Heat Stress
What is Heat Index ?
Generally, in weather reports, they give an actual temperature and then a “feels-like” temperature. This “feels-like” temperature is much higher than the actual temperature. This “feels-like” temperature is known as Heat Index. Heat Index is based on 2 variables:
- Actual Temperature
- Humidity
To identify the heat index, the following standard table can be used. According to this table, when the actual temperature and the humidity are known, the heat index can be identified.

Generally, when the heat index gets higher than 105, it is not good. So it is very essential to use not only the outdoor temperature but also the heat index when planning outdoor activities.
High heat index levels were prevalent from March to September 2019 in Northern, North-western, Southern and Eastern provinces. However, Nuwara-Eliya and Badulla districts showed very low levels of HI throughout the year.
Extreme HI values were observed from May to June in Jaffna district. People living in the area should be cautioned to minimize outdoor activities to prevent heat related morbidities.
The same pattern in heat index variation is continued for the first 4 months of 2020 as well.
The annual monthly variation of heat index plotted for 2019, 2020 and 1986 – 2018 depict a recurring trend in heat index variation where, the HI increases from January to May and decreases from May to December.
Annual Monthly Variation of Heat Index 1986 – 2020

HEAT INDEX RANGES
80 – 90 | Caution |
91 – 104 | Extreme Caution |
105 – 129 | Danger |
> 130 | Extreme Danger |