Hydration is necessary for proper blood circulation. Much of the electrolyte loss is potassium which is why bannas are a prefect bike food.
Humidity reduces the body's ability to maintain its core temperature by perspiration. When I lived in Dallas the humidity greatly reduced my riding as unlike in California where it was my aerobic conditioning or my leg strength and endurance it was my body core getting too hot that was the limitation. I have ridden across Death Valley and it was not nearly as bad as in Dallas on a high humidity day. At an Olympic marathon trial in Dallas a local runner died from heatstroke during the race, so conditioning was not enough.