Interesting Question: Does your blood get thicker in the cold, and does that have an effect on my cardiovascular performance?
I can’t say that I have ever seen any research that would imply that weather change would effect blood tempeture, and/or thickness (viscosity). Our body keeps us within a very narrow margin of temprature (somwhere around 98.7 degrees). What might effect cardiovascular performance in the cold is the shunting of arterioles close to the skin to maintain core body tempeture. This may decrease blood flow to some peripheral musculature, as well as, increase total peripheral resistance and blood pressure giving your heart more work to do. You may also be affected by an increase in shivering (shivering is little muscular contractions) from all muscles resulting in more oxygen utilized for things other than your legs. So…. all in all, there is definate reason to believe that really cold tempatures affect performance by “thickening your blood.” One thing that has a significant effect on blood thickness (look up viscocity) is hydration. Regardless of cold or hot weather, or your perception of sweat and waterloss… Drink water while working out!!!