This is a FACT!

When you are in a cold environment, your body tries to keep your core warm by constricting your blood vessels and reducing the flow of blood to your skin. This causes your fingers and toes to shrink, so if you are wearing a ring on your finger, it will become loose.

The opposite happens when it is warm outside. Your body tries to cool itself by dissipating heat through your skin, through the process of sweating. As the external temperature rises, your blood vessels dilate so that your blood flow increases and the excess heat in your body can be released into the environment through your skin. This causes your fingers and toes to expand, so if you are wearing a ring, it will suddenly become a lot tighter.