The rate at which rain falls, expressed as depth per unit time. It is a critical factor in determining whether rainfall will infiltrate or run off the land surface.
Rainfall intensity is the rate of rainfall expressed as the depth of water per unit time, most commonly measured in millimeters per hour or inches per hour. It is one of the most important characteristics of a precipitation event from a hydrological perspective because it determines whether infiltration capacity will be exceeded, thereby controlling the generation of surface runoff. High-intensity, short-duration storms are particularly important for urban flooding, erosion, and stormwater system design, while long-duration, moderate-intensity events may be more significant for river flooding and groundwater recharge. Intensity-duration-frequency (IDF) curves are fundamental design tools that relate rainfall intensity to storm duration for various return periods, and are derived from long-term analysis of rainfall records. The relationship between intensity and duration is generally inverse: shorter duration storms tend to have higher intensities. Rainfall intensity varies greatly in space and time, and its measurement requires dense gauge networks or radar systems. Climate change is projected to increase extreme rainfall intensities in many regions, even where mean annual precipitation may decrease, due to the increased moisture-holding capacity of a warmer atmosphere.