With a flurry of winter storms hitting the Northeast this week, we had some snow on the brain. So we decided to talk to an expert on the topic. David Robinson is a Rutgers University scientist, and when he's not performing his duties as the New Jersey state climatologist, he heads the school’s Global Snow Lab, which provides daily breakdowns of snow cover across the planet.
Q: How do you describe what you do?
A: Depends where I am. If I’m talking to fellow scientists, where I’m wearing more of my “research hat,” very colloquially I’ll say I’m a “snow guy.” In New Jersey, I’m the state climatologist, so that’s all everybody knows. I even have groupies …
A: (laughs) Yes, in a sense I do … but more like, there’s weather buffs who’d kill for my position.
But basically, I study snow cover. I’m not a meteorologist that studies the dynamics of snowfall; I’m mostly interested in snow on the ground and how it sticks there. I’m interested in the role snow cover has, as an important factor within the climate system, and also in what gets the snow [to keep] sitting on the ground.
It’s a bit of a chicken-and-egg question: is it cold because there’s snow, or is there snow beccause it’s cold?
Q: How does snow influence the weather after it’s fallen?
When snow is on the ground, it has a very high reflectiveness, which we call “albedo.” Our atmosphere is primarily heated indirectly from sun – sunlight hits the surface of the Earth, and that surface radiates warmth into the atmosphere. When snow is on the ground, instead of being absorbed, that sunlight is reflected into space, never to be available for heating. Snow has a profound influence on the Earth’s radiation budget.
Also, snow is kind of an insulator. When the ground is snow-covered is actually protective of winter wheat; it also keeps the soil from freezing as deeply as it might.
There’s also the very important role of snow as part of the global water budget and water cycle. It’s a reservoir for subsequent seasons. They’re dying out west right now because they’re not getting snow on the mountains.
Q: Yeah, I heard that a lack of snow cover is connected to the ongoing drought in California …
Right, the snowpack in the Sierra Nevadas. They had a snow last weekend, which helped some, but they’re still heading towards 20 percent of normal.
What’s happened is the jet stream has buckled to the north of western North America. Think of it as a string of pearls, and each pearl is a storm. Well, the string has been wavy this year, and there’s this bulge in the western part of North America. All the storms have been shuttled away to the north of California, and south of the storm track they’re locked into very mild air.
And in the West, they only have this window of opportunity from November to April to get precipitation. They’re already halfway through the season, and they’re nowhere close to normal.
Q: Are you seeing any general trends in the amount of snow cover?
There’s not so much year-to-year variability in winter, but in the fall it can be quite variabile. In spring is where we really see a decline over the past few decades. I first published a paper in 1990 showing spring snow cover is decreasing.
There’s also some interesting things going on in the last decade, where in May or June in particular, we’re seeing a really rapid loss of spring snow. Even the snow cover in arctic is moving out a couple weeks earlier in May and June.
Q: How do you get all of the data on the snow coverage across the globe?
We’re totally beholden to the National Ice Center. They’ve got trained analysts scrutinizing thousands of satellite images, taken by satellites that either orbit the Earth from pole to pole, or sit stationary over the equator.
When they’re looking at pictures, they have to distinguish between snow and cloud cover, and they can do that in a couple of ways. You can put the pictures in motion, and most often the clouds move faster than the snow melts. Some pictures use microwaves to see through the clouds. Then there’s certain patterns on the surface of the Earth, even just look of snow-covered ground, that if you’ve got the experience you can see.
No observational product is perfect, but this has been workhorse for climate community for decades.
Q: What’s missing from our monitoring of snow coverage?
We’re trying do to more work assembling information on snow depth -- I’m working with folks to put together a good snow depth database, on a regional basis.
Q: Can you use satellites to measure snow depth?
Sort of. The Earth naturally emits microwaves, which are longer than visible light waves. If you have snow cover, that kind of mutes the ability of microwaves to get up and be sensed by a satellite. So you can use a rough relationship between how much the waves attenuated, and the amount of water in the snowpack.
Deeper snow tends to be equivalent to more water, but you have to estimate the density of the snow. Plus, microwaves are fraught with physical limitations.
There are a couple other ways of doing it – measurements, fieldwork. People can be taught to measure snow – but still, you have to worry about drifting, grass trapping air underneath that may give an inflated idea of the depth. And you’re only taking point measurements anyway.
We’ve already had one preliminary study that shows while the snowpack is melting out earlier, it’s also shallower than it used to be, which may help promote earlier snowmelt. So maybe the snowpack is declining in volume. But we’re not 100 percent confident yet.
Q: Do you ever get together with all the other state climatologists?
Yes, we have an annual meeting of our organization – the American Association of State Climatologists. We’re a bunch of nerds.
(Note: this interview has been condensed and edited.)