Heatwave Action Plan 

Heatwaves have become increasingly common at our latitude these past years. Recent trends show that heatwaves are now more frequent and more intense: it is no longer a matter of ‘if’ we will have a heatwave period, but rather ‘when’ it will occur, ‘how hot’ it will be and ‘how long’ it will last.

In 2023, CERN developed an Organization-wide Heatwave Action Plan consisting of a set of preventive and corrective measures to mitigate the effects of sustained high temperatures on its Personnel during a heatwave. These measures are environmentally responsible and inspired by best practices used in the Host States. They will only apply during defined heatwave periods that depend on official meteorological forecasts, monitored closely by the HSE Unit to determine the need to activate the Heatwave Action Plan.

Weather forecast



* Average temperature over the course of 24h, taking into account temperatures over night as well.

¹Weather data by MeteoSuisse.admin.ch
²Weather data by Open-Meteo.com

IMPORTANT NOTE: the Heatwave Action Plan will only be activated upon official notification by MeteoSwiss: the HSE Unit will follow the official MeteoSwiss criteria for declaration of heatwaves.

The dynamic weather forecast table on this page provides a visual indication on the possible forecast. It shows the tendency for the next 14 days, where the first 5 days are taken from the official MeteoSwiss predictions and the remainder from an additional forecasting indicator for longer-term predictions. Simply refresh the page for it to automatically update.

Please see the section HWAP activation and information to the community for information on how the heatwave action plan’s activation will be announced to the CERN Community.


Beating the heat: symptoms and tips

Heat may have an adverse impact on our health and work. A rapid rise in ambient temperature compromises the body’s ability to auto-regulate its own temperature and metabolic processes. Heat can cause dizziness and headaches, a sore throat, cramps, dehydration, exhaustion as well as changes in our behaviour. It can also lead to more severe or lasting health troubles, such as respiratory infections for example.

Weather forecast


* Average temperature over the course of 24h, taking into account temperatures over night as well.

¹Weather data by MeteoSuisse.admin.ch
²Weather data by Open-Meteo.com


The majority of these adverse health effects generally occur during heatwave periods, which are defined when the average temperature over the course of a 24h day is high (above 25oC) and is sustained for several consecutive days.

We all feel the heat differently. Whenever high temperatures in the work environment become difficult to bear, the following tips can help avoid unpleasant symptoms:

  • hydrate yourself regularly (at least 1.5 litres of water per day);
  • keep windows and blinds closed during the day;
  • use an office fan;
  • take regular breaks;
  • seek a cooler environment, if possible. The maximum temperature difference between indoor and ambient (outdoor) conditions should not exceed 8oC;
  • minimise heavy physical tasks during the warmest hours, and wear light clothing, if it is compatible with your work.
  • When working outside, wear sunglasses and sunscreen and appropriate clothing.

If you are pregnant or breastfeeding or suffer from a chronic illness, we advise you to consult your doctor to define additional measures if and as appropriate.

Beat the heat
Here are five tips on keeping cool and staying well from the CERN Medical Service (Image: CERN)

When would the Heatwave Action Plan be activated and how will the CERN community be informed?

Reaching maximum temperatures at mid-day alone does not necessarily mean we are in a heatwave.

According to MeteoSwiss, epidemiological analyses show that there is a strong correlation between average daily temperature and the impacts of heat stress: it is not only the hottest temperatures during the day time that have a negative effect on the human body, but also high temperatures at night.

An average daily temperature above 25oC is the first indicator that a heatwave period might be around the corner. Average temperatures exceeding 25oC have to be forecast for at least three consecutive days for a heatwave to be officially declared. See scientific basis and criteria below.

In the event of a heatwave, the activation and end of the Heatwave Action Plan will be announced on relevant communication channels:

  • home.cern,
  • hse.cern,
  • Posters on restaurant screens (see below),
  • Messages on site entrance screens.

Lessons learned

Each winter, a Lessons Learned exercise will be organised with respective stakeholders to improve the heatwave action plan based on previous experience with the aim to review its effectiveness in terms of implementation.

Heatwave action plan in force
This poster will be shown on restaurant screens when the Heatwave Action Plan is in force.

Detailed measures of the plan

What we can do on an individual level

  • Foresee recuperation/hydration pauses every hour;
  • Discuss the possibility of flexible work organisation with supervisor, if necessary;
  • Keep windows and blinds closed during the day;
  • Minimise heavy physical tasks during the hottest hours;
  • Request the purchase of office fans;
  • Wear light clothing, if it is compatible with tasks and workplace;
  • If working outside, be equipped with sun-glasses and sunscreen, and appropriate clothing;
  • If your activities foresee work alone, discuss with your supervisor to find suitable adaptation modalities during the heatwave period to avoid it as far as possible.

Recognize the symptoms: if you experience any of these, take a recuperation/hydration pause immediately!

