A long cylindrical pipe of radius $20\,\text{cm}$ is closed at its upper end and has an airtight piston of negligible mass as shown. When a $50\,\text{kg}$ mass is attached to the other end of the piston, it moves down. If the air in the enclosure is cooled from temperature $T$ to $T - \Delta T$, the piston moves back to its original position. Then $\Delta T/T$ is close to (Assuming air to be an ideal gas, $g = 10\,\text{m/s}^2$, atmospheric pressure is $10^5\,\text{Pa}$)