The arrival of May often suggests the definitive end of winter’s chill, yet cold snaps frequently occur well into late spring. This persistent cold is not random but results from predictable, large-scale meteorological forces interacting with the season’s transitional nature. Understanding why cold air continues to intrude requires looking high into the atmosphere at the powerful currents that steer our weather and examining the physical properties of the Earth itself.
How the Jet Stream Directs Cold Air
The primary mechanism for delivering unseasonable cold is the polar jet stream, a fast-moving current high in the atmosphere. It acts as the boundary between cold polar air to the north and warmer air masses to the south. Although it flows generally from west to east, its path is rarely straight, dictating where weather systems form and where air masses are directed.
When the jet stream follows a relatively straight path, known as zonal flow, it typically leads to mild, consistent weather by keeping cold air contained near the pole. However, when the jet stream develops deep, exaggerated curves, the pattern is called meridional flow. These pronounced meanders feature southward dips (troughs) and northward bulges (ridges), allowing for a significant exchange and mixing of air masses.
A deep trough in the jet stream acts like a scoop, pulling polar air masses far south into temperate regions. These southward intrusions of cold air override local seasonal warming, causing temperatures to plummet well below average. The position of these troughs and ridges determines which areas experience the cold air and which areas see unseasonably warm air surging northward.
Atmospheric Blocking and Polar Air Masses
The persistence of a cold spell often results from atmospheric blocking, which causes a weather pattern to stall over a region for days or weeks. Blocking occurs when a strong, stationary high-pressure system locks into place, disrupting the normal west-to-east flow of the jet stream. This forces the atmospheric flow to detour around the block, creating a stagnant weather pattern.
A common example is an Omega block, named because the pressure pattern resembles the Greek letter omega. In this configuration, a high-pressure ridge is sandwiched between two low-pressure troughs, forcing the jet stream to take a long, circuitous route. If the high-pressure block is positioned to the north, it can hold a cold, low-pressure system in place to the south, preventing the cold air from migrating eastward.
The air mass involved in these cold intrusions originates from the higher latitudes. While the core of the Polar Vortex typically remains near the North Pole, its boundaries can be pushed southward by a wavy jet stream. When a blocking pattern captures this polar air and keeps it from moving, the cold temperatures linger and resist the warming influence of May’s increasing daylight hours.
Why Seasonal Warming Lags Behind the Calendar
Even though solar radiation is rapidly increasing in May, the atmosphere is still susceptible to cold air intrusions because of the Earth’s thermal inertia. Thermal inertia describes the delay in temperature response to changes in energy input. May is still considered a transitional season, and the planet has not yet fully absorbed the heat energy from the stronger spring sun.
Large bodies of water, such as oceans and the Great Lakes, have a high specific heat capacity. These water masses, still cool from the winter months, absorb the sun’s energy slowly, which moderates and cools the air flowing over them. This process creates a significant seasonal lag, delaying the full warming of the atmosphere until well into the summer months.
The land surface is also slower to warm than the atmosphere, particularly where soil moisture is high or snow cover persists in the far north. Since the entire system—land, water, and atmosphere—is still recovering from the winter deficit, background temperatures remain cool. This cool, transitional state means that when the jet stream pulls a polar air mass southward, there is little resistance, resulting in a pronounced and often unwelcome cold snap.