Heat Module overview

Ingredients

The calculation of the Heat Stress Overlays takes into account:

• Meteorological circumstances such as:
  • Sun angles, altitude and azimuth, dependent on date and time of day and project location.
  • air temperatures, (hourly, daily minimum and maximum);
  • global radiation, (hourly and average);
  • relative humidity (hourly);
  • wind speeds (hourly and daily average).
  • wind direction (hourly)
• Spatial situation, such as:
  • Land use, for fraction vegetation and rate of evaporation (Bowen ratio).
  • Terrain, foliage and building heights

Setup

Generally, the steps required to use and configure the Heat Stress Overlay are as followed:

1. Determine the location and area of interest and generate a project for that area;
2. Add the Heat stress overlay;
3. Open the Heat stress overlay wizard to kick off the configuration process.
   1. Select date(s) and time of day
   2. Supply daily weather station data
   3. Supply hourly weather station data
   4. Select source for Foliage height

Date and time of day

1. Configure for which date you want to generate results for. This can be one day or multiple consecutive days when using the Heat Overlay Wizard.
2. Configure per day which hours you want to generate results for. Each day-hour combination will become a result. This result is stored as a time-frame to relate it back to the day and time.

Generated sun parameters

For each day, and for each time of day, the following will be automatically generated when using the wizard:

1. The day and time pairs, along with the project location, will be used to automatically calculate the sun altitude and azimuth angles for each day-time pair.
2. The sun daily motion factor is a parameter used in the atmospheric temperature formula. It will be automatically selected from the sun motion factor table, based on the date (column) and time of day (row).

Weather station data

Next, manually determine which weather station you want to use as a reference. Download the historical hourly weather data for that station and use the data from certain columns as input. In a nutshell, using the weather data, you have to supply:

• Daily Values

1. the minimum and maximum temperature of the day;
2. the average wind speed, measured at the height of 10 meters;
3. average global sun radiation during sun hours;

• Hourly Values

1. The hourly temperature;
2. The hourly humidity;
3. The hourly sun radiation, as an average of the current and previous hour;
4. The hourly wind speed;

Foliage Height

Select what source will define the Foliage height:

• Buildings with functions related to foliage. The foliage height calculation model will determine the foliage height using the height of the vegetation (as buildings) and the foliage crown factor attribute;
• Areas with as foliage height attribute, defining the foliage height for a whole area;
• GeoTiff, defining a foliage height value for each cell in its raster.

Calculation of Physiological Equivalent Temperature

To calculate the Physiological Equivalent Temperature (PET), we first have to determine which formula (sun or shade/night) should be used. It is selected by:

• Day or night? Determined by the sun altitude angle;
• Is a cell shaded? Calculated by the Shade calculation model;

PET sun

The following factors influence the Physiological Equivalent Temperature for locations directly in the sun:

• The atmospheric temperature, calculated by the atmospheric temperature formula;
• The hourly global radiation from the sun, supplied as weather station data.
• The wind speed at 1.2 meter above ground, calculate by the Wind speed calculation model;
• The Wet-Bulb temperature, calculated by the Wet Bulb temperature formula;