A sheet of glass at a temperature of 600∘C is cooled by passing air over its surfaces. The temperature of the air is the same as that of the surroundings. To prevent cracking it has been found that the temperature gradient must not exceed 15∘C/mm anywhere in the glass during the cooling process. If the thermal conductivity of the glass is 1.5 W/mK and its emissivity is 0.8 , what is the approximate lowest temperature of the air that can initially used for cooling?