The impact of very hot weather on health cannot be ignored. Are we ready to deal with it? Let us first explain the heat-related alerts or warnings issued by the Observatory (Figure 1):

When the weather becomes hot, especially when temperature is high, humidity is high and winds are weak, the public should take appropriate precautions to reduce the risk of heat stroke. When such weather conditions are expected, the Observatory will issue the "Hot Weather Special Advisory" to remind members of the public to prepare for hot weather.

When Hong Kong is expected to be affected by very hot weather, the Observatory issues the Very Hot Weather Warning (VHWW) to alert members of the public to beware of heat stroke and take precautionary measures. Under prolonged heat condition or weather generally becomes extremely hot, the Observatory will also issue the following Special Weather Tips as necessary when the VHWW is in effect:

The above messages will be disseminated through the Observatory's website, the "MyObservatory" mobile application, TV and radio broadcasts. App users can also opt for receiving push notifications. People who are more vulnerable to heat stroke (Figure 2) should stay particularly vigilant or need more care. They should drink more water and take all necessary protective measures to prevent discomforts caused by very hot weather, and pay due attention to health conditions. Under extremely hot weather, vigorous physical activities should be avoided. Other measures against the heat and measures for sun protection should not be ignored. For more information on protection against the heat, please visit the following websites (with links):

In addition to temperature, other factors such as moisture, winds and solar radiation can affect heat stress and the risk of heat stroke as well. Therefore, the Observatory established the "Hong Kong Heat Index" (HKHI) which comprehensively considers temperature, humidity, airflow and solar radiation. According to the joint study by the Observatory and a local university, the public should take appropriate precautions when the HKHI at King’s Park is around 30 or above to avoid health effects caused by hot weather.

The HKHI is not temperature but an index that takes into account the aforementioned factors. Under the same temperature conditions, the HKHI will be lower with lower humidity, weaker solar radiation, or stronger winds. For example, on 14 September 2022, it was sunny and very dry in Hong Kong with a maximum temperature of 35.5 degrees Celsius recorded at the Observatory. The highest HKHI was only 27.5 as the daily average relative humidity was just 44% at King’s Park. On the other hand, the HKHI will be higher under conditions of higher humidity, scorching sun, and lighter winds. For example, Hong Kong had a higher relative humidity under the influence of a southwesterly airstream on 12 August 2019. The maximum temperature recorded by the Observatory was 34.0 degrees, not reaching the extremely hot level. However, the maximum HKHI at King’s Park reached 32.4 and the average relative humidity at King’s Park that day was 82%. The HKHI is composed of multiple elements and is affected by changes in multiple factors. Among them, the value of solar radiation varies due to cloud cover or showers with higher fluctuations. There are still limitations in accurately predicting the index by computer forecasting models.

In summer, everyone should pay more attention to heat related alerts or warnings from the Observatory and take all necessary protective measures against the heat and the sun. Those who are more vulnerable to heat stroke should pay special attention to prevent discomforts caused by the heat.