Humidity is a measure of water vapor in the atmosphere. There are measures of absolute humidity, which is the grams of water vapor present per cubic meter of air. Specific humidity is the grams of water per kilogram of air. Then there’s the one people are most familiar with, relative humidity, which is the percentage of water vapor present in the air relative to the maximum it could hold at its temperature. Knowing the humidity is useful for many things including assessing forest or soil moisture, predicting thunderstorm development and severity, calculating ice formation or estimating air quality.   

NASA’s ground, airborne, and satellite instruments measure humidity in a variety of ways and offer hundreds of datasets with the information and direct applications for air quality, disease, extreme heat, and tropical cyclone research.