Thermal Comfort Explained

Orbital Stack’s Thermal Comfort analysis provides objective metrics (e.g., SPMV*, UTCI) and vivid imagery that show how outdoor spaces meet their desired usage profiles. Thermal comfort is a complex concept as it accesses an individuals perception of temperature which are reliant on both physiological/environmental causes and psychological factors.

The thermal comfort analysis combines wind and solar simulations of your site geometry with a complex array of climate data, including temperature, humidity, cloud cover, activity level, clothing, wind direction and speed, summarizing the results in plots that indicate the overall thermal comfort of patrons in that space.

Thermal comfort can be evaluated using a variety of different approaches from the really simple metrics like Humidex and Windchill to much more complex methods. The following table lists some common metrics and shows the factors that each takes into account when judging comfort. Orbital Stack supports UTCI and SPMV*.

Index Observations
1) Windchill/ Humidex Temperature based. No clothing or activity.
2) Operative Temp Does not include the impact of RH or activity.
3) WBGT Used to define heat stress on a body.
4) UTCI Good for moderate climates. People dress for weather.
5) PET Defines conditions to an equivalent T.
6) PMV (INDOOR ONLY) Wide acceptance. Only good for indoors.
7) SPMV* Outdoors. Wider sensitivity.

Interpreting thermal comfort results

When preparing thermal comfort results in Orbital Stack, the analysis is performed on windows of time (usually a season and a subset of the day – eg summer mornings). The goal is to provide insights that do not span a set of wildly opposing conditions, but rather a relatively small and uniform period.

Even though that is the goal, results must still be taken in context. The raw SMPV* or UTCI metrics are based on the average condition during the analysis window. Comfort can still vary significantly across sample days and hours during that window. Because of the averaging, it has the potential to blur results by canceling warm samples against cold samples.

To alleviate this challenge, Orbital Stack also provides “Percent of Time In-bounds” plots. These evaluate each record within the analysis window against an upper or lower comfort bound (SPMV Comfortable or SPMV Acceptable) and measures the percentage of hours that are within the range. This has proven to be the most effective way to gauge the comfort of a particular location.

For more information check out SPMV Thermal Comfort Criteria Explained and UTCI Explained.