When sculpting in oil-based clay, the clay remains pretty consistant. It does get harder or softer with temperature, but it's oil content remains stable.
Working in water-based clay is quite different. The clay dries when exposed to air. Here in semi-arid Colorado it dries significantly faster than in humid Florida.
You will find that if you are holding clay in one hand an applying that clay to your sculpt with your other hand or a tool, that the clay in your hand dries very quickly - you'll see gray dust on your fingers and cracking clay on your hand - the contact of your skin to the clay is wicking the moisture from the clay. That makes it stiffer and more likely to crumble.
To counter this, if I'm working with smaller bits of clay, I'll take a ball of it and slap it onto an unfinished area of my sculpt - in this case a portion of the sky but it could as easily be the hair or the base of the shoulders if a bust. That way the clay is sitting on the clay and retains about the same moisture content...I simply take a pinch or use my tool to grab a swipe of the clay for application.
At this stage of the sculpt, areas are starting to dry out a bit. I am now starting to cover those sections with a damp paper towel to keep it from getting too dry when working in other areas. It will get progressively drier and firmer as I'm working, allowing for more accurate detailing.