How Heat and Humidity Affect How You Experience Cool
Apart from mosquitoes, heat and humidity are the main things that make us feel uncomfortable in the summer. Humans rely on evaporating sweat from their skin to cool their bodies. In fact, in order to regulate body temperature, human skin sweats automatically at 98.6° F (37° C).
So, when we sit near a fan or under a ceiling fan, the breeze helps evaporate the moisture from our skin. But while the breeze has no effect on the actual air temperature, it does work nicely with our own natural cooling system. In short, “fans cool people, not rooms.”
This is where humidity enters the equation. Because humid air already holds a lot of water, it takes longer for sweat to evaporate from your skin. Consequently, high humidity makes you feel warmer and more uncomfortable.
The combination of heat and the relative humidity is what’s known as the heat index. If the humidity is 65% and the temperature is 96° F (35.6° C), it will feel around 121° F (49.4° C), but if the temperature is higher at 104°F and the relative humidity is lower at 40% , the heat index will actually feel slightly cooler — 119° F (48.3° C). Sure, that’s still beastly hot, but your sweat evaporates faster in dry air, which is more effective at keeping you cool.
Keeping Cool is a Matter of Fashion
Here's the big problem with answering this question: We rely on the thermometer to tell us about what’s comfortable based solely on what is customary in our society. Over the past 190-something years, the thermal comfort zone has been moving up. In 1820, 50-55° F (10-12.8° C) was believed to be a healthy and comfortable temperature. By 1850, 62° F (16.7° C) became the new standard until it rose to 72° F (22.2° C) some thirty years later.
Why? For one reason, dressing habits. Layered clothing was the fashionable any time of year during most of the 19th century. Not so much now.
A series of psychological experiments by the Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) showed that people have been acculturated to believe that 72° F (22.2° C) is the optimum comfortable room temperature. It also showed that “The preferred temperature range for occupants dressed in summer clothes is 73° to 79°F (22.5° to 26° C).”
In fact, with one set of ASHRAE experiments, participants who did not know the room’s temperature said they were as comfortable at 68° F (20° C) as they were at 72° F (22.2° C).
A Family Science Experiment
The best way to find what your family thinks is comfortable is to do some testing. When everyone is home, secretly raise the thermostat setting for few hours every day. For example, raise it from 72° F (22.2° C) to 74° F (23.3° C). See how your family reacts. Over the next few days for the same period of time, raise the temperature to 76° F (24.4° C). A week later, push it to 78° F (25.6° C), and chances are you won’t get a noticeable reaction until it goes above that level.
Basically - when you find the maximum comfort level temperature, use that.
Adjustments are Healthy
When everyone is gone for the day, there’s no good reason to keep cooling your home at its comfort level. The same applies to night time when everyone is asleep, as your core body temperature lowers and heat radiates from your extremities. A National Institute of Health (NIH) study found the best sleep happens as the body reaches “thermoneutrality,” when environmental temperatures are at 86° F (30° C) (nude and uncovered) or 60 - 66° F (15.6 - 18.9° C) (wearing pajamas and covered by one sheet).
This suggests the key to getting a good night’s sleep during the summer is to raise your thermostat setting to a warmer setting than you might expect - 80° F (26.7° C), for example. Then use the appropriate amount of summer bedding and pajamas to feel comfortable. You can also help your sleep by using a ceiling fan to gently waft a breeze down onto you.
OK - So What IS the Right Temperature for My Air Conditioner in Summer?
To summarize: the right temperature for your air conditioner is:
- Not the thermostat’s temperature reading. It’s the temperature that feels comfortable.
- The one that saves you the most money. You can save 3–5% on air conditioning costs for each degree that you raise the thermostat. Raising your thermostat from 74° F (23.3° C) to 78° F (25.6° C) can save you up to $25 (USD).