Wet-Bulb Temperature
Also called: wet bulb
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.
How to Read It
- When the air is saturated, at 100% relative humidity, the wet-bulb temperature equals the air temperature because no more water can evaporate.
- Wet-bulb temperature is not wet-bulb globe temperature (WBGT). WBGT is a heat stress measure that also accounts for wind speed, sun angle, and cloud cover.
- Wet-bulb zero is the height where the wet-bulb temperature falls below 0°C. Heights of roughly 7,000 to 10,500 feet correlate with large hail at the surface when storms form in an unstable air mass.
Related Terms
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.
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.
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.
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.
Hail
Balls or irregular lumps of ice produced in severe thunderstorms. Hailstones form when updrafts carry raindrops into extremely cold areas of the atmosphere where they freeze. Repeated cycles through the updraft can create hailstones larger than softballs.
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.
Wet-Bulb Temperature: Guides and Explainers
These explainers 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 vs Feels Like Explained
What feels-like temperature means, how wind chill and heat index differ, the frostbite times on the NWS chart, and why Extreme Cold Warnings replaced the Wind Chill Warning.
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.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Tool
Wet Bulb Calculator
A free tool for working this out with your own numbers.
Guide
Understanding Snow Totals
Why snow forecasts give ranges instead of exact numbers: snow ratios, banding, and elevation effects.
Tool
Temperature Converter
Convert between Fahrenheit, Celsius, and Kelvin instantly.
Tool
Wind Chill Calculator
Estimate cold-weather exposure risk from temperature and wind speed.
Sources
Explore Other Terms
Epicenter
The point on Earth's surface directly above where a fault rupture begins. It is a single map location, not the whole earthquake: a large event can break hundreds of kilometers of fault, so severe shaking often extends far beyond the epicenter.
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.
Thundersnow
A rare phenomenon where thunder and lightning occur during a snowstorm. Thundersnow typically happens in intense winter storms and is often associated with heavy snowfall rates of 2-4 inches per hour.
Reflectivity
A measure of how much radar energy is reflected back by precipitation, measured in dBZ (decibels of Z). Higher values indicate heavier precipitation or larger particles. Values above 50 dBZ typically indicate heavy rain or hail.
Lifted Index
A stability index comparing the temperature of a rising air parcel to the environment at 500mb (about 18,000 feet). Negative values indicate instability. Values below -6 suggest strong instability and severe thunderstorm potential.
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.
Browse by Category
Get the WeatherAI App
WeatherAI is a weather app for iPhone, iPad, Mac, Apple Watch, and Apple Vision Pro. It is free to download.