Raindays

A rainday, or rain day, is a calendar day on which precipitation is recorded at a weather station. In practice the exact definition can vary by country, agency, or dataset. Some sources count a rainday whenever any measurable precipitation falls (greater than 0.0 mm), while others use a higher threshold such as 0.1 mm or 1.0 mm to exclude trace amounts and minor fluctuations.

Rainday frequency is a basic aspect of climate and is used alongside total rainfall to describe rainfall regimes. Regions with arid or semi-arid climates have relatively few raindays but may experience heavier falls on those days, whereas tropical and temperate regions can have many raindays scattered throughout the year. Seasonal patterns in rainday frequency help characterize monsoon systems, front-driven rain, and other circulation features.

Data on raindays are collected from daily precipitation records and are used in climate studies, agriculture, water resource planning, and flood risk assessment. For farmers, the number of raindays influences decisions about irrigation and crop selection. For hydrologists and planners, rainday frequency helps estimate soil moisture, runoff potential, and the likelihood of weather-related disruptions. In transport and outdoor event planning, knowledge of typical rainday patterns informs scheduling and risk management.

Limitations include the reliance on daily totals, which can obscure the timing and intensity of rain within a day, and variations in gauge coverage and measurement practices across regions. Users should consider local definitions and data quality when interpreting rainday statistics.