The jet stream forms primarily because of temperature differences between air masses. The greater the contrast in temperature between the cold polar regions and the warm tropics, the stronger the jet stream. This temperature gradient creates a pressure difference, which the atmosphere balances by generating strong, fast-moving winds.

Additionally, Earth’s rotation plays a crucial role. The Coriolis effect—caused by Earth’s rotation—deflects air moving north or south, turning it to the right in the Northern Hemisphere. This results in a west-to-east flow of the jet stream.

Although we often picture the jet stream as a straight, fast-moving river of air, it’s much more dynamic and variable.

The jet stream’s location shifts with the seasons:

• In summer, the polar jet stream tends to retreat northward as the polar regions warm, and its speed weakens.
• In winter, the temperature contrast between equator and poles increases, causing the jet stream to move southward and become stronger and more active.

Rather than flowing in a straight line, the jet stream often develops ridges (northward bulges) and troughs (southward dips), creating a wavy pattern. These undulations are known as Rossby waves and they influence how long weather patterns persist over an area.

Sometimes, the jet stream can form blocking patterns, where a high-pressure system gets “stuck” and forces the jet stream to divert around it. This can lead to prolonged periods of heat, cold, or rain in one region.

Recent research suggests that  rapid warming in the Arctic, known as Arctic amplification may be weakening the polar jet stream. As the Arctic warms faster than the rest of the planet, the temperature gradient between the poles and the equator lessens. This may cause the jet stream to slow down and meander more, potentially resulting in more persistent and extreme weather patterns.

The jet stream has profound impacts on North American weather, and understanding its behavior is key to long-range forecasting.

The jet stream steers low-pressure systems (storms) and determines where they travel. A southward-dipping jet stream can direct storms across the southern U.S., bringing rain and even snow to regions that are usually dry or warm. Conversely, a northward ridge can push storm tracks into Canada, leaving much of the U.S. dry.

The position of the jet stream separates cold polar air from warmer subtropical air. When the jet stream dips southward in a trough, it can usher in Arctic air, causing cold outbreaks in the Midwest or the South. When it lifts into a ridge, warm air can surge northward, leading to heatwaves even in typically cooler areas.

The waviness of the jet stream can lead to persistent weather patterns. For example, if a trough lingers over California, it may bring repeated storms and heavy rainfall. Alternatively, a ridge stuck over the Plains could lead to drought conditions due to prolonged dryness.

Spring and early summer are peak times for tornadoes and severe thunderstorms, particularly in the central U.S. The jet stream provides the upper-level winds necessary for storm development. When it aligns with warm, moist air from the Gulf of Mexico and cool air from the Rockies or Canada, the conditions become ripe for explosive storm development.