If you're planning to step outside for a quick run, and the temperature is over 90F, take the time to consider if your body is ready for that kind of treatment. Blazing hot days are not ideal for long runs. Ask yourself if an indoor run would be just as good and save yourself some torture. You can also plan a shorter route/run, just to save your body some pain later. Make sure that you're actually checking the temperature and resist coming to conclusions by stepping outside for a few seconds to “get a feel” of the temperature. This will not give you an accurate description of what it will feel like when you start running. Your body temperature increases significantly during exercise and can make the weather feel twice as hot than it actually is.