Winter Weather Continued
In the summer, when the north pole is tilted toward the sun, the northern hemisphere gets more direct sunlight and the days are longer than during fall and winter. During summer in the northern hemisphere, (it is winter in the southern hemisphere.) In winter, the tilt is away from the sun and sunlight strikes the northern hemisphere at a lower angle. Latitude is what determines both the length of the day and the angle of the sun (Waterton/Glacier International Peace Park straddles the 49th parallel along the Canadian border). The amount of sunlight striking the earth’s surface (solar insolation) and the length of the day is determined by the position of the sun in the sky. The reduced amount of winter sunlight striking the earth due to shorter days and angle of the sun causes colder temperatures. As the land and its air mass cools, surface waters turn to ice and precipitation freezes to cover the land with snow.
At northern latitudes and in mountainous terrain, winter comes early and stays late. At higher elevations the atmosphere is thinner and holds less warmth. Consequently, it’s colder, snow lasts longer, and the length of the growing season is greatly reduced. Elevations in Glacier range from less than 3200 feet in the Lake McDonald valley to 6646 at Logan Pass to more than 10,000 feet on the tallest mountains in the park. In Glacier National Park, the seasons are jokingly referred to as June, July, August and Winter. There is some truth to this as the high country may be snow-free for only about 3 months of the year. It is not unusual to see visitors skiing at Logan Pass in June and occasionally even into July. Winter lasts a long time throughout most of the park.
It is worth noting that the weather on the west and east sides of the park are different. The west side is greatly influenced by Pacific Northwest weather patterns. These weather systems provide more rain, milder temperatures and (generally) moister snow than the east side receives. The east side of the park is influenced by continental weather systems characterized by less precipitation and strong, gusty winds. During winter, cold fronts moving down from Siberia and Alaska though Alberta along the Front Range can flow over the passes and settle in western valleys. Sub-zero temperatures can last for days or weeks. Eventually, a warm moist Pacific air mass will move in. As it moves over the mountains, the moisture condenses and precipitation occurs. The process of condensation releases heat that was stored in the moisture-laden air (thus the east side of the mountains is warmer than the west side at the same elevation). As this warm air moves down the east slope, it picks up speed and creates winds that can exceed 80 mph. These warm winds are known as a “Chinook”, a Blackfeet word for “snow eater”. Chinooks can cause temperatures to rise from below zero Fahrenheit to above freezing within hours.
Winter poses many challenges to plants and animals. In northern latitudes it is the longest and most difficult season of the year. Temperature, snow depth, snow density as well as the duration of winter (a deep snow pack and late season snow extend the winter season) determine the severity of winter and play a role in how many animals survive. Many animals will die during winter. Winter kill refers to the combined effects of bad weather, malnutrition, starvation, disease and predation. Winter is a time for economy: food is scarce and energy must be conserved. It is truly a time of survival of the fittest.
Fallen snow is not always the same temperature. When the bottom layers of snow are much warmer than the top layers, water vapor creates a bottom or in-between layer that is granular and resembles sugar. This type of snow is yielding and allows small animals like mice, voles and shrews to readily tunnel through it. Because it contains a lot of air it also is good insulating snow for grouse to hunker down in on a cold night. Animals that paw through snow like moose, deer and elk can easily uncover grasses.
Air temperatures and wind alter snow crystals over time forming a hard, compacted snow mass with an even temperature throughout. This type of snow is difficult for mice to burrow through. It can cause a build-up of carbon dioxide in the lower layers as a result of decaying vegetation. Many of the small “mouse holes” seen on the surface are actually vent holes that allow carbon dioxide to escape. Without them, mice and other subnivean (under the snow) dwellers could die. Yet, this same snow allows snowshoe hares and deer to reach up higher in shrubs and trees in search of food. Melting and re-freezing causes snow crystals to reshape and form a very solid layer of snow. The strength of the snow varies, depending upon whether it is in the melt or freeze stage. Some animals can travel on the surface, while others not as well adapted will fall through and flounder, becoming easy prey for predators. An icy crust allows small animals to move with ease, but may cut a deer’s legs allowing bacteria and infection to spread in an animal already in a weakened condition.
