Relative Strain Index¶
The Relative Strain Index (RSI) is a dimensionless warm-season heat-stress index. It expresses the strain a warm, humid environment places on a young, healthy adult from just two variables: air temperature and the ambient water-vapour pressure of the air.
Index origin: Lee, D.H.K. & Henschel, A. (1966) Effects of physiological and clinical factors on response to heat, Annals of the New York Academy of Sciences 134(2):743–749. https://doi.org/10.1111/j.1749-6632.1966.tb43059.x
Implemented closed form and assessment bands: Asghari, M. et al. (2020) Feasibility of Relative Strain Index (RSI) for the Assessment of Heat Stress in Outdoor Environments: Case Study in Three Different Climates of Iran, The Open Ecology Journal 13:11–18. https://doi.org/10.2174/1874213002013010011
How to use¶
You need 2 m air temperature in Kelvin and relative humidity in percent. The
ambient water-vapour pressure e (in hPa) is derived internally from these two
inputs with calculate_nonsaturation_vapour_pressure, so no separate vapour
input is required.
It returns the Relative Strain Index, which is dimensionless.
The implemented form is RSI = (Ta − 21) / (58 − e), with Ta the air
temperature in °C and e the ambient water-vapour pressure in hPa. RSI is a
summer index meant for air temperatures up to about 35 °C, and it rises with
both temperature and humidity (through the vapour-pressure term e). At lower
air temperatures it stays near the comfort level across most of the humidity
range, though very humid conditions can still lift it past the comfort threshold.
Interpret the output¶
The index is read against five strain levels (Asghari et al. 2020, Table 2):
| Relative Strain Index | Interpretation |
|---|---|
| < 0.15 | Climate comfort |
| 0.15 – 0.25 | Discomfort for sensitive people (the elderly and children) |
| 0.25 – 0.35 | Discomfort for all people |
| 0.35 – 0.45 | Risk of exposure to excessive heat (heat stroke) for 50% of people or more |
| ≥ 0.45 | Risk of hyperthermia for all people |
Provenance and variants¶
The index originates with Lee & Henschel (1966); the specific hectopascal closed form implemented here, together with the five-level assessment table above, is taken from Asghari et al. (2020), which states the formula with units and was read directly from the open-access source. The same thresholds (0.15 / 0.25 / 0.35 / 0.45) recur in the Romanian bioclimatology literature (Ciulache 2006; Ionac & Ciulache 2008). Some secondary sources attribute the level table to "Błażejczyk 2011", but no Błażejczyk 2011 publication tabulating RSI could be found under that citation — the 2011 Miscellanea Geographica paper cited defines the unrelated Bioclimatic Contrast Index — so this guide cites the verified Asghari (2020) tabulation instead.
A second, different closed form appears in the literature,
(10.7 + 0.74·(Ta − 35)) / (44 − Pa), where Pa is expressed in another unit
(most likely mmHg). It could not be verified against the primary text and is
not implemented here; only the peer-reviewed hPa form is provided.
Note the domain edge: as e approaches 58 hPa (near-saturation around 35–36 °C)
the denominator 58 − e shrinks, so the index grows without bound (±infinity),
and for e above 58 hPa (very hot and near-saturated, beyond the ~35 °C validity
range) the denominator turns negative and RSI becomes negative — a spurious value
for a heat-strain index. These out-of-range values are returned as-is and are
deliberately not clamped, so mask or guard the result if your inputs can reach
that regime.