Chilly temperatures and frost warnings in late May often clash with the expectation of consistent spring warmth. This unexpected cold snap is not random, but results from temporary shifts in the atmosphere’s large-scale circulation patterns. The science behind this unseasonable cold involves an interaction between high-altitude air currents and polar air masses.
The Primary Meteorological Driver: Jet Stream Position
The polar jet stream is the atmosphere’s primary mechanism for separating warm and cold air. This fast-flowing river of wind is driven by the temperature contrast between the cold polar regions and the warmer tropical latitudes. When this difference is large, the jet stream flows in a relatively straight path from west to east, keeping cold air near the pole.
Late spring cold snaps occur when the jet stream develops a large, deep southward dip, known as a trough. This trough disrupts the typical zonal (west-to-east) flow, causing the jet stream to become more wavy or meridional (north-to-south). This amplified pattern temporarily allows the cold air mass normally confined to the Arctic to move far south into lower latitudes.
The Source of the Cold: Arctic Air Intrusion
The air mass responsible for the May chill originates in the Arctic, often representing remnants of the tropospheric polar vortex. The polar vortex is a large area of cold air and low pressure that swirls continually over the North Pole. Even as spring progresses, the polar region retains a significant amount of cold air.
The southward movement of this cold air is often facilitated by an atmospheric block, a high-pressure system that stalls over areas like Greenland or the North Atlantic. This blocking pattern acts like a barrier, forcing the jet stream and the Arctic air mass to take a sharp detour south. This process, known as an Arctic air intrusion, carries dense, frigid air into regions already experiencing seasonal warming. The air retains enough chill to cause frost, even in May.
Historical Patterns of Late Spring Cold Snaps
Sudden cold spells in May are not new; such events have been observed for centuries during the transition from spring to summer. These recurring periods of unseasonable cold are sometimes referred to as meteorological singularities. In European folklore, a historically recognized cold spell often occurs around May 11th through the 15th, linked to the feast days of the “Ice Saints.”
Farmers historically avoided planting sensitive crops until after this mid-May period to protect them from the last potential frost. This persistent pattern results from atmospheric instability during the spring. The large temperature difference between rapidly warming landmasses and still-cold oceans contributes to highly fluctuating weather patterns and large temperature swings. The existence of these traditional proverbs suggests that late-season cold fronts are a common feature of the Northern Hemisphere’s climate variability.
Weather Variability Versus Long-Term Climate Trends
An isolated cold snap in May is a classic example of weather variability, describing short-term fluctuations in atmospheric conditions over days or weeks. This is distinct from climate change, which refers to the long-term shift in average temperatures and weather patterns over decades. A single cold event does not negate the overall trend of global warming, characterized by rising global average temperatures.
Some research suggests that a warming Arctic, a feature of climate change, may contribute to a weaker and more erratic jet stream. A less stable jet stream can lead to more extreme “wobbles,” potentially increasing the frequency of both unseasonable cold plunges and extreme heat waves. While the overall frequency of cold extremes is decreasing globally, an Arctic air intrusion remains possible due to the complex, wave-like dynamics of the atmosphere.