Thundersnow

Usually, thunder and lightning are phenomena that are confined to summer. Part of winter's peace comes from the fact that even the most vigorous blizzard lacks the violent weather elements that an average summer storm has -- lightning, thunder, hail, tornadoes, and the like. We can venture outside in even the worst weather without risking being struck by lightning, and the most noisy storm is marked by nothing more than whistling of wind around the eaves -- a sound that can even be cozy if we are seated by a comfortable evening fire.

When conditions are right, however, a combination thunderstorm and snowstorm can occur. This is known as thundersnow, and results from one of several different causes. The whole United States witnesses only a few instances of thundersnow each winter, so actually seeing this weather is a rare experience. Because of its rarity, thundersnow has been difficult to study, but a lot of research has been done over the years and a clear picture of what causes thundersnow and what it means in terms of weather in general has emerged. So, next winter, keep your eyes open for unexpected flashes or a muffled rumble of thunder, and you may be able to number yourself among the small group of people who have experienced this meteorological event.

The genesis of thundersnow

Thunderstorms usually don't occur in winter because the atmosphere is usually too stable for the strong convection that builds thunderheads. Basically, in the absence of hot, moist, tropical air, it's hard to get the powerful upward air flow that results in thunderstorm formation in the warmer months. Layers of cool and cold air are much more stable, even when the weather is bad, and the cold air also can't hold enough moisture for the vertical cloud development that typifies a lightning storm.

It's for this reason that thundersnow is most common in late winter or early spring, when warmer, moister air is starting to trickle in from the south and make the atmosphere less stable. This warmth and moisture also provides 'fuel' for the building of a large enough cloud mass to make a thunderstorm. There are several ways that thundersnow can form, however, including the same mechanisms that make lake-effect snow, so there is a chance of this weather at any point in the winter in the correct areas.

Ways that thundersnow can come into being include --

• As part of lake effect snow -- near the Great Lakes, as well as some coastal regions of Japan, Britain, and Greece, the same processes that make lake effect snow can also produce thundersnow. The rapid rising of warm, moist air from the surface of a large body of water can be intense enough to build a thundersnow storm that will strike the snow belt area where lake effect snow ordinarily occurs. Thirty miles of open water is needed at least in order to mix enough warmth and moisture into the weather system to possibly create thundersnow.
• As a thunderstorm that moves into colder regions -- during the late winter, thunderstorms generated by a weather front sometimes move into much colder areas without raising the temperature above freezing. In this case, the rain falling from the thunderstorm may freeze into snow.
• When warmer, moister air is present under a layer of cold air, but is itself still cold enough not to melt falling snow, this can sometimes cause enough instability in the atmosphere for heavy convection to occur. The situation needed for this type of thundersnow is fairly complex and all factors must be properly balanced to result in the storm -- a type that chiefly occurs in late winter and early spring.

Special traits of thundersnow

Thundersnow storms are generally less violent than their summer counterparts, since although convection is present, it still isn't usually strong enough to develop truly intense thunderstorm conditions. Thundersnow includes only snow, not rain, but the snow tends to be more intense than an ordinary snowstorm, just as thunderstorm rain in the summer is more intense than that from regular nimbostratus. Some research suggests that thundersnow drops an average of 6 inches of snow over a 140-mile wide area in a short amount of time. A few thundersnow storms are strong enough to produce small hail, but most are not that violent.

Thundersnow is less spectacular than a summer thunderstorm because both the light and noise of the lightning strikes is suppressed by the thick snowfall. The fluffy snowflakes muffle sound so much that the thunder can only be heard within 1 to 2 miles, rather than the usual 12 miles. Lightning is also hidden by the snow, resulting in a bright but directionless flash that can only be seen for a limited distance as well.