The forecasts for October 5 predict 25°C on Monday and Tuesday, then 11 to 15°C on Thursday and Friday. A 15°C in May invites you to wear a T-shirt, while a 15°C on Thursday will bring out the coat. Yet the thermometer will display the same number: what changes is what the body and mind have experienced in the days prior.
Same number, two sensations.
- Comfort temperature follows the rolling average of previous days, not the current temperature
- An 15°C after 25°C demands a comfort target around 24–25°C, whereas an 15°C after a cold spell aims for 21–22°C
- The ASHRAE and EN 16798-1 standards create a gap of more than 3°C between the two cases for the same thermometer
The thermometer measures the air, not comfort
A thermometer says nothing about the clothes we wear or what we expected from the day. For a long time, standards relied on the PMV index, developed by Fanger in 1970, which sees comfort as a thermal balance between the body and its environment. This instantaneous physics fits well with a climate-controlled laboratory. A second approach, called adaptive, came to complement it based on field measurements.
The adaptive model rests on data collected in 160 buildings, a work associated with Dear and Brager. Its principle is that occupants of a naturally ventilated building adjust their clothing, their expectations, and the opening of windows. The temperature considered comfortable indoors thus follows that outside instead of remaining fixed. Berkeley researchers remind us that the narrow lab testing range was hard to maintain in real buildings, especially those with operable windows. The model emerged from this gap between theory and real life.
The American standard ASHRAE 55 gives the comfort temperature as 0.31 × the dominant outdoor mean temperature + 17.8°C. The European standard EN 16798-1 uses 0.33 × the rolling average + 18.8.
The word “rolling” matters, because it is an average over past days rather than the current temperature. ASHRAE calculates it over 7 to 30 days depending on the chosen method. In the EN 16798-1 approximate formula cited by REHVA, yesterday weighs 1, day before weighs 0.8, then the weight decreases to 0.2 for the seventh day. A Thursday at 15°C, preceded by two days at 25°C, would thus land on an average still influenced by the early-week heat.
The calculation: more than 3°C difference for the same 15°C
Let’s do a simple school calculation, using chosen averages for the example (these are not measurements). Suppose a rolling average of 20°C after hot days: the ASHRAE formula yields 0.31 × 20 + 17.8, or 24.0°C of comfort. For a May that stayed cool, take 10°C as the average: 0.31 × 10 + 17.8 gives 20.9°C. Both cases have the same 15°C outside, but not the same target.
With the European formula, you get 0.33 × 20 + 18.8 = 25.4°C in the first case, and 0.33 × 10 + 18.8 = 22.1°C in the second. The gap reaches 3.1°C on the ASHRAE side and 3.3°C on EN 16798-1.
More than 3°C difference in the target for the same outside 15°C.
These numbers illustrate a logic; they do not predict how you will feel on the street. The ASHRAE model applies only to spaces without heating or cooling systems in operation. Occupants must be able to adjust their clothing within a range of 0.5 to 1.0 clo. Their activity should remain moderate, between 1.0 and 1.5 met. Transposing these figures to the street is a matter of analogy, not measurement.
Clothing, expectations, physiology: what “adapting” covers
In May, 15°C follows winter, and one might think the expectation is for a thaw, making a T-shirt almost natural. In October, the same value comes after 25°C, i.e., 10 to 14°C cooler than the start of the week.
Clothing forms the most visible lever, since the model assumes occupants adjust it freely. On Monday and Tuesday, at 25°C, a light wardrobe was in order. On Thursday and Friday, between 11 and 15°C, it will likely no longer suffice.
The model groups clothing, expectations, and physiology under the term adaptation. The formula implies smoothing: the comfort target shifts with the average of recent days, not from one day to the next. A Thursday at 11–15°C, just after a start-of-week spell at 25°C, would thus land on a still-high target, hence the feeling of cold. In terms of actions, an extra layer in the morning, removed in the afternoon if the 15°C persists, compensates for the gap.
There remains physiology: the duration of acclimation is not quantified in the sources consulted.
Around the target, ASHRAE 55 allows a band of ±2.5°C for 90% acceptability and ±3.5°C for 80%. The 3.1°C gap calculated above between our two cases therefore matches the size of this entire band. The model, however, only applies if the dominant outdoor mean remains between 10°C and 33.5°C.
Sources: rehva.eu | cbe.berkeley.edu | tameteo.com
