It was about 1 pm when I started my run (the same 1.6 miles that I've been running), and it was hot. The shade temperature was in the mid 80s (F) and the sun temperature was probably close to 100 (F). Most of my run was in the sun. I forgot to carry water, and I hoped the glass I had just before I left home would carry me through.
That glass of water did carry me through most of the run, but I had a touch of dizziness during the last 500 or so yards. Two of the early signs of heat exhaustion are dizziness and headaches. When those happen, I usually stop running and head for a shady spot to rest for a few minutes before I walk home. Today, the dizziness was slight and didn't last long, so I continued my running and walking until I finished.
Runners need to be careful when they run when it's hot, because heat exhaustion is a precursor to heat stroke, and heat stroke is a precursor to death! Literally! In heat stroke, ones body can no longer control the internal temperature of the body, and the temperature goes up and up and begins to cook ones brain and other internal organs. It is important to know and recognize the symptoms of heat exhaustion and heat stroke. Here is my page about running in bad conditions.
I live in Utah, and the Jordan River Parkway, where I run, is at an elevation of 4300 - 4400 feet. That high altitude means runners get less oxygen with each breath and thus experience fatigue more than they would at a lower elevation. I was raised in Southern Utah (elevation about 5500 feet), but I've lived most of my adult life outside of Utah at relatively low elevations. When I moved back to Utah in December 1992, I experienced a lot of dizziness due to the higher altitude here. I would be walking down a sidewalk and would have an attack of dizziness. I would lose contact with the ground, and it would feel like I was walking in the air. It was hard to walk without feeling the ground. After about two years the dizziness went away. My low blood pressure (115/60) probably was a factor, too. Another factor about living in the desert is the low relative humidity of about 10%. That dryness means runners lose more moisture due to evaporation and thus need to drink more. It also means that runners cool more from evaporation of their sweat.