UC Berkeley

The heat index is an apparent temperature that measures how hot it feels when humidity is factored in with the actual air temperature. Defined in 1979 by Robert Steadman, the heat index is based on a model of human thermoregulation, and each value maps to a unique physiological and behavioral state of the human. The heat index thus defined is widely used by weather agencies to communicate the health risk associated with high heat and humidity. However, Steadman's model gives unphysical results at sufficiently high temperature and humidity, leading to an undefined heat index. In the 1970s, instances of an undefined heat index were uncommon, but global warming is increasing the frequency of conditions that leads to an undefined heat index. In this dissertation, the problem with the model is identified and fixed, allowing the heat index to be defined for all temperature and humidity. Similar to the existing heat index, the extended part also maps to physiological states of a human under extreme heat stress, such as hyperthermia or heat death, providing a tool to assess the regional health outcomes at different levels of global warming.

Before the problem with the model is fixed, weather agencies relied on a widespread polynomial extrapolation designed by the National Weather Service to estimate the heat index at high temperature and humidity. Now, with the extended heat index, we have an opportunity to reassess past heat waves in the United States. In this dissertation, three-hourly temperature and humidity are used to evaluate the extended heat index over the contiguous United States during the years 1984 to 2020. It is found that the 99.9th percentile of the daily maximum heat index is highest over the Midwest. Identifying and ranking heat waves by the spatially integrated exceedance of that percentile, the Midwest once again stands out as home to the most extreme heat waves, including the top-ranked July 2011 and July 1995 heat waves. The extended heat index can also be used to evaluate the physiological stress induced by heat and humidity. It is found that the most extreme Midwest heat waves tax the cardiovascular system with a skin blood flow that is elevated severalfold, approaching the physiological limit. These effects are not captured by the National Weather Service's polynomial extrapolation, which underestimates the heat index by as much as 10 K during severe heat waves.

Another long-lasting problem of the heat index is that it has never been validated against laboratory data. In this dissertation, we use the laboratory data obtained by physiologists at Penn State, showing that the model underlying the heat index can correctly predict the combinations of temperature and humidity that cause hyperthermia. This is the first time the heat-index model has been validated against physiological data from laboratory experiments. For light and moderate exertion in an indoor setting, the heat-index model predicts hyperthermia would occur at the heat-index value of 345 K, consistent with the experimental results. For the same setting and exertion, the heat-index model predicts the core body temperature would equilibrate at the fatal value of 315 K at the heat-index value of 366 K, establishing a heat-index threshold for the survivability of humans.

Global warming poses a direct threat to human health. Many previous studies of future impacts have relied on the wet-bulb temperature, defined as the equilibrium temperature of a wet thermometer under an infinite wind, to predict human survivability. However, the wet-bulb temperature is not an accurate metric for human heat stress because it does not account for human exertion, finite wind speed and radiation heat exchange. With the laboratory-validated heat index, we find that a commonly used survivability threshold of 35 °C for wet-bulb temperature would overestimate the occurrence of heat death, but would underestimate the occurrence of hyperthermia. For example, in a world warmer than pre-industrial by 10 K, the best estimate is that 30% of the world's population would be exposed once or more per year to a wet-bulb temperature above 35 °C, but less than 2% would be exposed to fatal conditions and over 60% would be exposed to conditions that would cause hyperthermia.