Heat alert trial - Summer 2022

Heat alert trial aims to help reduce health impacts from hot weather


What does the project involve?

The heat alert trial has been developed and produced by MetService, in partnership with University of Waikato and the Institute of Environmental Science and Research to warn people in New Zealand of unusually hot weather.  The pilot expands on the first phase of the trial (which was the first of its kind in New Zealand) that ran last summer. With extreme heat events worsening in response to human-induced climate change a national heat alert system is really important – this trial is an important step towards that.

As part of the trial, MetService will monitor heat events for 44 urban areas across Aotearoa. When a significant heat event is forecast for one of the locations, a heat alert banner for the affected area will appear on the respective forecast page on metservice.com, and as part of the written forecast for each location.  In early 2023 they will also appear on the MetService weather app.

An alert will be triggered by an extremely high temperature (record or near record) or when a run of very hot and humid weather is forecast. Hot, humid weather is typically less well tolerated than hot, dry weather.


Why is this important?

Extreme heat during summer can be draining and have an impact on people’s daily lives – especially their health and wellbeing.  Everyone is vulnerable to extreme heat. However, babies and infants, older people, those with pre-existing medical conditions or on certain medications are more at risk. International research shows that extreme heat and heatwaves can cause illness and death, but effective planning and actions can reduce its effects on health.

If a heat alert is triggered it means that people should take precautions, such as keeping hydrated, seeking shade, checking in with vulnerable people/children and looking after your pets and livestock.

People are also advised to use the alert information to inform their decisions, for example not going for a long run or having employees outside picking fruit.


Tips for managing in hot weather events  

If you or others feel unwell

If you feel dizzy, weak or have an intense thirst or headache you may be dehydrated.

Drink water and rest in a cool place. Seek help if symptoms persist. 

If you are experiencing painful muscle cramps, your body may need electrolytes as well as fluid.

Drinking oral rehydration solutions or zero sugar sports drinks may help.
Seek medical advice if heat cramps last for more than one hour. 

Keep medicines below 25 degrees C or in the refrigerator (read the storage instructions on the packaging).


Keeping yourself and others safe in hot weather:

Drink plenty of water - try to drink two litres per day

Stay out of the sun

Have plenty of cold drinks

If you need to be in the heat, stay in the shade where possible

Avoid drinking alcohol, caffeine, and hot drinks

Use sunscreen

Eat nourishing food with a high water content (such as salads and fruit)

Wear a hat

Dress yourself and children in light clothing, cool cottons and natural fabrics are best

Avoid extreme physical exertion

People and pets should not be left in stationary vehicles

Take a cool shower of bath

Check in on others, especially those at most risk, children, elderly or those with health issues

Spray or sprinkle water over skin or clothing, a damp cloth on the back of the neck can help

heat awareness3 pets and animals


Tips for keeping your home cool

Open windows in the evening and overnight, once temperatures have cooled

 Turn off non-essential lights and electrical equipment – they generate heat.

 Home insulation will help keep your home cooler in summer

 Move into a cooler room if possible, especially for sleeping

 Keep light-coloured curtains or blinds closed on windows facing the sun during the day.

Dark coloured curtains or blinds are best left open as they absorb the heat

 Use electric fans to keep cool 

Use the cool cycle on a heat pump. Clean heat pumps – vacuum the filters regularly.



What are the thresholds that define a significant heat event?

An alert will be triggered by an extremely high temperature (record or near record one day temp) or when a run (two day or more) of very hot and humid weather.

To define extremely high temperature (one day) thresholds for the 44 locations selected for the second trial, daily maximum temperature data were used.  MetService used all available daily data 1993-2021 for each location to estimate appropriate thresholds used in the trial. 

This temperature metric captures ‘actual’ peak temperature each day. When forecast ‘actual’ temperatures are forecast to meet or exceed the threshold for the weather station at that location on any given day, a heat alert will be issued.  

To define a run of very hot and humid weather (two days or more) temperature thresholds for the 44 locations selected for the second trial, hourly observation data were used from MetService weather station locations nationwide to calculate the daily maximum” feels like” temperature.

The “feels like” temperature is calculated using temperature, humidity and wind. MetService used all available hourly data 1993-2021 for each location to estimate appropriate thresholds used in the trial. 

When two consecutive days are forecast to meet or exceed the ‘feels like’ threshold for the weather station at that location, a heat alert will be issued.  


We've taken three factors identified by the Ministry of Health that contribute to heatwaves, to define a heat alert event:

  • Day-time temperatures - the increase in above average temperature can cause adverse impacts on health. Locations with higher average temperatures (Gisborne for example) have higher thresholds than locations with lower average temperatures. 
  • Humidity – High levels of humidity can make it feel hotter (perceived heat) and can negatively affect the body’s cooling mechanisms. Humidity has a marked effect for some locations , such as Auckland.
  • Duration – Heat alerts are based on a threshold being reached for at least two consecutive days. Extended periods of hot and/or humid weather increase the risk to and impact on health.


What happens when a threshold is met?

As part of the trial for the 2022/23 summer, a heat alert banner will be generated on metservice.com, on the respective forecast page of the location. MetService may also use social media to generate awareness of the heat event.

In the case of heat alerts within the Canterbury or Auckland region, MetService is providing data directly to Te Mana Ora WaitahaCanterbury (formerly Community and Public Health, Canterbury DHB) and the Auckland Council.


When will the trial run for?

From December 2022 through to 30 March 2023.