A snowflake is not a single ice crystal but an aggregation of many smaller crystals that have collided and stuck together during their fall through the atmosphere. Most snowflakes are small, often less than half an inch across, but conditions occasionally align to form massive aggregates. The quest to confirm the dimensions of the largest example ever observed has become a persistent point of fascination in meteorology.
The Record-Breaking Snowflake
The largest snowflake ever recorded is cited in major meteorological reference books and record-keeping organizations as having fallen at Fort Keogh, Montana, on January 28, 1887. A ranch owner named Matt Coleman reported the immense size of the flake, which he later described in the Monthly Weather Review Magazine. Coleman claimed the snowflake was 15 inches (38 cm) wide and 8 inches (20 cm) thick, a dimension often compared to the size of a dinner plate or a milk pan of the era.
This historical claim has stood as the universally recognized record for the largest aggregate snowflake for over a century. The account notes that Coleman observed multiple flakes of a similar, exceptionally large size during the same severe storm. The record is a testament to the potential for extreme natural phenomena under the right atmospheric conditions.
Meteorological Conditions for Massive Snow Aggregates
The formation of these massive snowflake aggregates depends on a specific and delicate combination of atmospheric factors. The most important condition is a temperature profile near the freezing point, often slightly above \(0^\circ\text{C}\) near the surface. This warmer air causes the edges of the falling ice crystals to partially melt, creating a thin, sticky film of water that acts like a glue.
This “stickiness” allows individual crystals to collide and bond together into a larger structure, a process called aggregation. A high moisture content in the air is also necessary to fuel the growth of the initial crystals and sustain the aggregation process as the flake descends. Crucially, the atmosphere must exhibit very low wind shear and turbulence, which prevents the fragile, loosely bound aggregate from being torn apart before it reaches the ground.
The Challenges of Measuring and Verifying Giant Snowflakes
The 1887 record, while officially recognized, presents significant challenges for modern scientific verification. The primary difficulty lies in the inherent fragility of large snowflake aggregates, which are only loosely held together and can disintegrate instantly upon hitting the ground or encountering a slight breeze.
Furthermore, the rapid melting rate of a large, wet aggregate makes precise measurement nearly impossible, especially with the rudimentary tools available in the 19th century. The 15-inch measurement was an eyewitness account, likely taken of the aggregate’s widest dimension before it had a chance to settle or melt. Modern meteorology relies on specialized equipment like the Multi-Angle Snowflake Camera (MASC) or laser disdrometers to capture images and dimensions of falling snowflakes. These modern instruments, which provide verifiable data, typically record the largest aggregates in the range of 1.5 to 2 inches (3.8 to 5 cm) across.
This contrast highlights the gap between historical observation and scientific documentation. While the size claimed in 1887 stretches the bounds of credibility for some scientists, the record remains in place because contemporary accounts supported the rancher’s astonishing claim.