It's Not The Heat,

It's The Humidity
A source of constant concern for any collection, is the relative humidity which is the amount of moisture in the air.

In regards to humidity and the preservation of artifacts consisting of leather and metal, there are two primary concerns. The first is that below 40% humidity leather will begin to dry out causing shrinkage, dry rot (also known as red rot), brittleness, splits, and cracks. The second primary concern is that mould and rust form at approximately 66% humidity with the "danger" zone beginning at 75%.

If a collection is stored in a basement, during warm weather it will tend to be more humid than the space above it, for the simple reason that it is cooler. Since cool air can hold less moisture than warm, the humidity in the basement will rise.