A cold summer almost always comes down to the jet stream, the high-altitude river of wind that separates polar air from warmer tropical air. When the jet stream dips unusually far south and gets stuck in that position, it funnels cold, dense Arctic air into regions that would normally be warm. This can last days or even weeks, making an entire summer feel off.
The frustrating part is that a cold summer in your area doesn’t mean the planet has cooled down. Global temperatures have continued to rise, with North America recording its fourth-warmest year on record in 2025 and Europe its second-warmest. What feels like a cold summer is typically a redistribution of heat, where one region gets stuck under cooler air while another bakes.
How the Jet Stream Creates Cold Summers
The jet stream flows west to east at roughly 30,000 to 40,000 feet, powered by the temperature contrast between warm air near the equator and cold air near the poles. When that contrast is strong, the jet stream blows fast and relatively straight. When it weakens, the jet stream slows down and develops large north-south waves, like a river meandering across a flat plain.
Those waves matter enormously for your weather. A southward dip in the jet stream pulls polar air deep into mid-latitude areas, dropping temperatures well below seasonal norms. A northward bulge does the opposite, pushing warm air into places that don’t usually see it. The key problem during a cold summer is that these waves can become nearly stationary, parking cold air over the same region for weeks at a time. Meteorologists call this a “blocking pattern” because it literally blocks the normal west-to-east flow of weather systems.
One common version is the Omega block, named because it looks like the Greek letter omega on a weather map. It features a dome of high pressure flanked by two areas of low pressure on either side. If you’re under one of those low-pressure zones, you get persistent cold, clouds, and rain. If you’re under the high-pressure dome in the middle, you get clear skies and warmth. Two cities a few hundred miles apart can have completely different summers because of where the block sits.
Arctic Warming Makes the Jet Stream Wobblier
The Arctic is warming roughly two to four times faster than the rest of the planet, a phenomenon called Arctic amplification. This shrinks the temperature difference between the poles and the tropics, which is the very engine that drives the jet stream. When that engine weakens, the jet stream slows and its waves grow larger and more persistent.
Three leading hypotheses explain how this plays out in summer. First, weakened storm tracks mean weather systems move through more slowly, so any cold spell lingers longer than it used to. Second, the jet stream itself can shift position, steering the usual track of storms and fronts into areas that wouldn’t normally see them. Third, the large, slow-moving waves embedded in the jet stream can amplify, creating deeper southward plunges that trap cold air farther from the poles.
This same mechanism can also produce extreme heat. When the wave pattern locks a northward bulge over a region, that area bakes. So paradoxically, the same atmospheric setup that gives you a cold summer can give your neighbor a record-breaking heat wave. The issue isn’t that the atmosphere is producing more cold overall. It’s that temperature extremes of both kinds are becoming more persistent because the jet stream is less efficient at mixing air between latitudes.
Ocean Cycles Play a Role Too
The tropical Pacific Ocean cycles between El Niño (warmer than average surface water), La Niña (cooler than average), and neutral conditions. These shifts influence weather patterns across the globe. After a brief La Niña in late 2024 and early 2025, the tropical Pacific returned to neutral conditions by spring 2025, and forecasters expect it to stay neutral through the Northern Hemisphere summer. Neutral conditions don’t push temperatures strongly in either direction, which means other factors like the jet stream and regional oscillations have more influence on whether your summer runs hot or cold.
In the North Atlantic, a pattern called the North Atlantic Oscillation has a summer mode that directly affects weather across the UK, Scandinavia, and parts of eastern North America. During its negative phase, the storm track shifts about 10 degrees of latitude southward, bringing cloudy, wet, and cooler conditions to northwest Europe. High pressure builds over Greenland while cyclonic (stormy) conditions settle over the British Isles and Scandinavia. If you’re in those areas and your summer has been gray and rainy, a negative summer NAO is a likely culprit.
Volcanic Activity and Upper-Atmosphere Effects
Major volcanic eruptions have caused some of the most dramatic cold summers in history. The most famous example is the “Year Without a Summer” in 1816, triggered by Indonesia’s Mount Tambora. That eruption, the largest in recorded history, ejected massive amounts of ash and tiny particles into the stratosphere, where they spread around the globe and blocked enough sunlight to drop Earth’s average temperature by about 3°C. Snow fell in New England that summer. Cold, gloomy rain blanketed Europe for months.
The 2022 eruption of Hunga Tonga-Hunga Ha’apai in the South Pacific was unusual because instead of mainly releasing particles that block sunlight, it injected enormous amounts of water vapor into the stratosphere. This produced stratospheric cooling of 0.5 to 1.0°C between 2022 and mid-2023, with even stronger cooling higher up in the mesosphere afterward. However, that cooling was concentrated in the upper atmosphere rather than at the surface where you’d feel it. The eruption’s effects on day-to-day summer weather have been modest compared to a Tambora-scale event, but they’ve added a subtle layer of complexity to the atmosphere’s behavior over the past couple of years.
Why Your Experience Doesn’t Match the Headlines
Global temperature records measure the average across the entire planet, including the oceans. A record-warm year globally can still contain pockets where individual regions ran cooler than normal for stretches of weeks or months. Your summer weather is determined by what’s happening in the atmosphere directly above you, and that’s driven largely by the position of the jet stream, the phase of regional ocean patterns, and whether a blocking pattern has settled over your area.
Cold summers are also partly a matter of perception. If the previous few summers were unusually hot, a return to near-normal temperatures can feel cold by comparison. And a summer doesn’t need to be uniformly cold to feel that way. A few weeks of persistent below-average temperatures during June or July, even if August recovers, can color your memory of the entire season.
The practical takeaway is that a cold summer is almost always a regional event driven by temporary atmospheric patterns, not a sign that global warming has reversed or that the climate has fundamentally shifted. The same jet stream dynamics that parked cold air over your region are likely pushing abnormally warm air somewhere else. It’s a redistribution problem, not a temperature problem, and understanding that distinction helps make sense of why record global heat and your chilly backyard barbecue can coexist in the same season.