The millimeter per hour (mm/h) is a metric unit that quantifies the rate at which a liquid or solid layer accumulates or recedes over a surface, measuring the vertical distance traveled in millimetres during one hour; it is commonly used to express precipitation intensity, irrigation flow, and erosion rates, making it essential for meteorologists forecasting rainstorms, engineers designing drainage systems, and scientists monitoring climate‑related water cycles, where precise mm/h data enable accurate modeling, resource management, and risk assessment in both practical field applications and advanced research.