In Short
- Guinness World Records credits a report from Fort Keogh, Montana, in 1887 describing snowflakes up to about 15 inches (38 cm) wide and several inches thick.
- The measurement comes from eyewitness accounts by a rancher and local observers, not from a preserved specimen or photograph.
- Scientists say very large “flakes” can form when many smaller snow crystals clump together, but 15 inches would be extraordinary and is hard to verify after the fact.
- So, the Montana monster snowflake is best seen as a famous anecdote that may be exaggerated, even if giant clumped flakes themselves are possible.
The Fort Keogh Story
The record goes back to January 1887, near a U.S. Army post called Fort Keogh in what is now eastern Montana. During a winter storm, a rancher named Matt Coleman reportedly saw snowflakes “larger than milk pans” and claimed that one measured about 15 inches across and around 8 inches thick.
Newspapers repeated the story, and it eventually found its way into record books.
Today, Guinness still lists the Fort Keogh storm as producing the largest natural snowflakes ever reported.
Could A Snowflake Really Be That Big?
When you picture a snowflake, you probably imagine a single star shaped ice crystal, only a few millimeters wide. But what people call “snowflakes” in everyday life are often clusters of many individual crystals stuck together. Under the right conditions, those clusters can grow several centimeters across.
Atmospheric scientists at North Carolina State University have pointed out that very large aggregates are theoretically possible if humidity is high and flakes collide and stick as they fall. They also note that measuring something soft and fragile during a storm, with nineteenth century tools and no photography, introduces a lot of uncertainty.
Legend, Record, Or Both?
The Fort Keogh story sits in an odd place between science and folklore. There is no preserved sample or instrument reading, only testimony recorded long after the storm. Yet the report has been repeated by record keepers and popular science writers for decades, turning it into a kind of official tall tale.
A safe takeaway is that very large, clumped flakes do occur, and that the Montana storm probably produced some memorable monsters. Whether one of them really reached a full 15 inches is a question we cannot definitively answer now.