Heat risk builds across hot days and nights. Read local alerts, check the hourly temperature and overnight low, and arrange access to a cooler place before heat illness begins.
What Is a Heatwave?
A heatwave is a period of unusually hot weather that lasts for several days and often continues through the night. The temperature does not have to reach one universal value. A level that is routine in one climate may be exceptional in another.
Weather services therefore define heatwaves against local conditions. They may compare forecast maximum and minimum temperatures with regional thresholds, the time of year, or measures that include humidity. Health agencies can use different criteria because the danger depends on more than the thermometer. Duration, warm nights, access to cooling, housing, age, health, and local adaptation all affect the outcome.
Official heat alerts use local criteria and may account for expected health impacts. Read the alert together with local health advice. Conditions can still be harmful before a formal threshold is reached, especially during outdoor work, exercise, illness, or prolonged exposure.
How Heatwaves Form
Many heatwaves develop under a persistent area of high pressure in the middle and upper atmosphere. Air sinks beneath the high, warms as it is compressed, and limits the rising motion that helps clouds and rain form. Clearer skies allow strong solar heating at the surface, while a slow or blocked weather pattern keeps the hot air mass over the same region.
Several processes can strengthen the heat:
- Warm-air transport can bring very hot air from a lower latitude or a dry continental interior.
- Dry soil leaves less water available for evaporation, so a larger share of the Sun’s energy heats the ground and nearby air.
- Light winds reduce the arrival of cooler air and offer less relief from heat at the surface.
- Clear skies increase daytime solar heating, although cloud can also make nights warmer by slowing heat loss.
- Blocking patterns can divert weather systems around the hot region for days.
A heat dome is a common name for this broad pattern of persistent high pressure and trapped heat. It is a useful description, but not a separate type of storm with a fixed boundary.
Why the Definition Changes by Location
Heat risk is tied to the local climate and to how well people and buildings are prepared. Homes in a usually cool region may retain heat and lack mechanical cooling. People who are not acclimatised may also experience strain at lower temperatures than residents of a consistently hot climate.
This is why warning systems often use regional thresholds. Some focus on daily maximum temperature, while others include overnight minimums, humidity, or expected health effects. The required duration also varies.
Use the local warning level rather than comparing one city’s temperature with another. A forecast map shows the weather; the warning system adds local risk and preparedness to that information.
Hot Nights Increase the Risk
The daily maximum gets most of the attention, but the minimum can be just as important. A cooler night gives the body, buildings, roads, and public spaces time to release some of the heat stored during the day. When the temperature stays high overnight, that recovery is limited.
Several hot days and nights create cumulative strain. Indoor temperatures can rise from one day to the next, particularly in top-floor apartments, poorly insulated homes, and rooms with direct afternoon sun. Sleep also becomes harder, which adds fatigue and can make it more difficult to recognise early symptoms of heat illness.
Check the hourly forecast and the overnight low, not only the daytime high. If your home is no longer cooling after sunset, plan access to a cooler place before the next afternoon.
Humidity, Sunshine, and Heat Stress
The body releases heat partly by evaporating sweat. High humidity slows evaporation, so the body has to work harder to maintain a safe temperature. This is why a humid day can create more heat stress than a hotter but drier day.
The heat index combines air temperature and relative humidity to estimate how hot conditions feel to a person in the shade under light wind. Direct sun, heavy clothing, strenuous activity, and some medications can raise the strain beyond that value. Read feels like temperature explained for more about apparent-temperature calculations.
For work or vigorous exercise in direct sunlight, wet-bulb globe temperature, or WBGT, is often more useful. It accounts for temperature, humidity, wind, solar radiation, and cloud cover. Heat index and WBGT use different scales, so their values should not be compared as if they were two thermometer readings.
- Air temperature: the standard shaded air measurement used in a weather forecast.
- Heat index: temperature and relative humidity, intended for general heat stress in shade and light wind.
- WBGT: temperature, humidity, wind, and solar radiation, intended for work or exercise, especially in direct sun.
Relative humidity changes as temperature changes, so it does not tell the whole story by itself. Humidity and dew point explained shows how both values describe moisture in the air.
