The glass cover (area 2 m^{2}, thickness 0.5 cm, k = 0.8 W/m·°C) of a solar collector has inner and outer surface temperatures of 30 °C and 27 °C. The outer surface loses heat by convection (h = 12 W/m^{2}·°C, air at 17 °C) and by radiation. The fraction of heat lost by radiation is