Reference
Glossary
The terms forecasters use when they talk about the polar vortex, the jet stream and the Pacific, each in a sentence or two, with a source.
The atmosphere
Exosphere
The outermost layer of the atmosphere, with no clear upper boundary, where atoms and molecules escape to space.91
Kármán line
The most common definition of where space begins: 100 km (62 miles) above the surface.2
Mesosphere
The layer from about 50 to 85 km (31 to 53 miles), where most meteors vaporize. Its top is the coldest place in the atmosphere.67
Ozone layer
Ozone in the stratosphere that absorbs ultraviolet light from the sun, heating the layer and shielding the surface.52
Stratosphere
The layer above the troposphere, up to about 50 km (31 miles). Temperature rises with height here, and the polar vortex forms here in winter.14
Thermosphere
The layer above the mesosphere, where temperature climbs steeply, auroras occur and the space station orbits.8
Tropopause
The top of the troposphere and the bottom of the stratosphere. The jet streams run just beneath it.13
The polar vortex
10 hPa
The pressure level, about 30 km (19 miles) up, where scientists usually track the strength of the polar vortex.15
Major warming
A sudden stratospheric warming in which the wind at 60°N and 10 hPa reverses from westerly to easterly.12
North Atlantic Oscillation
Also called: NAO
A pressure pattern over the North Atlantic. Its negative phase dominates the surface effects of a sudden stratospheric warming.16
Ozone hole
The severe springtime loss of ozone over Antarctica. The cold air inside the polar vortex is one of its key ingredients.10
Planetary waves
The largest waves in the atmosphere. The biggest of them can travel up into the stratosphere, where they break and disturb the polar vortex.1110
Polar vortex
Also called: Stratospheric polar vortex
A band of strong westerly winds that forms in the stratosphere, about 10 to 30 miles above the pole, every winter.4
Stretched vortex
A disruption in which the polar vortex is elongated instead of broken. Its link to cold outbreaks is still debated.1314
Sudden stratospheric warming
Also called: SSW
A large, rapid rise in temperature in the winter polar stratosphere, during which the polar vortex weakens, shifts or splits.114
Tropospheric polar vortex
The much larger circulation of cold air in the weather layer, which exists all year. News reports often call it "the polar vortex" too.10
The jet stream
Arctic Oscillation
Also called: AO
A seesaw in air pressure between the Arctic and the mid-latitudes. In its negative phase the jet stream dips south and brings cold-air outbreaks.22
Blocking
An arrangement of highs and lows that stops other weather systems from moving through. It can last several days.19
Jet streak
Also called: Jet max
A pocket of the fastest wind within a jet stream. Jet streaks help surface storms form and deepen.2120
Jet stream
A relatively narrow band of strong wind in the upper atmosphere, typically around 30,000 feet up, blowing from west to east.17
Omega block
Two cutoff lows with a blocking high between them, in the shape of the Greek letter omega.19
Polar jet stream
Also called: Polar front jet
The jet stream found between 50° and 60° latitude, near the boundary between polar air and milder air.173
Ridge
A bulge in the jet stream toward the pole. Weather under a ridge tends to be warmer and clearer.19
Rossby wave
Also called: Planetary wave, Longwave
One of the giant north-south meanders in the jet stream. There are usually four or five around the hemisphere at once.18
Subtropical jet stream
The jet stream found around 30° latitude. It sits higher than the polar jet.173
Trough
A dip in the jet stream toward the equator. Most stormy weather occurs between a trough and the ridge to its east.19
Wind shear
Winds at different heights blowing at different speeds or in different directions.23
El Niño and La Niña
Bjerknes feedback
The loop in which tropical Pacific winds and ocean temperatures reinforce each other, pushing the system toward El Niño or La Niña.35
ENSO
Also called: El Niño-Southern Oscillation
A natural climate pattern in the tropical Pacific with two opposite phases, El Niño and La Niña.24
Niño 3.4 region
The patch of equatorial Pacific, from 5°N to 5°S and 120°W to 170°W, that forecasters use to track ENSO.28
Oceanic Niño Index
Also called: ONI
The three-month average of how far the Niño 3.4 sea surface is from its long-term average. It was NOAA's official ENSO index until 2026.29
Relative Oceanic Niño Index
Also called: RONI
NOAA's official ENSO index since February 1, 2026: the Niño 3.4 temperature departure with the average departure of the whole tropical ocean subtracted.2830
Southern Oscillation
The atmospheric half of ENSO: a shift in air pressure between the western Pacific and the central and eastern Pacific.27
Trade winds
The winds that blow west along the equator, pushing warm surface water toward Asia.31
Upwelling
Cold water rising from the deep ocean to replace surface water that the wind has pushed away.31
Severe weather
Arctic amplification
The Arctic warming faster than the mid-latitudes. Whether it is making the jet stream wavier is contested.4950
Atmospheric river
A long, narrow band in the atmosphere that carries most of the water vapor moving out of the tropics.45
Blizzard
Winds or frequent gusts of 35 mph or more with falling or blowing snow cutting visibility below a quarter mile, for at least three hours.37
Bomb cyclone
Also called: Bombogenesis, Explosive cyclogenesis
A storm outside the tropics whose central pressure falls at least 24 millibars in 24 hours (at 60° latitude).3940
Derecho
A widespread, long-lived windstorm produced by a fast-moving line of thunderstorms.47
Enhanced Fujita scale
Also called: EF scale
The scale, EF0 to EF5, that rates a tornado by the damage it caused and the wind speeds that damage implies.43
Freezing rain
Rain that freezes when it hits the ground, coating roads, trees and power lines in ice.37
Heat dome
A persistent area of high pressure that traps heat over a region for days to weeks.46
Lake-effect snow
Narrow, intense bands of snow that form when cold air crosses the open water of the Great Lakes.41
Nor'easter
A storm along the East Coast of North America, named for the northeast winds it brings to the coast.38
Severe thunderstorm
A thunderstorm that produces a tornado, winds of at least 58 mph, or hail at least one inch across.44
Tornado
A narrow, violently rotating column of air that extends from a thunderstorm to the ground.42
Wind chill
How cold it feels on exposed skin, based on the rate of heat loss from wind and cold together.36
Sources
- NOAA National Weather Service, JetStream. Layers of the Atmosphere.
