Heat Safety in the High Sierra
A lot of people may think it strange to speak of heat safety in the Sierra Nevada Mountains. But it is not. This harsh environment of extremes often serves up high temperatures.
I have experienced low to high 80 degree tempertures hiking South on the Tahoe to Yosemite Trail from Round Top near Carson Pass all the way down to Jenkins Canyon during countless trips between mid-July to mid-August. Even some low 90 degree tempertures. Once you get some altitude approaching Highland Lakes things finally begin to cool off, but not the danger. Higher altitudes, though cooler, still carry the risk of exposure due to the intensity of light rather than the heat.
These high tempertures are mostly a function of elevation. The Tahoe to Yosemite Trail traces a low elevation route along sections of its Western Flank route.
Check out the low elevations along parts of the trail between Carson Pass to Lake Alpine, and Lake Alpine to Saint Marys Pass. Those low elevations are potential hot spots along your hike, depending on conditions. Heat waves during early Summer are not uncommon.
Take heat seriously. It poses a significant danger while hiking at altitude carrying loads that are already stressing out your metablolism. Maintaining hydration and not overheating must happen, and you must have a better system than how "you feel." The "feel" system fails..
I maintain hydration based on logical standards proven by experience and monitored for accuracy by the frequency and color of my pee. I pack 32 oz of water for a max of 9 miles of "regular" high altitude travel, and lower the mileage down to 7 miles for "warm" tempertures, and down to 5 miles for "hot" conditions.
My standard rewatering practice is to drink 32 ounces each time I refill my 32 ounce bottle. I drink more if conditions are warm, and even more for hot tempertures. The factor I find important for my rewatering practices is time.
When I am really hot it takes a few minutes for me to cool down sufficiently to take in the amount of water I need. It appears that overheated hikers tend to underestimate the water they actually need by not taking the time their bodies require to absorb that much water.
Clear water must go in and out on a regular basis or you will "feel" like crap if not crash hard. Another good marker of your level of hydration is the mositure level of your crap. If your turds look as dry as coyote turds you are seriously dehydrated, and you must significantly increase your water intake.
The National Weather Service has a great page about the Heat Index and simply describes the physical and atmospheric physics behind the Heat Index.