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.
More Atmospheric Conditions Terms
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.
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.
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 Barometric Pressure in Context
These explainers and product surfaces 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
What Is Storm Surge? Surge vs Storm Tide, and Why Category Doesn't Predict It
Storm surge explained: how surge, storm tide, and inundation differ, what decides how high the water gets, why a Category 2 can flood worse than a Category 4, and which NHC products to act on.
Weather Safety
Hurricane Preparedness Week: A 7-Day Plan to Get Ready
A complete guide to NOAA's Hurricane Preparedness Week themes, with day-by-day actions to determine your risk, build a kit, harden your home, and finalize a written hurricane plan.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Guide
Barometric Pressure Explained
What barometric pressure means, how it predicts weather changes, and how to read pressure trends.
Guide
How to Read Weather Radar
Learn the basics of radar interpretation so you can tell where storms are headed and how intense they may become.
Tool
Pressure Tendency Calculator
Calculate 3-hour barometric pressure tendency from rising or falling readings.
Guide
Lightning Safety Guide
Know when lightning becomes dangerous, how far away it can strike, and when to move indoors.
See Barometric Pressure in Action
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