The experience of unseasonably cold weather in June often feels confusing, especially when summer should be starting. This temporary departure from expected seasonal warmth is a common atmospheric phenomenon known as a negative temperature anomaly. Meteorology explains that this is driven by large-scale shifts in atmospheric circulation patterns. These patterns are the primary drivers that transport cold air masses far south of their typical northern boundaries.
The Meandering Path of the Jet Stream
The upper atmosphere contains a powerful, fast-moving river of air known as the polar jet stream, which is the primary steering mechanism for weather systems across the mid-latitudes. This current flows from west to east, typically positioned over Canada and the northern United States during summer. The jet stream forms where the cold, dense Arctic air mass meets the warmer, lighter air from the south.
The jet stream’s path constantly meanders, creating large atmospheric waves called Rossby waves. When the jet stream develops a deep, southward dip, it is called a trough, which acts as a channel for cold air. A large trough can curve unusually far south into areas expecting warmer temperatures. This deviation allows frigid air masses, normally confined to higher latitudes, to penetrate deep into temperate regions, resulting in the unseasonable June chill.
Atmospheric Blocking and Cold Air Intrusion
The persistence of cold weather is often due to atmospheric blocking. This occurs when a stable, high-pressure system—called a blocking high or anticyclone—becomes stationary and halts the normal west-to-east progression of weather systems. The blockage forces the jet stream to take an exaggerated, amplified path around the obstruction.
When a blocking pattern sets up, it can lock the jet stream into a deep, southward-plunging trough for days or weeks. This sustained configuration continuously draws cold air masses from the north, preventing the typical seasonal warming trend. The source is often a persistent pool of Arctic air, pulled southward by the sustained pressure patterns. The longevity of the block converts a brief cold snap into a prolonged period of unseasonable temperatures.
The blocking high acts as a traffic jam in the atmosphere, rerouting the flow and keeping the cold air entrenched over a specific region. The stubborn nature of the June cold is explained by the combination of the jet stream delivering the cold air and the atmospheric block maintaining that pathway. These systems are synoptic-scale features, meaning they cover vast geographical areas, and their stability leads to extended periods of the same weather.
Weather Anomalies Versus Climate Trends
Cold weather in June is an example of a weather anomaly, which is a short-term departure from the average conditions expected for a specific location. Weather refers to the state of the atmosphere over hours or days, including temperature, precipitation, and wind. A negative temperature anomaly means the measured temperature is cooler than the long-term baseline average, typically calculated over a 30-year period.
These cold snaps are fluctuations within the Earth’s natural weather variability and do not negate the broader, long-term warming of the global climate. Climate, in contrast to weather, is defined by the statistical patterns of weather over decades. While the global average temperature shows an upward trend, this warming does not mean that every location will be warmer than average every day.
Unseasonably cold periods are natural occurrences within a complex and variable climate system. They represent a temporary deviation from the expected seasonal norm, driven by localized atmospheric mechanics. Viewing these events through the lens of short-term weather variability provides the context needed to understand why a specific region can be cold in June, even as the global climate continues to warm.