Jet Stream
A jet stream is a relatively narrow band of strong wind in the upper atmosphere, typically around 30,000 feet, that blows from west to east. The polar jet sits between 50° and 60° latitude and the subtropical jet near 30°. Jet streams follow the boundaries between cold and warm air and steer the storm systems below them.
Why It Matters
The jet stream follows the boundary between cold and warm air. In the Northern Hemisphere, a location north of the polar jet is on the cold side and a location south of it is on the warm side, so when the jet dips south, cold air comes with it.
Jet streams exist because of temperature contrast. The contrast is greatest near 30° and between 50° and 60° latitude, and the stronger the temperature difference, the stronger the wind. Earth's rotation turns that flow from west to east.
Jet Stream Facts
| Property | Value |
|---|---|
| Height | 4 to 8 miles up, typically around 30,000 feet |
| Direction | West to east |
| Peak wind speed | More than 275 mph |
| Polar jet | Between 50° and 60° latitude, in both hemispheres |
| Subtropical jet | Around 30° latitude |
| Strongest | In winter, when temperature contrasts are greatest |
Source: NOAA JetStream: The Jet Stream
Frequently Asked Questions
How fast is the jet stream?
Wind in the core of a jet stream can exceed 275 mph. Speeds vary widely along its length and are highest in winter.
Why does the jet stream blow from west to east?
Because of Earth's rotation. Air moving toward the poles keeps the eastward momentum it had nearer the equator, while the ground beneath it is moving more slowly, so the air races ahead toward the east.
Does the jet stream move with the seasons?
Yes. Jet streams shift toward the poles in spring and toward the equator in autumn, and they are strongest in winter. In summer, the Northern Hemisphere polar jet is typically near the border between the United States and Canada.
Related Terms
Polar Vortex
The polar vortex is a large area of low pressure and cold air surrounding each of Earth's poles. It always exists, weakening in summer and strengthening in winter. During Northern Hemisphere winter it often expands, sending Arctic air south with the jet stream and producing cold outbreaks across the United States.
El Niño and La Niña
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.
Cold Front
A cold front is the boundary where a cooler, denser air mass is advancing and replacing warmer air. The cold air wedges under the warm air and forces it upward, which often produces a band of showers or thunderstorms, followed by a wind shift and cooler, drier air.
Wind Shear
A change in wind speed or direction over a short distance, either horizontally or vertically. Strong wind shear is essential for supercell and tornado development but is also a significant hazard to aircraft during takeoff and landing.
Bomb Cyclone
A bomb cyclone is a midlatitude storm that intensifies rapidly through a process called bombogenesis: its central pressure falls by at least 24 millibars in 24 hours, measured at 60° latitude. The threshold shrinks toward the equator, to about 17.8 millibars at the latitude of New York City.
Atmospheric River
An atmospheric river is a long, narrow corridor of concentrated water vapor moving through the lower atmosphere, typically 250 to 375 miles wide and often more than 1,000 miles long. When one comes ashore and is lifted over mountains, it produces prolonged heavy rain and snow. Atmospheric rivers are rated from AR 1 (weak) to AR 5 (exceptional) by strength and duration.
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Explore Other Terms
Virga
Precipitation that falls from clouds but evaporates before reaching the ground. Virga appears as wispy streaks extending from the base of clouds and is common in dry or desert climates where the air below clouds has low humidity.
Heat Index
The heat index is how hot it feels when relative humidity is combined with the air temperature. Humid air slows the evaporation of sweat, so the body cools less efficiently and feels hotter than the thermometer reads. At 100°F with 55% relative humidity, the heat index is 124°F.
Velocity
Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.
ECMWF
ECMWF is the European Centre for Medium-Range Weather Forecasts, an intergovernmental organization supported by 35 states, and the name commonly given to its global weather model, often called the European model or the Euro. Its physics-based Integrated Forecasting System (IFS) runs on a global grid of about 9 km and forecasts up to 15 days ahead.
VFR
Visual Flight Rules allow flight using visual references when the applicable visibility and cloud-clearance minimums are met. Those minimums vary by airspace class, altitude, and day or night under 14 CFR 91.155. Separately, the VFR weather category means a ceiling above 3,000 feet and visibility greater than 5 statute miles.
Tropical Storm
A tropical cyclone with maximum sustained winds between 39-73 mph. At this stage, the storm receives a name. Tropical storms can produce dangerous flooding, winds, and tornadoes even without reaching hurricane strength.
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