Houston is humid, especially in the morning. As the heat rises through the day, the humidity drops. OK, it doesn't drop to Tuscon levels, but it drops double digits. Nonetheless, morning is still the best time to run as this morning proved.
The run/walk (staying in the aerobic zone) this morning was seven minutes faster than the first one. The first run was late in the morning when it was 90F (32C). This morning it was 68F (20C). During the first run, I could not get back to 70% HRmax and eventually could get my HR lower than 76% HRmax. This is due to thermodynamics. As the body tries to cool, the heart beats fast enough to get blood to the surface for cooling and regulate the core temperature. Normally, perspiration appears on the skin and evaporates producing a cooling effect. In high humidity, the perspiration does not evaporate and therefore, little cooling takes place. Reduced cooling on the skin causes the heart to beat faster to pump more blood to the surface for some cooling. This is why runners prefer 50F (10C) for running because the HR will bother little with its core temperature responsibilities.
I was able to run/walk throughout the three miles (the first day I walked the third mile, maintaining a HR of 76% max). The first four cycles I was able to get my recovery HR to 70%, and then it began to rise, eventually to 73% HRmax. The cooler temperature helped me be seven minutes faster; I am certain I have not changed physiologically enough to warrant a 12% improvement in two days. :-)
This is a process. A long one to manage, but the outcome is always positive. Runners who stay at it for years go from being beginners and grow into accomplished runners. I will, too.
Thanks for coming along; keep moving.