What supervisors can do

Everyone perceives and responds to heat differently. If your supervisees express discomfort in their office in hot weather, check the temperature of the office if you can and follow proposed measures from the Swiss State Secretariat for Economic Affairs (SECO)

Further, in line with your Department/Large Experiment’s implementation plan (see also 'organisational administrative and technical measures below):

  • Discuss the risk linked to excessive heat with your Supervisees, giving special attention shall be provided to workers at risk (based on a medical certificate);
    • Note: CERN Safety Regulation on Responsibilities and organisational structure in matters of Safety at CERN (SR-SO), stipulates that “responsibilities in matters of Safety regarding employed Members of the Personnel follow the hierarchical line”. The Supervisor shall ensure Health & Safety of their supervisees during excessive heat, namely with respect to the protection of pregnant and breastfeeding workers (maximum indoor temperature at workplace is 28oC) and other workers at risk.
  • Should your supervisees work in a building or area that might meet the criteria for being considered ‘critical’ with respect to heat, HSE recommends that you discuss with your TSO of the building or zone concerned who will inform the concerned DSO. The DSOs will, once per year, collect the requests and present an updated proposal to the DSOC for recommendation to the SCE Department.
  • Identify the suitable ventilated A/C meeting rooms that will be available during heatwaves;
  • Foresee the purchase of equipment, e.g. office fans, Temp measurement devices;
  • Request the purchase of office fans;
  • Recall the importance of declaring any incident (dangerous situation, near miss, etc.) using the EDH Incident Declaration Form
  • Discuss possible adjustments to working hours/telework according to the organisation administrative measures below.

Avoid fire risks: heat also affects your portable Lithium battery devices!

Lithium ion batteries that power our portable devices are sensitive to heat and can become a fire hazard. Maximum charging temperatures of most common portable lithium-ion batteries are defined in the respective user manuals and is typically around 35oC.

  • During a heatwave, don’t leave your device to charge unsupervised;
  • Keep it away from direct sun or additional heat sources;
  • Check it regularly to avoid possible malfunctions (see this article for more details).

Organisational administrative measures

During a heatwave period, if required, specific flexible measures are proposed to the Personnel, such as the possibility of extended teleworking (under OC 7, per chapter II. 7 – heatwaves will be considered a “specific circumstance”) ) as well as possible adjustments to the eight-hour working day between 7 a.m. and 8 p.m. (outside shift work provisions). Supervisors may allow recuperation/hydration pauses every hour.

  • Please contact your supervisor to understand possible modalities.

Organisational technical measures

Departments and Large Experiments implement specific measures based on respective activities and needs. In this vein they are encouraged to:

  • Review activities affected by the heatwave and adapt the modalities of their execution, if required (change of operating mode, change of working hours, avoidance of lone working, reserve the afternoon booking of the main air conditioned (and ventilated) amphitheatres, auditoriums and meeting rooms to allow temporary access to perform office-related tasks.
    • Check with your DAO/Experiment secretariat to see which rooms might be available
  • Carry out increased surveillance and awareness, by the Supervisors and Safety officers, of:
    • specific indoor and outdoor activities (e.g. avoid lone working activities in areas with extreme heat, visits to ‘critical buildings’, etc.)
    • Critical buildings (with respect to heat) identified by SCE

NOTE: Installation on a large scale of A/C is not foreseen, given legal constraints but also environmental considerations (A/C units lead to increased energy consumption and greenhouse gas emissions) as well as health and safety aspects (A/C units are noisy and may create a breeding ground for microorganisms, such as Legionella).

Scientific basis and criteria

The HSE Unit will monitor the average daily temperatures using the weather forecasting models from MeteoSwiss to inform of the imminence of a heatwave, which in turn will see the entry into force of the HWAP.

Heatwave period

Swiss forecasting ECMWFWeather forecasting models

In the field of meteorological prediction models, there is an important balance to strike between notice given and uncertainty: the longer the forecast, the higher the model uncertainty (see figure - courtesy of ECMWF). In order to avoid possible false alarms and ensure an acceptable uncertainty level, MeteoSwiss base their alerts on a 5-day forecast.

Why do we use the average temperature metric instead of the heat index?

Until 2021, the state-of-the art metric to declare heatwaves was the Heat Index (HI), which included the combination of absolute dry-bulb temperature and relative humidity at a given time of the day. Since the summer of 2021 experts have changed the metric to the average temperature over the course of a 24 hour day (Tmean). Why? MeteoSwiss explains:

  • The threshold values and criteria for the heat warning concept have been established on the basis of new scientific findings from the Swiss Tropical and Public Health Institute (Swiss TPH). Epidemiological analyses show that there is a strong correlation between Tmean and the impacts of heat stress, especially when it comes to mortality. Even a single day with high temperatures can be dangerous for the human body. For this reason, an additional warning level (Level 2) was introduced in order to warn the public of short, intense hot spells.
  • It is not only the hottest temperatures in the daytime that have a negative effect on the human body, but also high temperatures at night. If the nights are not sufficiently cool, the body struggles to recover and is therefore less able to cope with heat stress, as demonstrated by a project of the National Centre for Climate Services (NCCS). Tmean is a very appropriate parameter for use in heat warnings, because it is an average of values over 24 hours and therefore includes night-time temperatures.
  • According to the Swiss TPH, humidity in the Swiss climate affects well-being but not mortality. Humidity is indirectly accounted for by Tmean, in that when humidity is high there is a lesser degree of cooling at night. A higher minimum temperature will therefore result in a higher mean temperature over 24 hours. Tmean thus incorporates the new epidemiological findings for Switzerland.

Situation 2023

As a reference, the heatwave period was declared during a total of 16 calendar days in 2022, and 10 calendar days in 2023.

Heatwave situation 2023