What is Wet Bulb temperature?
In a recent study with Matt Huber, we showed that it doesn't take that many degrees of global warming to permit peak heat summertime heat stress to (occasionally) become unsurvivable, in many parts of the world that are currently highly populated.
We came to this conclusion by considering a meteorological quantity called the wet-bulb temperature. You measure this quantity with a normal thermometer that has a damp cloth covering the bulb. It is always lower than the usual or "dry-bulb" temperature; how much lower depends on the humidity. At 100% humidity (in a cloud or fog) they match. In Sydney and Melbourne, even during the hottest weather, the wet-bulb usually peaks in the low 20's C. The highest values in the world are about 30-31C, during the worst heat/humidity events in India, the Amazon, and a few other very humid places. This map shows peak annual afternoon wet-bulb temperature attained in the present climate (this is a better-quality version of the figure in our PNAS paper).
(Aside: It is surprising how often I hear people say "...it was 38C and 80% humidity," or something like that. So far these claims have invariably been incorrect---usually, the high temperature was in the afternoon while the high relative humidity was in the morning or evening. If they had coincided, the wet-bulb temperature would have been well over 30C, but I do not find any evidence of this and there are good reasons (explained in the paper) why the atmosphere would not let it happen....in today's climate at least. The warmer the tropical oceans become, however, the higher this "thermostat" will be reset.)
Here is a model prediction of what the above map would look like in a world of roughly 10C warmer global mean temperature (also an improved version of the figure from our paper):
Heat stress is more conventionally measured by other quantities such as the "wet-bulb globe temperature" (WBGT), "Apparent temperature," "humidex," and so on. These are essentially empirical approximations of how a person would feel, that take the humidity and the temperature into account and "translate" them into a single, "feels like" temperature. None of these numbers has any theoretical basis or direct relation to heat-transfer laws, and they are only good approximations within a certain range of temperatures and humidities (often excluding the most extreme conditions).
The wet-bulb temperature is probably not a very good predictor of the "feels-like" temperature for most common conditions, which is why it is not used for this. However, it can be used to establish an absolute limit on metabolic heat transfer that is based on physical laws rather than the extrapolation of empirical approximations. That is why we focused on it instead of the usual measures.
Here is a table comparing temperature, relative humidity, approximate-WBGT and wet-bulb temperature. The approximate WBGT value assumes typical outdoor wind and sun conditions, and would be lower indoors. The wet-bulb temperature is dependent of wind or sunlight. These are computed at a pressure of 1013 hPa.*
Temperature = 30C (86F):
Temperature = 40C (104F):
The last two entries in the 40C table are far above any reported wet-bulb temperature values in the present climate, but would become possible in humid tropical areas with roughly 6C or 10C of further global warming, respectively. WBGT values in excess of 28C raise heat-stress concerns for strong and extended exertion, and those above 35C are considered dangerous for any exertion. Sustained wet-bulb temperatures of about 35C would surely be deadly even for a person at rest; the corresponding WBGT limit is somewhere in the high 40's / low 50's (the exact value depends a lot on the temperature/humidity combination).*Thanks to J. Buzan for finding errors in a previous version of this table. Last updated 7 March 2012.