UHIE Analysis details
Alternative – 1 : UHIE calculation details
The following guidelines must be followed while using the GRIHA UHIE calculator:
- The analysis is to be carried out for 21st March.
- GnPr of 1%, surface albedo of 1, and hard paved area of 100% can be considered in the base case. All other parameters, that is, hourly average air temperature on any reference day (°C), daily average solar radiation (W/m²), average height-to-building ratio, total wall surface area (m²), and SVF will remain the same in the base case and design case.
- SVF for stand-alone buildings with no surrounding buildings within a radius of 100 m can be assumed to be 1. All buildings in the immediate surroundings of the project within a 50 m radius have to be considered in the calculation of SVF as per Figure 2. The example, as shown in Figure 3, can be referred to for the calculation.
- For sites with more than one building, all buildings have to be considered in the calculation of wall area, SVF, albedo, and average height-to-building ratio.
- For sites with area > 5000 m2, UHIE analysis has to be done for at least 3 points (buildings of interest) onsite.
Definitions of terms used in the calculator
- Average of ambient daytime hourly air temperature: It is the average of hourly air temperature taken for the reference day in any typical season. This average must be for the sunshine hours (for example, morning 6:00 am to evening 6:00 pm) and must be taken for 21st March. For example, the ambient average daytime temperature for Delhi is 24.6°C, Mumbai is 26.7°C, and Bengaluru is 27.3°C, etc.
- Daily (daytime) average solar radiation (W/m2): The daily average solar radiation for the location should also be for the day of 21st March.
- Hard paved area (within 50 m radius) in percentage: This is the total percentage of hard paved area (buildings as well as paved area) in the study area.
- Average height-to-building FAR: The average height-to-building area ratio represents the thermal mass in the environment and it refers to the ratio of average of heights of all buildings (in the study area) to the total of floor areas of all the buildings (in the study area).
- Total wall surface area (m2): This is the total wall surface area of all the buildings that fall in the study area.
- Green plot ratio: The green plot ratio refers to the green density in the area and is calculated by the following formula:
GnPR = (Total tree leaf area + Turf area) ÷ Area of circle (site = 50 m radius)
Total tree leaf area = No. of trees × Canopy area × LAI
The LAI is a number ranging from 0 to 6; 0 depicts barren site, grass has LAI of 1, and 6 depicts extremely dense tree vegetation. On an average, most mature trees in India have a stand-alone LAI ranging from 3.5 to 5.5. The following are the representative LAI of some of the Indian trees:
- Sky view factor: SVF is the extent of sky observed from a point as a proportion of the total possible sky hemisphere. Its value would vary between 0 and 1 based on the extent of sky visible at the point of observation, 1 being the sky is visible at the fullest extent and 0 being the least (refer to Figures 2 and 3). Once we have the heights of the buildings and the width of road, the SVF can be calculated using the simple formula given below.
The formula for calculating SVF: SVF = cos (arctan ((H1 + H2)/W))
- SVFs should be calculated for two directions that must be perpendicular to each other. The average of the two should be considered for the calculation.
Alternative – 2 : UHIE simulations
The following guidelines must be followed while performing UHIE simulations:
- The analysis is to be carried out for 21st March, 9:00 am to 12:00 am (midnight); however, the
simulation results have to be shown for only 3:00 pm (daytime UHIE) and 9:00 pm (nighttime UHIE).
- Modelling has to be carried out for both the base case and the design case.
- Surface albedo of 1 and hard paved area of 100% can be considered in the base case. All other parameters, that is, hourly average air temperature on any reference day (°C), daily average solar radiation (W/m²), wind speed and direction, average height-to-building ratio, total wall surface area (m²), and SVF will remain the same in the base case and the design case.
- All geographical terrain > 3 m height must be modelled in addition to any surrounding water features.
- ‘Simple forcing’ can be used for the purpose of this simulation.