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.
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.
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.
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.
Temperature Inversion: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
What Causes Thunder and Lightning? The Science Behind Thunderstorms
Learn how lightning forms through charge separation in thunderstorm clouds, the different types of lightning, what causes thunder, and how to stay safe during electrical storms.
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
Moisture, Instability, Lift, and Wind Shear: The 4 Storm Ingredients
Learn the four thunderstorm ingredients meteorologists look for: moisture, instability, lift, and wind shear, plus how they combine to create severe storms.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Guide
Lightning Safety Guide
Know when lightning becomes dangerous, how far away it can strike, and when to move indoors.
Tool
Temperature Converter
Convert between Fahrenheit, Celsius, and Kelvin instantly.
Tool
Wind Chill Calculator
Estimate cold-weather exposure risk from temperature and wind speed.
Tool
Dew Point Calculator
Convert between temperature, humidity, and dew point for planning and forecasting.
Explore Other Terms
IFR
Instrument Flight Rules govern flights conducted under instrument procedures and can be used in clear weather too. Separately, the IFR weather category means a ceiling from 500 to less than 1,000 feet and/or visibility from 1 to less than 3 statute miles, with lower values classified as LIFR. Legal operating requirements depend on airspace, regulations, equipment, and pilot qualifications.
Post-Tropical Cyclone
A post-tropical cyclone is a former tropical cyclone that no longer has enough tropical characteristics to be classified as one. It can still bring heavy rain and strong winds. The two kinds are extratropical cyclones, which draw their energy from temperature contrasts between air masses, and remnant lows, which have winds below 34 knots (39 mph).
Ice Storm
A storm that produces significant ice accumulation from freezing rain, typically 0.25 inches or more. Ice storms can down trees and power lines, causing widespread outages lasting days to weeks. They are among the most damaging winter weather events.
Small Craft Advisory
A Small Craft Advisory is a National Weather Service alert for winds or seas that are hazardous to small boats in coastal and Great Lakes waters. The wind and sea thresholds are set separately for each geographic area. There is no precise definition of a small craft: any vessel that the conditions could affect counts.
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.
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