When snow gets deep, deer will yard up (stay in one location) since bounding through snow requires a lot of energy. By yarding, they pack down a network of trails that permits them to reach areas containing winter food. At the same time, there are risks associated with it. During long, hard winters, there is the risk of overbrowsing their winter range. And there is an increased risk of spreading diseases when many animals are confined to a relatively small area.
Moose and elk can “plow” through deep snow. Moose are especially well adapted for it with their long legs. However, moose will frequently follow already established trails, while elk tend to follow in trails made by a strong lead animal. These modes of travel are known as trailing, and they are a means of reducing energy output.
The depth, density and hardness of the snow can help or hinder animals depending upon the situation. A build up of snow on branches of trees may break and snap or bend young trees. Heavy snow on trees can restrict tree travel for pine martens and squirrels, making it more difficult to catch prey or to escape predators. Willows and alders bent by the weight of heavy snow provide food and shelter for snowshoe hares. Where the branches of spruce and fir catch falling snow, snow depth becomes unequal on the forest floor. In open areas snow is deeper than beneath trees. Trees with full crowns collect most of the snow on branches. The small amount that reaches the ground quickly melts or evaporates leaving a “snow shadow” or tree well. Many small animals avoid tree wells during the coldest part of winter since they offer little insulation or protection from predators but, if the branches are heavy enough with snow and press close enough to the ground, they form cozy hideouts for snowshoe hares. As winter merges into spring, tree wells are the first place that juncos and other returning birds search for food.
Plants in Winter
The leaves of deciduous trees and shrubs change color as daylight hours wane. Soon the leaves will be shed. Leaf scars are sealed with a corky layer and next year’s buds are covered with scales to conserve moisture. Winter is similar to drought as water is unavailable when it is frozen as ice or snow. Woody shrubs and trees survive the winter in a state of dormancy. Evergreen trees and shrubs have thin or small needle-like leaves with waxy coatings to conserve moisture. The conical shape of many evergreen trees makes them more resilient to heavy snow loads. Since their branches slope out and downward, the weight of snow pressing down allows snow to fall off. Evergreen trees will photosynthesize (make sugars from water, carbon dioxide and sunlight) at the first available light in spring.
Animals in the High Country
The pika will remain active all winter in its den hidden among rockslides. It will feed on “hay” made up of grass that was cut, dried and stored during summer. It has distinctive adaptations that allow it to survive the long and extreme winter conditions. Its small round ears lay flat along its head; an inconspicuous tail and short legs reduce surface exposure and heat loss; and fur insulates the soles of its feet and provide good traction.
Mountain goats are the largest mammals remaining active in the high country year-round. Their heavy wool undercoats with long hollow guard hairs provide protection from the cold and wind. Mountain goats can subsist on lichens and mosses if they cannot find adequate browse. In winter, goats move to more south or southwest facing slopes where winter sun melts snow more quickly and prevailing winds blow it away. The ptarmigan is the only bird that remains at or above treeline throughout the winter. This alpine cousin to the grouse changes its brown plumage to white as autumn light diminishes and winter snow begins to blanket the mountains. Feathered feet act as snowshoes which allows it to walk on snow. Sharp claws help it to scratch for food beneath the snow. Ptarmigan will feed on willow buds and the needles of subalpine fir. Warmth, and protection from winds and sub-zero temperatures is attained by diving into the snow.
Cold, wind, and blowing snow of the high country offer challenges greater than most animals can adequately cope with. For animals that remain active during winter, lower elevations offer easier access to food and more protection from the elements.
