Snowfall in spring is common enough in many temperate climates that it has become an annual expectation for some regions. The timing of this late-season snowfall often depends less on the calendar date and more on the precise atmospheric conditions aligning above a specific location. The definition of when “spring” begins is complex, which further complicates the public perception of what constitutes an anomalous weather event.
Defining the Seasons and Snowfall
The calendar date that signals the start of spring is not universally agreed upon, as meteorologists and astronomers use different systems. Astronomical spring begins with the vernal equinox, which occurs around March 20th in the Northern Hemisphere and is based on the Earth’s orbit. This date can vary slightly each year. Meteorological spring, however, is a fixed period encompassing March, April, and May in the Northern Hemisphere. This standardized definition allows scientists to consistently calculate and compare seasonal weather statistics, making it the more practical measure for analyzing late-season snow events. For snow to physically form and reach the ground, the air temperature must be cold enough, but this requirement is better understood through the concept of wet-bulb temperature. This measurement accounts for both the air temperature and the humidity, determining the lowest temperature air can reach through evaporative cooling. Snow generally requires a wet-bulb temperature near or below the freezing point to sustain the ice crystals as they fall.
Meteorological Conditions That Permit Late Snow
Late-season snow is the result of a specific and temporary reintroduction of winter-like air masses into an otherwise warming environment. One of the primary drivers is cold air advection, the horizontal transport of cold air into a region of warmer air, typically pulled down from polar latitudes. This intrusion of cold, dense air lowers the temperature throughout the atmospheric column, setting the stage for frozen precipitation.
The necessary atmospheric lifting mechanism is often provided by an upper-level trough, an elongated area of relatively low pressure high in the atmosphere. Ahead of this trough, winds spread out and diverge, creating a void that air from the surface must rise to fill. This large-scale upward motion lifts the moist, low-level spring air into the colder altitude layers. As this air rises, it expands and cools rapidly in a process called adiabatic cooling. If the upward motion is strong enough, the air mass will cool past the freezing point, allowing the moisture to condense and freeze into snow crystals. The juxtaposition of a deep upper-level trough with a moisture-rich, low-level air mass is the recipe for a significant late-season snowfall, even if the surface temperature is slightly above freezing. These events are temporary “cold snaps” because the sun angle is already high in the spring sky, quickly warming the atmosphere once the cold air mass moves away.
How Geography Influences Spring Snow Events
The likelihood and duration of spring snow are significantly modulated by local geographical features. Latitude is a major factor, with higher latitudes experiencing spring snow as a routine continuation of winter weather rather than an anomaly. Conversely, mid-latitude regions are prone to the most dramatic late-season shifts, where an early warm spell is abruptly followed by a cold air intrusion.
Elevation plays a role, as temperatures naturally decrease with increasing height, making mountains effective cold-air reservoirs. Even in temperate zones, snowpack can linger on high-elevation slopes well into late spring or early summer. Mountain ranges also induce orographic lift, forcing air masses to rise and cool as they encounter the terrain, which enhances precipitation and favors snow formation if temperatures are cold enough.
Proximity to large bodies of water, such as the Great Lakes, can also extend the snow season due to the lake-effect mechanism. This effect can re-engage in spring if a cold air mass moves over the still-cold lake water, which remains chilled from the winter months. The temperature difference between the water and the air provides the necessary moisture and instability for localized, heavy snow bands to form downwind.
Impact on Plant Life and Ecosystems
A late-season snow event introduces both risks and benefits to the newly emerging plant life. The most significant threat comes from the accompanying “killing frosts,” which can occur when temperatures drop below freezing after plants have broken dormancy. Tender new growth, such as fruit tree blossoms, young shoots, and emerging leaves, are vulnerable to cell damage from freezing, which can result in extensive agricultural losses.
The physical weight of heavy, wet spring snow is also damaging, as it can snap branches weighed down by new foliage. However, snow itself can serve a beneficial function by acting as a temporary insulator for dormant ground cover. The air pockets within the snow blanket prevent heat from escaping the soil, protecting seeds and shallow roots from the extreme cold of the air mass above. When the snow melts, it provides a slow-releasing source of moisture that is beneficial for soil saturation and plant growth at the start of the season.