40s rapidly melt the snowpack in State College during the first part of the week? Well, dew points were relatively low through Thursday, a factor that sometimes gets overlooked (obviously). Now here's the $64000 question: Why do low dew points typically suppress the rapid melting of a snowpack? And if you want more food for thought, consider that a warm rain, at temperatures and dew points above the melting point of ice, is much more effective when it comes to rapidly melting a snowpack...of course, the higher the temperature and dew point, the more rapid the melting (stay tuned).
I'll go one step further. Are you sitting down? Here goes. During a warm rain, invisible water vapor rapidly melts most of the snow, not the visible raindrops. I realize that this statement flies in the face of the popular notion that a warm rain is directly responsible for rapidly melting snow. As you will soon learn, it's not so. Intrigued yet? Read on.
To explain why invisible water vapor is a rapid melter of snow (and visible raindrops are not), let's start with some basic concepts (after my last blog on sudden stratospheric warming probably gave you a headache, I feel obliged to tackle some fundamentals here). For starters, let's talk about sublimation. To keep things simple, suppose that there isn't any melting. Furthermore, let's assume that there isn't any net condensation onto the snowpack, which is a reasonable assumption when dew points are low. I'll also assume that there's no settling (compaction) of the snow cover. Nonetheless, the depth of the snow cover would still decrease due to sublimation. In this situation, ice goes directly to the vapor phase (do not pass go; do not collect $200). In other words, sublimation results in the relatively slow ablation of snowpacks (ablation is the general term for the reduction of snow or ice from the earth's surface).
When a warm rain falls over a snowpack, surface dew points increase and there's net condensation on the snowpack. In other words, water vapor condenses onto the cold snowpack (keep in mind that the snow is relatively cold compared to the overlying air because the temperature of the snowpack stays at 0 degrees Celsius). For the record, net condensation means that the rate of condensation is greater than the rate of evaporation.
The Arboretum At Penn State on Sunday morning, January 13. Officially, there's a trace of snow cover. Courtesy of The Arboretum at Penn State.
As a result of net condensation, condensational heating occurs, providing heat energy that melts the snow. Over the years, people have asked me for a simple demonstration of condensational heating. Alas, I've not come up with a good one. Its inverse, evaporative cooling, is easy to demonstrate (simply lick the back of your hand and blow on it). In my view, however, the rapid disappearance of the snowpack in my yard this weekend is testimony to the impact of condensational heating.
Of course, I haven't shown you that "warm raindrops" are not largely responsible for the rapid meting of a snowpack. If you're interested, check out this paper (pdf file) written by Craig Bohren, a professor emeritus at Penn State. It's mathematical, which is why I didn't address it here.
So what is most important to the rapid melting of snow is that net condensation occurs. For the record, the rate of evaporation of meltwater on the surface of the snowpack (at 0 degrees Celsius) is pretty much set in stone. So the amount of net condensation onto the snowpack depends pretty much on the rate of condensation, which is a function of dew point and temperature. As the dew point increases above a snowpack, the number density of water vapor also increases. If rapid melting by condensational heating is the goal, a relatively high number density of water vapor is crucial (that's why low dew points don't typically promote rapid melting, despite sunshine). Moreover, as the temperature of the water vapor increases, the rate of condensation also increases. So higher dew points and temperatures mean that there's greater net condensation, and, ultimately, greater condensational heating. Bye, bye, snow!
I should point out that the wind can accelerate the loss of snowpack during a warm rain. Here's the story. There is a very thin, stagnant boundary layer that separates the snowpack and the overlying air. This boundary layer acts as a barrier to the net transport of water vapor onto the surface of the snowpack during a warm rain. As the wind speed increases, the thickness of the boundary later decreases, thereby promoting a greater net transport of water vapor to the snow surface. That means a greater degree of net condensation and greater condensational heating.
As a caveat to this discussion, rain doesn't have to be present for dew points to increase over a snowpack. Indeed, moist (and warm) advection also increases surface dew points over a snowpack.
If you walk away from reading this blog with the understanding that a warm rain does not directly melt snow (to any significant degree) and that it's the net condensation of invisible water vapor onto the snowpack that ultimately matters, then I've done my job. In deference to the role of a warm rain, it does provide ideal conditions for the dew points to increase, paving the way for greater condensational heating and the rapid melting of snow.
Many thanks to Craig Bohren for his observations and input.
Comments will take a few seconds to appear.