Thermocline

“A  thermocline (sometimes metalimnion) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline may be thought of as an invisible blanket which separates the upper mixed layer from the calm deep water below.” http://en.wikipedia.org/wiki/Thermocline

I like to think of an octopus at night, reaching out when it gets chilly and arm by arm by arm by arm cuddling his invisible blanket around himself.