What is Thundersnow? Boston's White Christmas Brings Snow, Thunder and Lightning

About three inches of snow was expected to fall on the city of Boston on Monday, but residents were greeted with a bit of thunder and lightning too on Christmas morning.

Several people on Twitter expressed their shock to hear thunder while it was snowing. Although uncommon, Boston's thundersnow on Monday wasn't a first for Massachusetts. Back in February, thunder was heard during Winter Storm Niko.

Thundersnow—a weather event in which thunder and lightning occur while it's snowing— can develop any time the air near the ground is below freezing and rises to the shallow layer of a snow cloud, according to the National Weather Service (NWS). During a thunderstorm, air rises all the way to a cloud's top layer but the activity during thundersnow occurs in a cloud's shallowest layer about 20,000 feet from the ground and 5,000 feet thick in cloud density.

Similar to snowstorms, thundersnow develops in flat, layered snow clouds. However, during thundersnow events, snow clouds develop upward bumps called turrets that rise above the rest of the flat snow cloud. As the turrets move above the rest of the snow cloud, cold winds in the upper layer begin to cool the air. The cooling of the air above the initial snow cloud along with turrets rising above the flat layer of the cloud causes the snow cloud to become unstable.

This instability, referred to as "elevated convection," can result in thunderstorms or showers, but if a cloud's temperature is below 14 degrees, snowflakes will fall instead of rain. And as the particles start to interact, they can create electrical charges, resulting in thunder and lightning.

In Massachusetts, lightning strikes and bouts of thunder were detected near Needham, Wellesley and Newton around 9:30 a.m. ET. NWS confirmed the phenomenal event on Twitter.

"Thanks for all your reports of thunder snow this morning. We have seen the lightning on lightning detection, mainly in clouds but a few cloud to ground near and W/NW of Boston," NWS wrote.