Exercising vigorously in hot, humid conditions can lead to serious overheating and dehydration. Slow your pace when the temperature rises above 70°F. On days when the thermometer is expected to reach 80°F, exercise during cooler morning or evening hours or at an air-conditioned gym. Watch for signs of overheating, such as headache, dizziness, nausea, faintness, cramps, or palpitations.