One of the constants in our daily home routine is a game of tennis ball toss-and-retrieve with our Labrador retriever, Bella, led by my wife or me; or—on a lucky day for Bella—each of us in turn. We tend to lose more tennis balls in winter, thanks to their ability to disappear if they strike the snow at a steep angle. Sometimes Bella will sniff them out, but—winter scenting conditions and icy tennis balls being what they are—sometimes not. They’re also lost when my wife wields the tennis ball Chuck-It device and an errantly-thrown ball lands on the snowy garage roof.

But snow conditions have changed in the last couple of weeks, and as a result we’re losing fewer tennis balls. On top of the earlier snows—which together had totaled 15 inches and more—there recently came several more inches of what the forecast called a “wintry mix,” which sounds more like a seasonal salad than weather. It had a consistency somewhere between snow and a snow cone, and the temperature drop that followed left the drifts with a skin of ice on top, and the heaps piled up by plows as rock-hard ice sculptures.

Now, the tennis balls we throw for our Labrador don’t disappear into the snow. Unless one falls into a track, or some other depression in the snow, they bounce and roll on top. As for Bella, except in sheltered areas where some snow remains comparatively soft, she—at 65 pounds—can run on top of the snow, too.

It’s a different story for Minnesota whitetail deer, most of which weigh more than twice what our Labrador does. Whether ice-encrusted or soft, moving around in the snow we’ve accumulated is no walk-in-the-park for a whitetail. Though the surface area of a deer’s foot set flat on the ground might surprise you, the lightest whitetails probably weigh 85 to 100 pounds, and adult does and bucks average 140 to 170. That much weight—on four thin legs and relatively small, pointed feet—is too much to be supported by the snowpack; icy or not.

The depth of snow and a deer’s ability to navigate through it play a very-well-known role in determining whether a whitetail will survive a Minnesota winter, and this one has started out on the wrong foot over much of the state. Early December brought snowfall of 12 to 15 inches over a wide area of Central and Northern Minnesota, and more than two feet—24 inches and more—in a swath of Northeast Minnesota, including Duluth. More has fallen since.

Snow is a factor in deer survival, because—when there is too much of it—snow can restrict a deer’s movement between feeding and security areas, as well as in efforts to escape predators. These predators include the Eastern gray wolf, and to a lesser extent coyotes. Coyotes are most successful in preying on whitetail deer when fawns are young and especially vulnerable, and also in winter when snow conditions restrict deer movement and weaken them physically.

While predation is the most dramatic cause of death suffered by whitetails, it is often aided and abetted by the stress of a severe winter, weakening deer and making them more vulnerable. Sometimes winter itself is the killer, with the vector of death starvation—deer using more energy to fuel their metabolism under the stress of extreme cold, and expending more just to reach that food. And, some that survive the winter will be too weakened to bear healthy fawns.

There is an index used by many biologists to measure the severity of a winter, called—logically enough—the winter severity index, or WSI. Though severe winters can have a negative impact on any number of creatures—wild or domestic—the WSI seems particularly suited to guesstimating the negative effects of a winter on deer. For one, it measures snow depth as a negative, when in point of fact for some creatures—ruffed grouse being the obvious example—deep snow is actually advantageous.

The WSI works like this: count the number of days with a snow depth of 15 inches or more, then add to it the number of days with a temperature of zero degrees or lower. For example, a single day with 15 inches of snow on the ground and a temperature that drops below zero, would add “2” to the WSI; one count for the snow depth, and one for the low temperature.

According to the legend on the Minnesota Department of Natural Resources (DNR) WSI map, an end-of season WSI of less than 100 indicates a mild winter, while a WSI above 180 indicates a severe winter, one in which deer can be expected to be lost. On January 2nd the state snow depth map posted by the DNR showed that the upper one-half of the state had snow depths that mostly range from 15 to 30 inches, with some small pockets in this half of the state having as little as 12 inches. A very substantial amount of it—perhaps one-third of this upper half of the state—showed 18 to 30 inches of snow.

It should be noted that snow depth is not completely static throughout an entire winter. Snow will compact some over time, especially during spells of warm, sunny weather. Wind can also move snow, reducing depth in one area, and deepening it in another by depositing drift snow.

On the current DNR map, all of Minnesota now shows a WSI index labeled “50 or lower.” But it should be noted that we’re a scant month into the winter season; not even a month, if you count December 21st the start of winter. Some years we have little snow during the entire month of December, but this year much of the state has had 15 or more inches of snow for the better part of a month. We also do not typically begin to receive our annual complement of sub-zero temperatures until early in the New Year.

So, while the WSI is far from the danger point at this juncture in the winter of 2019-2020—at any location in the state—we have roughly three months of potentially winter-like weather ahead of us. Any of those months could add 30 points to the WSI based on snow depth alone, to say nothing of the number of 24-hour periods when the temperature drops below zero.

No cause for alarm or panic now, but a cautionary start to the winter for our whitetails.