Insects in Winter
Insects that have incomplete metamorphosis (egg, nymph and adult) usually over-winter in the egg stage. For insects that develop through complete metamorphosis (egg, larva, pupa and adult), the egg and pupa are the most likely stages for over-wintering since they are immobile and they have a protective coating that helps them withstand the cold. But some insects like the woolly bear caterpillar over-winters as a larva. The woolly bear stops eating in late summer and finds a sheltered place under leaves and grass. In spring, it forms a cocoon and emerges as an Isabella moth. Insects that over-winter as adults usually find a sheltered place: under leaves, in crevices in trees, under bark, rocks, plants, in buildings, or they descend into the ground and remain dormant. Staggered timing of life cycles ensures that food will be available when they reach the eating stage. Insects react to cold temperatures by slow, stiff movements and a lowered metabolic rate. They lose a high percentage of water and produce glycol, a substance that acts as a kind of antifreeze. On warm days adult insects move around as their bodies warm up sufficiently.
Migration: A Winter Survival Strategy
Large animals will also migrate as winter snows arrive. Generally it is the elk and mule deer that migrate from higher elevations to lower elevations. Prior to the actual onset of winter, reduced daylight hours trigger a response that is registered in the “master control” gland(hypothalamus) in the brain. The hypothalamus then secretes hormones that activate other systems throughout the animal’s body. Animals react in various ways. Moose, elk and deer begin to rut. The interval between the mating season and giving birth ensures the young will be born in the spring when food is abundant. Other reactions to shorter days is the urge to eat more thus building up layers of fat that will help animals make it through winter. Beavers and red squirrels cache extra food. Animals that remain active all winter will grow a thicker coat of fur. Deer, elk and moose have winter coats comprised of hollow hairs that trap air for better insulation. Other animals develop thick undercoats. Snowshoe hares, weasels and ptarmigan turn white. Photoperiod triggers hormonal changes that are influenced by cold and snow. These hormones cause changes in hair color. Weasels undergo a complete molt. Each hair is lost and a new white hair replaces it. Only the tip of the hair turns white on snowshoe hares, while the base remains gray. Timing is critical. A white snowshoe hare or weasel (ermine) makes an easy-to-spot target for a predator. Snowshoe hares as their name implies, have snowshoes. Extra fur on the bottom of their feet in winter helps distribute their weight so they can move on top of the snow with ease.
Active or Dormant: Another Survival Strategy
For many years, debate raged as to whether or not bears were true hibernators because they maintain a fairly high body temperature. Hibernation is defined as a physical state where an animal’s body functions slow down in order to conserve energy through a season of no food and water, and cold temperatures. Bears are now considered to be highly efficient hibernators. They sleep for months without eating, drinking, urinating or defecating. Bears develop thick coats of fur and have less surface to mass ratios than smaller hibernators so they stay warmer (the larger the animal, the greater the heat production capacity it has. Larger animals therefore stay warmer than small animals).
Body size is one of the main factors in determining which strategies animals use for coping with winter. Body size determines storage capacity for energy and the costs of maintaining it. Body size also determines the ability to migrate. The larger the body size, the more options the animals has. For animals that do remain active in winter, snow is a mixed blessing. It can offer shelter and protection. Snow acts as insulation, holding in earth-warmed air and keeping out cold air. Snow creates a stable environment beneath it (subnivean layer) in which temperatures may range from about 20 degrees F to 30 degrees F, while air temperatures can fluctuate from 30 degrees F below zero to 45 degrees F above zero.
The subnivean world allows plants, insects and animals to escape from temperature extremes and wind. This is important for small animals like mice, voles and shrews. Since their body surface is large in proportion to their size, they lose heat rapidly and it takes considerable time (and energy) to replace it. Their small size does not allow them to carry a thick enough coat to withstand continual exposure to cold. The bark of trees and shrubs as well as seed-heads from plants flattened by snow provide much of the food for these small animals. Their tracks indicate they spend time on top of the snow (supranivean layer) in search of food. These forays make them vulnerable to predators.
Last updated: February 24, 2015