Blood viscosity is in no way affected by the heat or cold of the surrounding environment, so blood does not thin in hot weather. Experiences of extreme temperatures differ from the effect they have on the blood and other internal anatomical elements of the human makeup.
While the body may feel cooler or warmer depending upon its surroundings, homeostasis ensures that the blood and organs remain at a consistent temperature. Blood viscosity is one of the factors protected by homeostasis, as the maintenance of a consistent level temperature is essential to human health.
Blood thinners are medications designed to impair or otherwise reduce the ability of blood to clot. They have many surgical and medical applications, but they do not make a person feel warmer or cooler because there is no significant linkage between blood viscosity and temperature.
Blood circulation is affected by internal thermo-regulation, with blood vessels relaxing and contracting in heat and cold respectively. This allows for the dissipation of body heat in the event that external temperatures are high and for the preservation of heat when external temperatures are low, thus preserving the system's efficiency. This is one of the only significant points of interaction between blood and external stimuli.