The idea that large, fluffy snowflakes signal the end of a snowstorm is a common piece of winter folklore. This observation often appears true, leading many to believe flake size directly signals the event is winding down. However, the connection is not direct causation, but a correlation rooted in specific meteorological conditions. Understanding the science reveals why this pattern frequently occurs.
The Conditions Required for Large Snowflakes
Large snowflakes form through aggregation, the clumping together of many smaller ice crystals as they fall. This process requires specific atmospheric conditions, most notably temperatures near the freezing point. When air temperature in the cloud layer and lower atmosphere hovers close to 32°F (0°C), ice crystals develop a thin film of liquid water on their surface.
This liquid film acts like a microscopic glue, causing the individual crystals to easily bond upon collision, forming larger aggregates. These clumps can grow quite large, sometimes reaching the diameter of a coin or larger. In contrast, very cold temperatures create dry, powdery snow where crystals are less likely to stick, resulting in many small, fine flakes.
Sufficient moisture is also necessary for aggregation and for the large flakes to grow before they hit the ground. When the air is moist and near freezing, the flakes absorb water vapor and collide with supercooled water droplets, enhancing their size. The large, heavy aggregates fall faster, which is why they sometimes appear as a brief, intense burst of heavy snow.
Is Snowflake Size a Reliable Indicator of a Storm Ending?
Large flakes frequently fall as a storm ends because the necessary conditions often occur when a storm system is exiting an area. A typical snowstorm is caused by a low-pressure system that draws in warmer air. As the system moves away, this slightly warmer air mass can briefly sweep over the location.
This temporary influx of warmer, moister air raises the temperature enough to facilitate snowflake aggregation. The shift from small, cold-weather flakes to large, wet flakes is a byproduct of this final, warming stage before precipitation ceases. Since this stage often coincides with the low-pressure system cutting off the main moisture supply, the large flakes appear right before the precipitation stops.
It is important to recognize this pattern as a correlation, not a strict rule. Large, wet flakes can also fall during the middle of a storm if a localized, temporary warm layer moves in. The size of the flake is merely an indication of the current temperature profile in the atmosphere, not a guaranteed forecast of the storm’s duration.
More Reliable Signs That a Snowstorm Is Winding Down
Forecasting the end of a snowstorm relies on tracking the movement of the entire weather system, not just the local temperature. One of the most dependable indicators that a low-pressure system is departing is a sustained rise in barometric pressure. During a storm, pressure is low, but as the system moves out, high pressure builds in, pushing the storm-causing air mass away.
A noticeable shift in wind direction also signals the storm’s conclusion. The wind will typically shift from an easterly direction to a westerly or northwesterly one as the low-pressure center passes. This change brings in a mass of colder, drier air that suppresses precipitation and marks the end of the event.
The most straightforward sign is a sustained, measurable decrease in the rate of snowfall. If the intensity begins to steadily diminish, or if the precipitation changes phase entirely, the storm is likely over. A shift from snow to ice pellets or rain indicates that the necessary cold air is no longer present in the vertical column, meaning the warm sector of the storm is dominating the area.