The body reacts to cold and heat by adjusting the blood stream to different parts of the body, like hands, fingers and feet. But the body can not adjust the blood flow to your head. This is to ensure brain function at all times. The colder it gets, the more heat you lose from the head by radiation, or by conduction in the water. You stay warmer with an effective hood. Waterproof H1 hoods including the unique HAVS (Hood AirVenting System) are constructed with these scientific facts in mind and are proven to keep you warmer when you need it most. 3-D shaped Anatomical Non-Bib version for drysuits Glideskin seal HAVS system (double vents) I-Span superstretch nylon Bonded Hi-Quality Nylon thread Reflective logo