It is possible for lightning to occur when it is snowing, a phenomenon known as thundersnow. This unusual weather event combines the electrical discharge of a thunderstorm with the atmospheric conditions that produce frozen precipitation. While most people associate lightning and thunder with warm-weather storms, the physical requirements for an electrical storm can still be met even when temperatures are well below freezing. This rare occurrence requires a specific balance of atmospheric ingredients.
Defining Thundersnow
The formal term for a snowstorm accompanied by lightning and thunder is “thundersnow.” The sensory experience is distinctly different from a summer thunderstorm. The sound of thunder during a snowstorm is typically muffled and travels a much shorter distance than it does in warmer air. This acoustic dampening occurs because the dense layers of snow crystals effectively absorb the sound waves. Instead of a sharp crack, the thunder often sounds like a low, deep rumble that is only audible within a mile or two of the lightning strike. The visual aspect, however, is often dramatic, as a flash of lightning can brilliantly illuminate the falling snow.
How Lightning Forms During Snowfall
The core mechanism for creating lightning during snowfall is charge separation within a cloud, the same process that drives any thunderstorm. This requires a strong vertical movement of air, known as convection, which is difficult to achieve in the stable, cold air of winter. The necessary instability is created by a steep temperature drop, or lapse rate, with increasing altitude, allowing for strong updrafts of relatively warmer air.
These powerful updrafts carry tiny ice crystals and supercooled water droplets high into the cloud, while downdrafts carry larger, heavier particles like graupel (soft hail). Collisions between these rising and falling particles transfer electrons, causing a separation of electrical charge. Lighter ice crystals acquire a positive charge and are carried to the top of the cloud, while heavier graupel acquires a negative charge and settles in the lower section.
When this electric potential difference becomes strong enough, the cloud discharges the built-up energy as a lightning bolt. This release instantly heats the air along the lightning channel, causing it to expand violently and create thunder. For this electrical charging to happen, the cloud must be deep enough to contain a “mixed-phase” region where ice, graupel, and supercooled water droplets all coexist and can collide. The top of the snow-producing cloud typically needs to reach temperatures of at least -30 degrees Celsius.
Rarity and Geographic Conditions
Thundersnow is a rare event because the specific conditions required—a deep, convective cloud and a steep lapse rate—do not frequently coincide with temperatures cold enough for snow. The intense atmospheric instability of summer thunderstorms is uncommon in winter.
It is often observed near large, relatively warmer bodies of water, such as the Great Lakes, where lake-effect snowstorms are common. When frigid air flows over the warmer water, it picks up moisture and heat, creating intense vertical lift and the rapid convection necessary for lightning to form. Thundersnow can also happen during powerful winter low-pressure systems, like Nor’easters, where the convergence of cold air aloft and warm, moist air from the ocean provides the requisite instability and lifting mechanism. Other regions reporting thundersnow include the Intermountain West and parts of the Northeastern United States, typically occurring in March.