For those hitting the trails a cooler, cloudy day can look and feel deceiving, according to Maricopa Integrated Health System Emergency Vice Chairman Frank Lovecchio.
He says hikers need to use the same cautions as they would on an extreme heat day, if not more.
"For most people, if you're outdoors in the middle of the day you need to drink at least a quart or liter of water every hour," said Lovecchio.
Lovecchio says people heading out need to consider the heat index. A 100 degree day with 50 percent humidity will actually feel more like 118 degrees.
"If the humidity jumps up to 30 to 40 percent or so, you can add about 3-4 degrees to that," said Lovecchio.
On a hot, dry day the sun evaporates sweat. This causes your body to cool according to Lovecchio and when there's high humidity it won't happen as fast.
"If the outside air is wet or has high humidity, humidity is a relation to the amount of moisture or water in the air. If the outside air is full of water, the water is not going to evaporate and you can't take advantage of evaporation and cooling."