A short note on the inclusion of sultriness issues in perceived temperature in mild climates

Abstract

Sultriness is a specific case in thermal perception under warm/humid environments and frequently applied in German operational weather forecast and is assumed if the dew-point temperature t _d exceeds a threshold of 16.5 °C. In perceived temperature (PT), which is based on the ASHRAE two-node-model, a Central European climatology was prepared of the classical predicted mean vote (PMV), exclusively linked to thermal stress, and the so-called rational PMV* that additionally accounts for the humidity in the meteorological environment and is closely linked to thermal strain. The standard deviation of the difference ΔPMV = PMV* − PMV over PMV reveals a similarity to the appearance of sultriness that suggests as side effect of the PT parameterisation a thermophysiologically based definition of sultriness: “Under warm/humid conditions a subject adapted to mild climatologic conditions perceives sultriness if the actual value of ΔPMV exceeds the long-term mean by more than its single standard deviation”. This definition accounts for all environmental and subject related variables influencing the thermal state of the body and is in accordance with all in the literature described properties in the perception of sultriness. The two definitions coincide more or less at daylight hours for t _d values markedly beyond the threshold. However, in the t _d threshold region, the PT-derived definition offers stronger differentiated patterns and is significantly less frequent at nighttime than the mono-causal definition. For given PT values, the sensible heat flux via the skin shows an increase under a sultry environment, whereas the latent is reduced; skin and core temperatures as well as the skin blood flow are also less. In any case, PT-derived sultriness is linked with an increased thermal strain on the body temperatures, which is a measure of discomfort caused by increased humidity and/or dissatisfaction with the thermal environment. This confirms the thermal uncomfortable feeling accompanying the perception of sultriness. Therefore, the PT-derived sultriness can be presumed to be a more appropriate measure for the perception of sultriness, because it accounts for all environmental impacts on thermoregulation.