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.
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 temperature at which air becomes saturated and water vapor begins to condense into liquid. A higher dew point indicates more moisture in the air. Dew points above 65°F feel humid, while above 70°F is considered oppressive.
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.
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.
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
A measure of how hot it feels when humidity is factored in with the actual air temperature. High humidity prevents sweat from evaporating efficiently, making the body feel hotter. Heat illness can occur at lower values too, depending on sun, exertion, exposure time, and individual health.
Wind Chill
A measure of how cold it feels on exposed skin when wind is factored in with the actual air temperature. Wind accelerates heat loss from the body, and the NWS formula uses air temperature and wind speed: at 0°F with a 15 mph wind, the wind chill is -19°F. For that 0°F and 15 mph combination, the NWS chart indicates frostbite can occur on exposed skin in about 30 minutes. Check the chart for other combinations.
Explore Other Terms
Tornado
A violently rotating column of air extending from a thunderstorm to the ground. Tornadoes can produce winds exceeding 300 mph and cause catastrophic damage along their path. They are rated on the Enhanced Fujita (EF) scale from EF0 to EF5.
Supercell
A severe thunderstorm characterized by a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm and can produce tornadoes, large hail, damaging winds, and flash flooding.
Freezing Rain
Rain that falls as liquid but freezes on contact with surfaces at or below 32°F (0°C), creating a coating of ice called glaze. Freezing rain is extremely dangerous as it can make roads impassable and cause widespread power outages from ice-coated power lines.
Sleet
Frozen raindrops that bounce when hitting the ground. Sleet forms when snowflakes partially melt falling through a warm layer, then refreeze in a cold layer near the surface. Unlike freezing rain, sleet is frozen before it hits the ground.
NEXRAD
Next-Generation Radar, a network of 159 high-resolution Doppler weather radars operated by the National Weather Service. NEXRAD detects precipitation, wind patterns, and severe weather signatures up to 250 miles from each station.
MRMS
Multi-Radar Multi-Sensor, an advanced system that combines data from multiple radars, satellites, and surface observations to create a seamless, high-resolution national weather picture updated every 2 minutes.
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Learn Relative Humidity in Context
These explainers and product surfaces connect this definition to real-world forecasting and decision-making.
Weather Safety
The Heat Index Explained: How Hot It Really Feels and How to Stay Safe
Learn what the heat index is, how temperature and humidity combine into a 'feels like' temperature, how to read the NWS heat index chart, and how to stay safe in extreme heat.
Weather Education
What Does 'Feels Like' Temperature Mean? Wind Chill and Heat Index Explained
Understand wind chill, heat index, and wet-bulb temperature, why weather apps can disagree, and how to use apparent temperature alongside local safety guidance.
Weather Education
Why Dew Point Matters More Than Humidity
Learn why dew point temperature is a better measure of moisture comfort than relative humidity, how to interpret dew point readings, and when high dew points become dangerous.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Tool
Dew Point Calculator
Convert between temperature, humidity, and dew point for planning and forecasting.
Guide
Dew Point vs Humidity
Learn why dew point is often the better way to judge comfort and storm moisture.
Feature
Conversational AI
Ask about rain timing, alerts, and what the forecast means in plain language.
Glossary
Humidity
The amount of water vapor present in the air.
See Relative Humidity in Action
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