Seeing snowflakes fall when the calendar indicates spring has begun is a common experience. This confusion arises because the date marking the start of astronomical spring—the vernal equinox—does not align with meteorological reality. While many look forward to consistent warmth, the atmosphere often retains the capacity for winter weather well into the season. Snow in March or April results from a specific convergence of atmospheric mechanics that still favors cold air and precipitation.
The Specific Atmospheric Recipe for Spring Snow
Snow can fall even when the surface air temperature is several degrees above freezing, sometimes as high as 40°F (4.4°C). The precipitation begins as snow high in the atmosphere where temperatures are well below freezing, a condition easily met even in spring. The challenge is for these flakes to survive the descent through the warmer air layer near the ground without melting completely into rain.
This survival depends on a phenomenon known as evaporational cooling. As a snowflake falls into air that is above freezing but not fully saturated with moisture, a thin layer of meltwater evaporates into the surrounding air. Evaporation requires heat energy, which is drawn from the air and the snowflake itself, causing a localized cooling effect.
This cooling process can chill the column of air from the cloud base to the ground by several degrees, sometimes enough to bring the air temperature down to freezing. Meteorologists use the wet-bulb temperature, which combines air temperature and humidity, to accurately predict the precipitation type. If the wet-bulb temperature remains near or below freezing, the snowflakes will survive the fall and reach the ground intact, even if the surface air temperature is slightly above \(32^\circ\text{F}\) (or \(0^\circ\text{C}\)).
Large-Scale Drivers of Late-Season Cold Air
The cold air necessary for spring snow results from large-scale patterns in the upper atmosphere. The Jet Stream, a fast-flowing river of air high above the surface, dictates where cold air resides. This current separates cold polar air masses from warmer mid-latitude air, but it does not always flow smoothly from west to east.
Instead, the Jet Stream often develops large, slow-moving meanders called Rossby waves. When these waves become amplified, they can buckle dramatically, allowing deep troughs of cold air to plunge far south of their typical polar boundary. This southward intrusion of frigid air creates the necessary cold reservoir for a spring snow event.
Blocking Patterns
The Jet Stream’s meandering can create a persistent feature known as a blocking pattern. An example is the Omega block, named because the pressure contours resemble the Greek letter Omega (\(\Omega\)). This pattern features a high-pressure system sandwiched between two low-pressure systems. These blocks are common in the spring and can stall the normal progression of weather systems for days or weeks, locking unseasonably cold air in place.
Polar Vortex Displacement
The weakening or displacement of the Polar Vortex, a large area of cold, low-pressure air circulating over the Arctic, can also bring late-season cold. A significant warming event in the stratosphere can disrupt this vortex, pushing fragments of the frigid air mass southward. While the resulting air mass is not as intensely cold as a mid-winter event, it is often sufficient to produce widespread spring snowfall when combined with an active storm system.
Characteristics and Impact of Spring Snowfall
The unique characteristics of spring snow relate directly to the warmer air temperatures it falls through and the higher sun angle. This snow is typically “wet snow,” meaning it has a high liquid water content. It often has a snow-to-liquid ratio closer to 5:1 rather than the 10:1 ratio common for drier, mid-winter snow.
This high water content makes the snow significantly heavier, impacting the environment. The wet, sticky flakes adhere easily to surfaces, including tree branches that may have already begun to bud or leaf out. The cumulative weight of this heavy snow can exceed the structural capacity of the branches, leading to widespread limb breakage and damage to deciduous trees.
Spring snow is especially hazardous for early-blooming agricultural crops and fruit trees. A sudden, sharp freeze accompanying the snow can kill flower buds, resulting in the loss of that season’s fruit, such as peaches, cherries, or apples. Even without a severe temperature drop, the sheer weight of the snow can damage delicate emerging foliage and flowering parts.
Despite the disruptive nature of spring snow, its persistence is usually low due to the increasing intensity of solar radiation. As the season progresses, the sun rises higher in the sky, causing a more direct angle of incoming solar energy compared to mid-winter. This higher sun angle delivers more energy to the ground and the snowpack, causing rapid melting even when air temperatures remain near or below freezing.