The idea that the first widespread fog of the season predicts the arrival of snow within a fixed number of days is a common piece of weather folklore. People often recall a specific number, such as 90 or 100 days, linking the appearance of dense, low-lying moisture to the first significant snowfall. This popular belief suggests a predictable meteorological link between two distinct atmospheric events. To determine the scientific validity of this adage, we must examine the specific atmospheric requirements for both fog and snow formation. This analysis will clarify why the connection is more of a seasonal coincidence than a reliable forecast mechanism.
The Origin of the Adage
These weather predictions are rooted in a long tradition of folk meteorology, arising from generations of seasonal observations. Before modern forecasting tools existed, people who depended on the weather for farming or travel used noticeable natural cues to anticipate the coming seasons. The appearance of a dense fog in the late autumn or early winter signaled that the air was becoming cool and moist enough to support winter conditions.
This wisdom is based on pattern recognition, not on a physical cause-and-effect relationship between the two events. Observers noted that after certain atmospheric conditions—like the first deep fog—the broader seasonal shift toward colder air masses was underway. The predictions often vary by region, with some folklore tying August fogs to the number of snowy days in the subsequent winter. These adages serve more as markers of the changing season than as accurate meteorological forecasts.
Atmospheric Conditions for Fog
Fog is essentially a cloud that forms at ground level, and its formation requires the air temperature to cool down to the dew point, causing water vapor to condense into tiny droplets. The two types of fog most relevant to the transition into winter are radiation fog and advection fog.
Radiation fog forms on clear, calm nights when the ground rapidly loses heat through radiation, cooling the layer of air immediately above it. This process often creates a temperature inversion, trapping the cold, moist air near the surface, which requires very light wind conditions to remain stable.
Advection fog, by contrast, forms when a mass of warm, moist air moves horizontally over a much colder surface. As the air passes over the cold surface, its temperature drops until saturation is reached. Both types of fog depend on high moisture content and a mechanism to cool the air to its saturation point, but they typically form under stable, non-stormy conditions.
Atmospheric Conditions for Snow
Significant snowfall requires three primary elements: a sufficient source of moisture, temperatures below freezing throughout the atmospheric column, and a lifting mechanism. The moisture is supplied by a large-scale weather system, such as a low-pressure system drawing humid air into the region. Temperatures must be at or below the freezing point from the cloud base down to the ground to ensure the ice crystals survive the descent.
The most critical difference from fog is the need for atmospheric lift, which forces air to rise, expand, and cool, leading to cloud formation and precipitation. This lift can be caused by a frontal system, where an advancing cold air mass wedges under warmer air, or by orographic lifting, where wind is forced up the side of a mountain. These dynamic, large-scale systems are fundamentally different from the calm, stable air required for most fog formation.
Why the Correlation Is Often Coincidental
The core reason the fog-to-snow adage fails as a reliable prediction lies in the conflicting atmospheric stability required for each phenomenon. Radiation fog, one of the most common types of pre-winter fog, depends on calm, stable air and a local cooling mechanism. Significant snowfall, however, requires the strong vertical motion and large-scale atmospheric instability provided by a powerful storm system.
A fog event signals that the air mass has become sufficiently moist and cold for winter, but it does not create the necessary conditions for a storm. The moisture that forms the fog is usually confined to the lowest layer of the atmosphere and is often too shallow to fuel a major snow event. The first snow ultimately depends on the arrival of a major weather system, which could be days, weeks, or even months after a fog event. While both require high moisture, the dynamic nature of a snowstorm is a complete break from the static conditions of a typical fog.