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.
More Atmospheric Conditions Terms
Barometric Pressure
The weight of the atmosphere pressing down on a given point, measured in inches of mercury (inHg) or millibars (mb). Rising pressure typically indicates improving weather, while falling pressure suggests approaching storms or precipitation.
Dew Point
The dew point is the temperature to which air must be cooled to become saturated, so that water vapor begins to condense into dew or fog. The higher the dew point, the more moisture is in the air. In summer, a dew point of 55°F or lower feels dry and comfortable, and 65°F or higher feels muggy.
Humidity
The amount of water vapor present in the air. Absolute humidity measures the actual water content, while relative humidity expresses how close the air is to saturation at its current temperature, shown as a percentage.
Relative Humidity
The ratio of the current absolute humidity to the maximum possible humidity at the current temperature, expressed as a percentage. 100% relative humidity means the air is fully saturated. Relative humidity changes with temperature even if moisture content stays the same.
Temperature Inversion
A layer of the atmosphere where temperature increases with altitude instead of decreasing. Inversions trap pollutants and moisture near the surface, causing fog, smog, and haze. They also cap thunderstorm development.
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.
Wind Chill
Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips heat from the body faster than still air does, so the wind chill temperature is lower than the air temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and exposed skin can freeze in 30 minutes.
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.
UV Index
The UV index is a forecast of the intensity of skin-damaging ultraviolet radiation expected to reach the ground at solar noon, when the sun is highest. It is 0 at night and rises to 11 or higher, and the higher the number, the faster unprotected skin and eyes are harmed. Sun protection is recommended at 3 and above.
Wet-Bulb Temperature
Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. It is always at or below the air temperature, and the drier the air, the larger the gap between the two. Forecasters use it to judge heat stress and how much falling rain or snow can cool the air.
Wind Shear: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
What Is a Derecho? The Inland Hurricane Explained
Learn what a derecho is, how it differs from tornadoes and hurricanes, why these long-lived windstorms are so destructive, and how to stay safe when one approaches.
Weather Education
Typhoon vs Hurricane vs Cyclone: Same Storm, Three Names
A typhoon and a hurricane are the same kind of storm with the same 74 mph threshold. What changes is the ocean basin, the agency, and how the wind is measured. Here is the full naming map.
Weather Education
What Is an Invest in Weather? What 90L, 91L, and 92L Actually Mean
An Invest is a data-collection label, not a forecast. Here is what the National Hurricane Center means by Invest 90L, how the numbering and basin codes work, and how an Invest becomes a named storm.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Tool
Wind Speed Converter
Convert between mph, km/h, knots, and m/s for wind reports and forecasts.
Seasonal Hub
Tornado Season Guide
Peak tornado timing by region, safety planning, and how warnings work.
Guide
The 4 Storm Ingredients
Moisture, instability, lift, and wind shear: how the four thunderstorm ingredients combine into severe storms.
Glossary
Supercell
A supercell is a thunderstorm with a deep, persistently rotating updraft called a mesocyclone.
Explore Other Terms
Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
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.
Kp Index
The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.
S-Wave
The secondary or shear wave, which arrives after the P-wave and moves rock perpendicular to its direction of travel. S-waves shake far more strongly than P-waves and cause much of the damage in an earthquake. They cannot travel through liquids, which is how scientists established that Earth's outer core is molten.
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
Graupel
Soft, small pellets of ice formed when supercooled water droplets freeze onto falling snowflakes. Also called snow pellets, graupel is crumbly and opaque, unlike the hard, clear ice of hail. It often occurs in spring and fall storms.
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