Snow can fall during summer months, though it’s uncommon in most populated areas. While many associate summer with warm temperatures and sunshine, specific geographical and atmospheric conditions can lead to snowfall even in the midst of the warmer season. This phenomenon is an exception to typical seasonal weather patterns.
Defining Summer and Snowfall
Summer can be understood in two main ways: astronomically and meteorologically. Astronomical summer in the Northern Hemisphere typically begins around June 20 or 21 with the summer solstice and concludes with the autumnal equinox in late September. In contrast, meteorological summer in the Northern Hemisphere spans June, July, and August, aligning with the warmest quarter of the year.
For snow to form and fall, specific atmospheric conditions are required. Snow develops when the air temperature is at or below freezing, which is 0°C or 32°F. Moisture must also be present in the atmosphere for ice crystals to form and cluster into snowflakes. While surface temperatures can be slightly above freezing, snow will generally not form if the ground temperature is at least 5°C (41°F).
Locations Where Summer Snow Occurs
Summer snowfall is primarily observed in specific regions globally, often due to their unique geographical characteristics. High-altitude mountainous areas are common locations. Examples include the Rockies in North America, particularly in places like Glacier National Park in Montana and parts of the Sierra Nevada mountains in California, where snow can persist or fall anew into July. The Alps in Europe, including areas like Stelvio Pass in Italy and Chamonix in France, also experience summer snow.
Other major mountain ranges such as the Himalayas and the Andes in South America, including regions in Argentina and Chile, are known for summer snowfall. Even parts of Africa, like the Drakensberg mountains in South Africa, can see snow during their winter months, which align with the Northern Hemisphere’s summer. Beyond mountains, polar and sub-polar regions, such as the Arctic Circle, experience snow throughout the year, even during their summer, though precipitation may also fall as rain further south within these areas. Australia and New Zealand also have alpine landscapes, like Australia’s Snowy Mountains and New Zealand’s Southern Alps, where snow falls during their winter season, which corresponds to the Northern Hemisphere’s summer.
Meteorological Factors for Summer Snow
Several meteorological factors combine to create the unusual conditions for summer snow. One significant factor is cold air advection, which involves the horizontal transport of colder air into a region. This can occur behind a cold front, bringing unseasonably cold air masses into an area that would typically be warm.
High altitude cooling plays a substantial role, as temperatures naturally decrease with elevation. Air thins at higher altitudes, becoming less efficient at retaining heat, leading to colder conditions where snow can form and persist. For instance, the temperature typically drops by about 0.7 degrees Celsius for every 100 meters of altitude gained. This allows for freezing levels to exist high in the mountains even when lower elevations experience warm summer temperatures.
Orographic lift is another key mechanism, particularly in mountainous regions. This occurs when moist air is forced to rise as it encounters a mountain barrier. As the air ascends, it expands and cools, causing the moisture within it to condense and form clouds, leading to precipitation. If temperatures are low enough at these higher elevations, this precipitation falls as snow.
Unstable atmospheric conditions, such as strong low-pressure systems or frontal passages, can also contribute to summer snow. Cold fronts, for example, involve a colder air mass displacing a warmer one, forcing the warm air to rise, cool, and potentially produce precipitation. These dynamic weather patterns can bring the necessary cold air and moisture together, defying typical summer warmth.