Snowfall regularly occurs over the sea, particularly in polar regions and mid-latitudes during winter. However, the conditions that govern the formation, descent, and ultimate survival of snowflakes over a liquid surface are different from those found over land. The ocean’s unique thermal properties introduce complexities that determine when and where frozen precipitation actually reaches the water.
The Basic Requirements for Snow Over Water
Snow formation requires sufficient moisture and air temperatures at or below the freezing point of water. Snow begins as ice crystals high in the atmosphere. For it to reach the ocean as snow, the air temperature must remain cold enough throughout the entire column of air from the cloud to the sea surface. The altitude at which the temperature drops to 0°C (32°F) is known as the freezing level.
For snow to survive the journey downward, the freezing level must be very close to the surface. Even if the temperature at the surface is slightly above freezing, snow may still fall without completely melting due to evaporative cooling. As the snowflakes begin to melt, the phase change draws heat from the surrounding air, which cools the air around the flake and slows the melting process. Generally, if the air temperature at the surface is warmer than 5°C (41°F), snow is highly unlikely to reach the water without turning completely into rain.
How Ocean Temperature Affects Snowfall
The immense body of water acts as a thermal regulator, significantly influencing the potential for snowfall. The ocean retains heat much longer than land and is often warmer than the air mass moving over it during winter. This temperature difference drives the transfer of latent heat and moisture from the sea surface into the atmosphere, a process relevant to “sea-effect” snow.
This oceanic influence can both enhance and inhibit snowfall. The evaporation of warm water vapor provides the moisture necessary to form heavy snow clouds, similar to lake-effect snow. When a very cold air mass sweeps over warmer ocean water, the air rapidly absorbs moisture and rises, creating intense, localized snow bands. This can lead to significant snowfall over the open water or on downwind coastlines.
Conversely, the heat released by the ocean often keeps the lowest layer of the atmosphere just above the freezing mark. This warmer air near the surface acts as a melting layer, causing ice crystals to partially or completely melt into rain before they reach the water. While the ocean supplies the moisture for heavy snow, its warmth can also destroy the precipitation it helped create. Consistent ocean snowfall occurs when the water surface is near freezing, such as in polar seas, minimizing the melting layer effect.
Distinguishing Snow from Other Icy Maritime Events
Mariners often encounter several forms of frozen moisture distinct from true snow, which is composed of crystalline ice flakes. Graupel, sometimes called soft hail or snow pellets, forms when a snowflake collects supercooled water droplets that freeze onto it, resulting in a soft, round pellet. Another form is freezing rain, which falls as liquid water but freezes immediately upon contact with any surface that is at or below freezing, creating a layer of glaze ice.
A specific hazard at sea is freezing spray, which is not precipitation falling from the sky but a product of water and wind conditions. Freezing spray forms when strong winds lift sea spray from wave crests into the air, and the resulting supercooled saltwater droplets freeze instantly upon hitting a vessel or structure. This phenomenon requires cold air, typically below -1.7°C (29°F), cold water, and high wind speeds. The resulting ice accumulation rapidly builds up on decks and equipment, posing a stability risk to ships.