- NASA Science. Earth's Atmosphere: A Multi-layered Cake.
- LibreTexts Geosciences. Atmospheric Processes and Phenomena, 10.6: Jet streams.
- NOAA Climate.gov (archived). Understanding the Arctic polar vortex, March 5, 2021.
- UCAR Center for Science Education. The Stratosphere.
- UCAR Center for Science Education. The Mesosphere.
- UCAR Center for Science Education. Layers of Earth's Atmosphere.
- UCAR Center for Science Education. The Thermosphere.
- UCAR Center for Science Education. The Exosphere.
- Bulletin of the American Meteorological Society. Waugh, Sobel and Polvani, What Is the Polar Vortex and How Does It Influence Weather?, 2017.
- Reviews of Geophysics. Baldwin et al., Sudden Stratospheric Warmings, 2021.
- Journal of Climate. Charlton and Polvani, A New Look at Stratospheric Sudden Warmings. Part I: Climatology and Modeling Benchmarks, 2007.
- Science. Cohen et al., Linking Arctic variability and change with extreme winter weather in the United States, 2021.
- Nature Communications. Davis et al., Limited surface impacts of the January 2021 sudden stratospheric warming, 2022.
- NOAA Climate.gov Polar Vortex Blog (archived). Welcome to the Polar Vortex Blog!, December 5, 2023.
- Journal of Geophysical Research: Atmospheres. Domeisen, Estimating the Frequency of Sudden Stratospheric Warming Events From Surface Observations of the North Atlantic Oscillation, 2019.
- NOAA National Weather Service, JetStream. The Jet Stream.
- NOAA National Weather Service, JetStream. Longwaves and Shortwaves.
- NOAA National Weather Service, JetStream. Basic Wave Patterns.
- EUMeTrain (EUMETSAT). The Jet Stream.
- NOAA National Weather Service Glossary. Jet Streak.
- NOAA Climate.gov (archived). Climate Variability: Arctic Oscillation, August 30, 2009.
- NOAA National Severe Storms Laboratory. Tornado Types.
- World Meteorological Organization. El Niño / La Niña.
- NOAA Climate.gov ENSO Blog (archived). What is the El Niño–Southern Oscillation (ENSO) in a nutshell?, May 5, 2014.
- UK Met Office. El Niño, La Niña and the Southern Oscillation.
- IRI, Columbia University. ENSO Essentials.
- NOAA National Weather Service. Public Information Statement 26-05: Relative Oceanic Niño Index, January 13, 2026.
- NOAA Climate Prediction Center. Oceanic Niño Index (ONI).
- NOAA Climate Prediction Center. ENSO: Recent Evolution, Current Status and Predictions (weekly update), November 9, 2026.
- NOAA National Ocean Service. What are El Niño and La Niña?.
- NOAA Pacific Marine Environmental Laboratory. What is El Niño?.
- NOAA Climate.gov ENSO Blog (archived). The Walker Circulation: ENSO's atmospheric buddy, August 1, 2014.
- Australian Bureau of Meteorology. El Niño and La Niña.
- NOAA Climate.gov ENSO Blog (archived). The Rise of El Niño and La Niña, October 23, 2020.
- NOAA National Weather Service. Wind Chill Chart.
- NOAA National Weather Service. Winter Storm Warnings, Watches and Advisories.
- NOAA National Weather Service. What is a Nor'easter?.
- NOAA National Weather Service Glossary. Bomb.
- NOAA National Ocean Service. What is bombogenesis?.
- NOAA National Weather Service. Lake Effect Snow.
- NOAA National Severe Storms Laboratory. Tornado Basics.
- NOAA National Weather Service, Norman OK. The Enhanced Fujita Scale (EF Scale).
- NOAA National Weather Service, Binghamton. Severe Weather Definitions.
- NOAA. What are atmospheric rivers?.
- NOAA NESDIS. Extreme Heat and Severe Weather Plague Parts of North America.
- NOAA Storm Prediction Center. About Derechos.
- NOAA National Weather Service. Tornado Watch vs. Warning.
- Geophysical Research Letters. Francis and Vavrus, Evidence linking Arctic amplification to extreme weather in mid-latitudes, 2012.
- IPCC. Climate Change 2021: The Physical Science Basis, Chapter 10, Cross-Chapter Box 10.1, 2021.
Last updated November 12, 2026