The combination of air temperature, radiant temperature, humidity, and air movement affect summer comfort. To be comfortable, the human body prefers to cool itself with convection and radiation. Sweating is the body’s last resort when radiation and convection aren’t sufficient.
Three factors determine how the human body cools itself.
- Air temperature and airflow determine the rate that a human body loses heat by convection.
- Radiant temperature determines the rate that a human body loses heat by radiation.
- Relative humidity determines the rate that a human body loses heat by evaporating sweat.
Air temperature and air movement
Moving air carries heat away from the skin and increases the evaporation of sweat. Rapidly moving air cools well by itself, and together with air conditioners, evaporative coolers, and whole-house fans.
Whenever the outdoor air temperature is cool and dry, ventilating with outdoor air removes heat from a building and saves energy. Outdoor air ventilation works best at night.
Radiant temperature is almost as important as air temperature. Absorbed summer sunlight raises wall and ceiling temperatures. These surfaces then radiate heat to people. The radiant temperature of walls and ceilings around you affects your comfort. If the surfaces are cooler than your skin, they absorb your body’s radiant heat. If the surfaces are warmer, they radiate heat to your body.
Relative humidity is the air’s percent of water-vapor saturation. Air at 100% relative humidity is saturated and can hold no more water vapor.
Dew point is the temperature at which condensation begins. At 100% relative humidity, the dew point is the same as the air temperature. Below 100% relative humidity, the dew point is less than the air temperature.
Humidity affects the choice of a cooling strategy during hot weather. At low relative humidity and low dew point, evaporative cooling and ventilation are effective cooling methods. Nighttime ventilation is effective below 70% relative humidity. Many first-world people demand air conditioning during hot weather, when the dew point is above 68°F (20°C) or when the relative humidity outdoors is above 70%.
At 70% relative humidity or above, the air feels either hot and sticky, or cold and clammy. These changing conditions aren’t comfortable to most first-world people. Air conditioners must remove moisture from indoor air, in addition to reducing temperature.