Snow thunder, formally known as thundersnow, is a rare meteorological event where lightning and thunder occur during a heavy snowstorm. This phenomenon is a wintertime thunderstorm where the precipitation falling to the ground is snow instead of rain. While the physics of electrical discharge are consistent with typical summer storms, the atmospheric conditions required in freezing weather are highly specific and unusual. Because the winter atmosphere generally lacks the widespread energy of warmer seasons, thundersnow is an uncommon occurrence.
The Atmospheric Ingredients Required
For thundersnow to develop, the atmosphere must possess a strong, shallow layer of vertical instability, known as convection, even with surface temperatures below freezing. This instability is often created by an extremely steep environmental lapse rate, meaning the air temperature decreases very rapidly with altitude. This rapid cooling of air aloft provides the driving force that pushes air parcels upward.
This setup is most frequently observed in two distinct scenarios: intense synoptic-scale winter storms, such as powerful Nor’easters, or within heavy lake-effect snow bands. In the lake-effect scenario, frigid air flows across the relatively warmer water of a large body of water. The temperature difference between the warm water surface and the air at about 1,500 meters (the 850 hPa level) must be significant, often 25° Celsius or more, to generate the necessary upward motion.
The rapid transfer of heat and moisture from the warm water surface into the overlying cold air mass triggers explosive vertical development, forming tall, narrow cumulonimbus clouds. These clouds are the engines of all thunderstorms and still extend high enough to contain the necessary ice particles, though they are usually shallower than summer counterparts. The strong updrafts within these clouds concentrate the energy into a localized, intense burst of heavy snow and electrical activity.
In contrast, thundersnow in strong Nor’easters is associated with a narrow band of intense precipitation near the center of the low-pressure system. This lift is provided by large-scale atmospheric dynamics, such as the advection of warm, moist air over the cold surface. Regardless of the trigger, the common factor is the presence of vigorous, localized vertical air movement strong enough to initiate the electrification process within the cloud.
How Charge Separation Occurs in Snow
The generation of lightning relies on the same non-inductive charging mechanism found in summer thunderstorms, where collisions between ice particles cause a separation of electrical charge. Within the mixed-phase region of the snow cloud (roughly 0°C to -40°C), three types of ice particles coexist: small ice crystals, snowflakes, and soft hail known as graupel. Graupel forms when supercooled water droplets freeze onto an existing ice crystal.
The strong updrafts cause lighter, positively charged ice crystals to be carried upward toward the cloud top. Simultaneously, the heavier, negatively charged graupel particles fall toward the lower and middle sections. These differential movements result from countless high-speed collisions between the particles.
During a collision, the graupel particle acquires a negative charge, while the lighter ice crystal gains a positive charge. This continuous process physically separates the positive and negative charges into distinct regions within the cloud: positive charge accumulates at the top, and negative charge forms a main pocket below.
Once the electrical potential difference between these charged regions or the cloud and the ground becomes too great, the air’s insulating capacity is overcome. This results in a rapid discharge of electricity observed as lightning. The lightning superheats the surrounding air, causing it to expand explosively and creating the shockwave perceived as thunder.
The Unique Auditory Experience
A distinguishing feature of thundersnow is the unique auditory experience, often described as a muffled, subdued, or low-frequency rumble. Unlike the sharp crack of a summer thunderstorm, the sound rarely travels more than a few miles from the lightning strike. This rapid dampening is primarily due to the physical characteristics of the snow itself.
Falling snowflakes and the dense layer of snow on the ground act as highly effective acoustic absorbers. Snow is composed mostly of air trapped within intricate ice structures, which efficiently scatters and absorbs sound waves, preventing them from traveling far.
Furthermore, the atmosphere during a winter storm often features a temperature inversion or a stable layer of cold air near the surface. Sound waves tend to refract away from warmer air toward colder air. Since the air near the ground is typically colder than the air higher up, the sound waves travel upward and away from the surface. This combination of acoustic absorption and atmospheric refraction results in the characteristic quiet, close-range rumble of thundersnow.