Climate Variability in Calabar, Nigeria, 1990–2020: Rainfall, Temperature, Humidity Trends and Drainage Planning Composite Index

Calabar receives more than 3,000-mm rain on average per year, and its drains were not sized with that in mind. This study asks how much the local climate has varied over decades and whether that variation can be summarized in a form useful to drainage planners. Monthly rainfall, maximum temperature, and relative humidity for 1990–2020 were obtained from the Meteorological center, Airport Road, Calabar, the same station record used by Egbe et al. (2026) used for their ARIMA rainfall forecasts. The annual series were examined using the coefficient of variation, standardized anomalies, Mann–Kendall test, and Sen’s slope. Rainfall averaged 3,094 mm a year (CV 14.8%) and showed no significant trend (Z = 0.95, p = 0.34). The maximum temperature rose by approximately 0.024 °C a year (Z = 2.55, p = 0.011). The relative humidity drifted upward at 0.037 percentage points a year, but the trend narrowly missed significance (p = 0.052). The three variables were only weakly correlated (|r| ≤ 0.24). To combine them, we propose a climate variability–urban drainage risk index (CV-UDRI), a weighted sum of the three standardized anomalies with rainfall weighted most heavily. The index ranks 1995, 2012, and 2005 as the most stressful years and 2020, 1993, and 2004 as the least stressful years. The two highest ranks do not change under the tested alternative weightings. The index is only a climatic screening measure only. It says nothing about drain capacity, land use, or flood occurrence, and is intended to sit alongside that information in later work