The crew stopped tripping in the hole to fill the pipe. The rig is a cold weather rig equipped with a “warming hut” on the derrick board. Filling the pipe was going to take about fifteen minutes so the derrickman unhooked his positioning rope and entered the warming hut. The derrickman stated he disconnected his positioning rope so he could close the hut door. After the pipe was filled he heard the blocks running up and exited the hut to get a stand of drill pipe. When he left the hut, the derrickman forgot to reattach the derrick board positioning rope to his harness. As he reached to latch the drill pipe in the elevators, he fell from the derrick board. His fall was arrested by the board’s self retracting lifeline (i.e., SRL) which left him hanging below the derrick board. The crew swung the air hoist line to him and he was able to get his feet on the weighted ball and get back on the board. The SRL and harness were replaced and taken out of service due to the impact.