Stand them up
An upright reel presents a narrow edge to the sun. A reel laid flat presents a full flange face, collects water in the centre, and heats the cable touching the wood to the highest temperature in the yard.
Get them off the ground
Dunnage, gravel pads or pallets keep the flange out of standing water. Wood wicks moisture upward, and a reel sitting in a puddle pulls water toward the cable.
Create shade and airflow
A shade structure, a container wall or even reels stacked in each other's shadow helps. Air movement matters as much as shade, because a sealed pocket of hot air against a jacket ages it faster than open sun.
Record the exposure
Note how long each reel has been outside and photograph any chalking. A buyer will ask, and a documented answer keeps the review on facts instead of assumptions.