Heat is what it's all about. Tapas, which means heat, is a niyama in the 8 limbs. This heat is the constancy that supports our discipline. It can be a purification by burning - represented in physical space, or the fire that lies inside us. Tapas is the fuel that helps us show up day after day for this practice, even when we don't feel like it.
Some of the sadhus (holy men, sages) go into such austerities that they will sit in the dead winter in nothing but a loincloth, or