Lightning can occur during a snowstorm, a phenomenon meteorologists term “thundersnow.” While it is considerably rarer than a standard thunderstorm, thundersnow involves the same fundamental atmospheric conditions that lead to the separation of electrical charges and the eventual discharge of lightning.
Defining Thundersnow
Thundersnow is essentially a thunderstorm where the precipitation reaching the ground is snow instead of rain. This occurs because the necessary ingredients for a thunderstorm—instability, moisture, and lift—are present, but the entire atmosphere is below freezing from the cloud base to the surface. It is considered a rare event, with fewer than ten storms per year producing thundersnow in the United States.
The heavy snowfall and the dense, multi-layered winter atmosphere act as an acoustic dampener. This muffling effect causes the thunder to sound more like a low, continuous rumble rather than the sharp crack heard during a summer storm. Because the sound is absorbed, the thunder from a thundersnow event is typically only audible within a two to three-mile radius.
The Necessary Atmospheric Conditions
Thundersnow requires the air to be lifted vigorously upward to create a deep, convective cloud. This strong vertical lift is often provided by an intense surface low-pressure system, a potent cold front, or the lifting of air over mountainous terrain. These forcing mechanisms are needed to overcome the stability typically found in a cold-weather environment.
The atmosphere must also exhibit significant instability, which is characterized by a steep lapse rate, meaning the temperature drops rapidly with increasing height. This vertical temperature profile allows a rising parcel of air to remain warmer and therefore less dense than the surrounding environment, causing it to accelerate upward. While this instability is more common in summer, in winter it is often achieved when extremely cold air aloft moves over a relatively warmer surface, such as a large body of water, leading to lake-effect thundersnow.
The Physics of Charge Generation
Non-inductive charge separation occurs within the cloud through the mechanical collision of different types of ice particles inside the rapidly rising air column. Specifically, larger, softer ice pellets known as graupel collide with smaller, lighter ice crystals.
During these numerous, high-speed collisions, electrical charge is transferred between the particles. The graupel tends to acquire a negative charge and, being heavier, falls toward the lower part of the cloud. Conversely, the lighter ice crystals acquire a positive charge and are carried aloft by the powerful updraft to the cloud’s upper reaches. This separation of positive and negative charges creates a strong electrical field that eventually becomes powerful enough to overcome the air’s insulating properties, resulting in a lightning discharge.