This study investigated the weather effect on thermal performance of a retrofitted extensive green roof on a railway station in humid-subtropical Hong Kong.  Absolute and relative (reduction magnitude) ambient and surface temperatures recorded for two years were compared amongst antecedent bare roof, green roof, and control bare roof.  The impacts of solar radiation, relative humidity, soil moisture and wind speed were explored.  The holistic green-roof effect reduced daily maximum tile surface temperature by 5.2 °C and air temperature at 10 cm height by 0.7 °C, with no significant effect at 160 cm.  Green-roof passive cooling was enhanced by high solar radiation and low relative humidity typical of sunny summer days.  High soil moisture supplemented by irrigation lowered air and vegetation surface temperature, and dampened diurnal temperature fluctuations.  High wind speed increased evapotranspiration cooling of green roof, but concurrently cooled bare roof.  Heat flux through green roof was also weather-dependent, with less heat gain and more heat loss on sunny days, but notable decline in both attributes on cloudy days.  On rainy days, green roof assumed the energy conservation role with slight increase instead of reduction in cooling load.  Daily cooling load was 0.9 kWh m⁻² and 0.57 kWh m⁻², respectively for sunny and cloudy summer days, with negligible effect on rainy days.  The 484 m² green roof brought potential air-conditioning energy saving of 2.80 × 10⁴ kWh each summer, equivalent to electricity tariff saving of HK$2.56 × 10⁴ and upstream avoidance of CO₂ emission of 27.02 t at the power plant.  The long-term environmental and energy benefits could justify the cost of green roof installation on public buildings.