Heat stress advisory: an index for time spent outdoors
A figure gets quoted whenever the week turns punishing, and households hear it as a verdict on the air in their rooms. It was built for people spending hours in the sun. Placed correctly it still earns one use here.
By Team Snowflake | Updated 8 Aug 2026
What goes into the reading
The heat stress advisory reports an index built from three sensors sitting side by side. The published figure combines all three by fixed weights. It comes out in degrees, and that is where most of the misreading begins.
The three are a natural wet-bulb reading, a black globe reading, and an ordinary dry air thermometer. The Meteorological Service Singapore sets out the combination as seven parts wetted sensor, two parts globe, one part dry air. Plain air temperature is the smallest of the three contributions to a number most people hear as a temperature.
NEA makes the distinction plainly on the page that publishes the figure. The measure takes in air temperature, humidity, wind and solar radiation together, and it reads differently from air temperature alone. Two afternoons matching on a thermometer can sit at very different points on this scale.
The wetted sensor carries the humidity and wind work, and wet-bulb temperature is covered separately here. Only its share matters at this level. Seven parts out of ten means the index is driven mostly by what evaporation can still achieve in that air, and only lightly by how hot the air is.
The globe reading is the input with no indoor counterpart. It is a thermometer inside a matt black sphere, which heats under sun and under radiating surfaces the way any dark object does. Move the same sphere into shade and its reading drops. That one input sets the character of the whole index.
Why the sphere matters more than it looks
A black globe measures load that a normal thermometer is built to exclude. Weather thermometers sit inside ventilated screens precisely so sun and hot surfaces cannot reach them. The globe is the opposite instrument. It exists to capture what the screen keeps out.
So the index climbs on a still, cloudless afternoon while air temperature barely shifts. Sun landing on a body and air that has stopped moving both enter the figure. Neither shows up on a household thermometer, and neither belongs to a shut room.
This also explains why a single national number would be useless. Radiant load and wind change over short distances, so the reading is taken at many points and published point by point. A field in the open and a shaded corridor a few streets away report very differently under identical weather.
Who the advisory was written for
NEA launched the advisory in July 2023 with the environment ministry, developed in consultation with the Ministry of Health's Heat Stress Guidelines Expert Panel. It was built for the general population weighing up prolonged outdoor activity. Hiking and running are the examples its own explainer reaches for.
The scope statement is short and it is published beside the figure. The advisory applies to prolonged outdoor activities under direct sun exposure. That sentence carries two conditions, and both have to hold before the figure means what it says.
It also names what falls outside. Brief outdoor movement at ordinary exertion sits beyond the advisory, because neither the duration nor the effort is present. Walking to a bus stop sits well outside what the guidance was calibrated on.
The advisory sorts the reading into named risk levels and attaches guidance to each. Where those boundaries fall, and what is asked at each level, belongs to NEA and the health panel behind it. Numbers repeated second-hand are exactly how this material gets quoted wrong, so this page points at the source and stops there.
People at work are covered by a separate instrument
MOM publishes a heat stress framework for outdoor work, and it is a different document with a different audience. It addresses occupiers and employers, and it concerns work done primarily in the open without permanent shade. The obligation lands on whoever engages or employs, and not on the person standing in the sun.
The framework names the required measures and the workplaces that must monitor the reading on site. On-site meters are mandated at construction sites above a stated contract value, at shipyards, and in the process industry. MOM sets out what each measure is and inspects workplaces against them. Those particulars belong in the framework itself.
Both instruments key off the same underlying index, and that shared root is what makes them easy to run together. A member of the public reading an advisory and an employer meeting a legal duty are doing quite different things with one number.
Does the advisory describe the air indoors?
It does not, and the reason is built into the instrument. The stations sit outdoors, placed in built-up locations with heavy human traffic. Stadiums, a reservoir and the meteorological station at Changi all appear on the published list.
The solar term is the part a shut room cannot supply. A black globe indoors, out of the sun, is measuring something quite unlike the same globe on an open field at full noon. Point the index at a room with drawn curtains and a running unit and its two largest inputs have gone.
An indoor question has its own readings, and they are different ones. What the air in a room holds is a humidity question, and the indoor humidity target guide covers the range a home here should hold. Why one room at the same setting feels worse than the next is perceived temperature. Where water will form on a surface is dew point.
