For many of us at Competitive's headquarters in Northern Utah, the range of temperatures where a thermal jersey like Castelli's long sleeve Puro Jersey for men determines when we hang the bike up for winter cross training and take it back down for early spring intervals. Castelli recommends it for temperatures down to 59 degrees Fahrenheit, but we find that as long as temperatures stay north of 40 degrees all we need is a base layer and a little sunshine to stay perfectly comfortable in the saddle. Once these conditions pass for the year, we break out the Nordic skis or retire to the trainer. The secret to the jersey's protective qualities lies in Castelli's use of Warmer fabric for the main body and Warmer X Stretch for the side panels and sleeves. Warmer places equal emphasis on insulation and breathability, because evacuating sweat and water vapor from a cyclist's microclimate during is as important to staying warm as trapping body heat. The Warmer X Stretch is a more elastic version that fits tighter under and across the arms, eliminating flapping caused by excess material to maintain the Puro's race worthy fit a fit that Castelli originally tailored for Garmin's cold weather training back when they were still in blue.