This is from another blog (Yahoo Answers) and is written by Big Shot.


"Actually, increase in the viscosity of blood has to do with an increase in the number of red blood cells (RBCs) Increase in red blood cells:

An increase in red blood cell mass is known as polycythemia. Normal physiological increases in the RBC count occurs at high altitudes (and this, as one may stand to reason, is associated with colder weather) At high altitudes, less atmospheric weight pushes air into the lungs, causing a decrease in the partial pressure of oxygen and hypoxia. Your body responds to this decrease by making more RBC's, helping get more oxygen to your brain and other body parts (RBC's carry oxygen molecules).

So yes, higher altitudes, colder weather, thicker blood. Lower alititude, warmer weather, thinner blood."