Keepin Warm...  Why is it that when we are outside on a 75 degree day we are quite comfortable, yet when we are scuba diving in 75 degree water we quickly become cold? The answer is because water conducts heat away from our bodies about 25 times faster than air of the same temperature. Air can actually be a very good insulator. While diving, we also lose the warmth provided to us from the sun's rays. The cold water also causes our bodies to expend a lot of energy trying to maintain our core temperature at 98.6 degrees. The longer we are in the water the more energy our bodies expend trying to keep warm. To prevent excessive heat loss, divers must wear some type of exposure protection. The type of exposure protection each diver needs depends on his or her own personal tolerance level to the cold water. Types of exposure suits used are dry suits, wetsuits, and suits designed to wear either wet or dry. A dry suit seals out the water from around our body and allows air to be trapped in between the diver and the suit. 
Since air can be a good insulator, it makes sense that a dry suit along with proper undergarments will help reduce our heat loss while diving. Dry suits are equipped with valves to allow you to add additional air as you descend or exhaust expanding air as you ascend. There are dry suits designed for tropical waters and cold water alike.
Choice of dry suit is primarily determined by the type of material it is made from and the thickness and type of undergarment you are planning to wear.
If we are still wet in a wetsuit, how does a wetsuit help us stay warm while diving? To help answer that question we must explain that a wetsuit is made of neoprene rubber. When the neoprene is created it is formed with tiny bubbles of gas trapped within it. These trapped gas bubbles are why your wetsuit has that sponge like feel and are what gives it its insulating ability. It is also important to note that these gas bubbles compress under pressure, so the deeper you go the less effective the wetsuit becomes. For a wetsuit to work it must be in good condition. A wetsuit that has seen many dives or has not been stored properly will begin to lose its effectiveness. 
The wetsuit must also fit the user snugly. A properly fitted wetsuit should allow only a thin "film" of water between you and the suit, not a "layer" of water. A loose fitting suit will let cold water move through the suit flushing out the water already warmed by your body. Finally, the suit should not be so tight that the veins in your neck and forehead are popping out, significantly restricts your movement or your face begins to turn as red as a beet!
A new emerging type of exposure protection is Lavacore. 
Lavacore is a unique fabric designed to be worn wet or dry or for hot or cold environments. It has the thermal properties of a 1.5mm neoprene suit and is neutrally buoyant! Lavacore helps control your core temperature longer, whether worn as a standalone garment, or under your wetsuit or clothing. Lavacore has a three layer design. Lavacore's outer layer is a water repellant lycra. Its inner layer of poly fleece is warm, moisture wicking and has an anti-microbial treatment. It's middle layer is micro porous to act aas a wind barrier. Lavacore also makes a great dry suit undergarment and recommended for other sports like skiing, kayaking, rafting, hiking, etc.
Keeping warm before, between and after your dives is just as important as during your dives. On windy days put on a dive parka, chammyz or change into dry clothes. You should always eat a good meal and
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hydrate. In addition be sure to get plenty of rest to allow your body to recover from energy expended due to the loss of heat.
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