Dry Heat vs Humid Heat
Why 100 F in Phoenix is different from 90 F in Houston, and which is more dangerous despite the lower number.
Phoenix in August: 105 F, 15% relative humidity. Houston in August: 90 F, 80% relative humidity. Which is more dangerous?
The Houston conditions, by a wide margin. Heat index for Phoenix is 100 F (Extreme Caution). Heat index for Houston is 121 F (Extreme Danger). The 90 F Houston day will kill more people than the 105 F Phoenix day.
Why humidity is the killer variable
Your body cools by sweating, and sweat cools you by evaporating off your skin. Each gram of water that evaporates removes 580 calories of heat from your body.
In dry air (15% humidity), there is plenty of room for water vapor. Sweat evaporates fast. Cooling is efficient. You can be 105 F outside and your skin stays close to body temperature.
In humid air (80% humidity), the air is already loaded with water vapor. Sweat does not evaporate, it just sits and drips. Your primary cooling mechanism is shut off. Your core temperature climbs.
This is why deserts have higher peak air temperatures but fewer heat deaths per capita than the Gulf Coast. Phoenix averages 30 heat deaths per year on a population of 1.6 million. Houston averages similar numbers on 2.3 million, adjusted for population, the rates are close, but Houston has it on cooler-temperature days.
The wet-bulb temperature angle
Wet-bulb temperature captures this exactly. It is the temperature evaporative cooling can reach.
- Phoenix 105 F at 15% humidity: wet-bulb 68 F. Hot but survivable. - Houston 90 F at 80% humidity: wet-bulb 84 F. Approaching dangerous.
The lethal wet-bulb threshold is 95 F for healthy adults. Most of the year, neither city is close. During extreme events, the Gulf Coast briefly approaches it; Phoenix essentially never does.
What to do differently
In dry heat: drink water aggressively because you cannot tell you are sweating (it evaporates instantly). Tourists in Phoenix die of dehydration more than heat stroke.
In humid heat: shower or wet down to physically cool your skin. Sweating alone will not do it. Sustained outdoor activity is genuinely dangerous, even for fit acclimatized adults.
Both: an air conditioned space is the difference between survival and not, especially at night when humid climates do not cool down.