Extreme temperatures and humidity levels can be particularly brutal if you’re working outdoors or in an unconditioned workshop. But it’s not just yourself you need to worry about.
Wood, particularly soft woods, are susceptible to extremes of humidity and heat. Lignin, which is more or less the “glue” within the fibers of wood that keep its shape, can weaken in extreme heat and direct sun and cause boards to “move” and warp and even split.
A couple of tips – don’t store lumber in direct sunlight, and keep it in your shop for a couple of days before you start working on it to let it adjust to the conditions in your shop.
Even just a few minutes in near 100-degree heat (which I experienced recently on a video shoot) and direct sunlight can warp a board. Heat and moisture are two ingredients used in forming wood to shape (i.e., steam bending), and once the lignin has been broken down and the wood shaped, you can’t “re-bend” it back to shape.