A high advisory reading carries no information about a machine. It describes outdoor exposure at a point on the island, averaged over a short window just past. Whether a coil is fouled, a charge is low or a drain is blocked gets answered at the unit itself, with the panel off and instruments on the circuit. No public index stands in for that.
The air-quality index has the same shape of problem, and PSI bands covers it: an outdoor figure that households read as an indoor instruction. Recognising the shape once is usually enough to catch it the second time.
| The reading a household sees | What it describes | What it cannot settle |
|---|---|---|
| The reading a household seesAir temperature from a forecast | What it describesDry air inside a shaded, ventilated screen | What it cannot settleHow hard the sun is loading a person outside |
| The reading a household seesThe heat stress figure | What it describesOutdoor exposure under direct sun at one station | What it cannot settleAnything about the air in a closed room |
| The reading a household seesRelative humidity | What it describesHow full the air sits against its own capacity | What it cannot settleHow that same air behaves once it is cooled |
| The reading a household seesThe air-quality index | What it describesOutdoor pollutant levels averaged across a region | What it cannot settleAnything thermal at all, inside or out |
The instruments that do answer an indoor question
A thermometer and a hygrometer on a shelf report the two figures a bedroom complaint turns on. Both are cheap, both sit in the room being complained about, and neither depends on a station somewhere else.
Reading them at the moment the room disappoints beats any national figure pulled from an app afterwards. A note of what the pair said, alongside the setting on the remote, turns a general impression into something a technician can work from.
Where it does touch an aircon owner
The condenser is outdoors, so anyone working on it is outdoors as well. That is the one place where a measure built around outdoor exposure carries into an aircon job. Coil cleaning on a ledge, a leak trace along an external run, a fan motor swap: all of it happens in the open.
The duty attaches to the arrangement and not to the weather report. MOM places the obligation on occupiers and employers, so the party engaging or employing the person outside is who the framework is aimed at. Which role a given household occupies turns on particulars this page cannot see.
Two adjacent subjects sit on separate pages. Telling a planned outdoor job apart from an improvised one, and identifying which named role planned the reach, belongs to aircon work at heights. The money side of an injury on site sits with contractor insurance for aircon work.
A visit moved because of the conditions outside is the framework doing its job. It can read as an excuse from where the customer stands. The scheduling call belongs to the party carrying the duty, and pressing against it asks a customer to overrule a judgement they do not hold.
What a household contributes is description. Where the unit sits, whether the approach is shaded or fully open, and how much of the work has to happen past the window line all change what the job involves. A booking form shows none of it.
- A condenser on an exposed ledge with nothing overhead
- A roof or podium deck where the plant sits in the open
- A shared platform reached across unshaded ground
- Pipe or trunking work running along an external wall
Shade sits on a different axis from height
Two properties of the same outdoor position matter here, and they do not travel together. How far a person could fall decides one set of duties. How much sun and still air the spot holds decides another. A ground-floor condenser in a walled service yard scores low on the first and high on the second.
So a description of the position beats a description of the storey. A shaded plant corridor and a west-facing parapet on the same floor are different environments by this measure, and the difference repeats on every future visit. Saying which one you have costs a sentence and changes what a visit has to account for.
Where the live figure and the rules are published
The current reading is public and free to check. NEA publishes it on the weather information portal and in the myENV app, station by station across the island. Each published figure averages a short window just past, so it tracks the present far more closely than a daily index manages.
The same readings are released as an open dataset, so station-level history can be pulled directly. The network has grown since launch: it began with nine sensors, and the published station list now runs several times longer. Sites were picked for heavy human traffic, which is why stadiums and parks appear on it.
The workplace framework sits with MOM on its own page, alongside the measures and the readings each is keyed to. An employer or occupier needing the detail should read it at source. Quoting outdoor work while unable to point at the governing document tells you how closely it was read.
Other indices exist that attempt to express how heat feels to a person, and they are assembled from a different mix of inputs.
What this page stops at
This page places the measure and names who publishes it. It carries no thresholds, no risk levels and no guidance on what anyone should do at any reading. Those belong to the agencies that calibrated them, and they are the part most often repeated wrong.
Nothing written here is health advice or a stand-in for the advisory itself. NEA publishes the public advisory and MOM publishes the workplace framework. Both are open to read, and both are the right place to take a question this page has only located.
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