Cities Can Stay Hotter
Built-up areas often retain more heat than nearby rural land. Dark roofs and paving absorb solar energy, buildings restrict airflow, and there may be less shade and evaporative cooling from plants. Concrete, brick, and asphalt then release stored heat after sunset.
This urban heat island varies from one street to another. A tree-lined park can be much cooler than an exposed square, while a top-floor room can stay hot long after the outdoor temperature falls. Waste heat from vehicles, industry, and air-conditioning systems adds to the local effect.
The nighttime difference is especially relevant during a heatwave because it reduces the period available for recovery. Local conditions matter even within the same forecast area.
Heatwaves and Climate Change
Natural weather patterns still determine where and when an individual heatwave develops. A warmer climate raises the baseline on which those patterns act. This makes extreme heat more likely and allows some events to reach temperatures that would have been less likely in a cooler climate.
The size of the change differs by region, and the effects depend on humidity, soil moisture, urban growth, and adaptation. Climate trends describe changing risk over many events. They do not replace the weather forecast for a particular place.
How Heat Affects the Body
Your body gains heat from the environment and from its own activity. It loses heat through radiation, moving air, and the evaporation of sweat. Trouble begins when heat gain exceeds the body’s ability to cool itself.
Heat can worsen heart, lung, kidney, metabolic, and mental health conditions. Dehydration can also affect circulation and kidney function. Risk rises with exposure time and physical effort, so a healthy, fit person can still develop a serious heat illness during hard work or exercise.
Heat exhaustion
Possible signs include heavy sweating, headache, nausea, dizziness, weakness, intense thirst, cramps, and cool or clammy skin. Stop the activity, move to a cooler place, loosen unnecessary clothing, cool the skin, and take small sips of water if the person is fully alert and able to drink.
Seek medical advice if symptoms worsen, do not improve, or include vomiting. Local health guidance should take priority because first-aid advice can vary with the person’s condition.
Heatstroke
Heatstroke is a medical emergency. Confusion, slurred speech, seizures, fainting, loss of consciousness, or severe changes in behaviour are critical warning signs. Skin may be hot and dry or still sweaty, so the presence of sweat does not rule heatstroke out.
Call emergency services immediately and begin rapid cooling while help is on the way. Move the person to a cool or shaded area, remove excess clothing, wet the skin, and use cold wet cloths, fans, or a cool bath if available. Do not force an unconscious or confused person to drink.
Who Faces Higher Risk?
Anyone can be affected, but risk is higher for:
- older adults, infants, and young children;
- pregnant people;
- people with heart, lung, kidney, metabolic, or mental health conditions;
- people taking medicines that affect sweating, circulation, alertness, or fluid balance;
- outdoor workers, athletes, and anyone doing strenuous physical activity;
- people who live alone or have limited mobility;
- people without reliable access to a cool space;
- residents of hot upper-floor rooms or dense urban areas;
- people who are not yet acclimatised to the heat.
Do not stop prescribed medication or change fluid intake for a medical condition without professional advice. A doctor or pharmacist can explain whether heat affects a medicine and how to store it safely.
How to Stay Safe During Extreme Heat
- Follow local warnings. Check the forecast and any instructions from weather, health, and emergency authorities.
- Plan around the coolest hours. Move exercise and heavy work where possible, and take more frequent breaks in shade or a cooled space.
- Drink regularly. Water is suitable for most people. Follow medical advice if you have been told to restrict fluids.
- Cool the body. Use cool showers, damp cloths, light clothing, and a cooler room.
- Reduce indoor heat. Shade sun-facing windows during the day. Open windows when the air outside is cooler, but close them when outdoor air is hotter.
- Use fans with care. A fan moves air but does not lower the room temperature, and it may not prevent heat illness when indoor air is very hot. Follow local health advice and move to an air-conditioned or designated cool place if your home stays hot.
- Check on others. Contact relatives, neighbours, and friends who may need help reaching a cooler place.
- Never leave a person or animal in a parked vehicle. The interior can become dangerous rapidly, even with a window partly open.
- Protect against sunlight. Seek shade, wear suitable clothing, and check the UV index when you need to be outdoors.
Heat can also coincide with stagnant air and smoke. Check the air quality index before strenuous outdoor activity, especially if you have a respiratory or heart condition.
How Heatwaves Are Forecast
Forecast models track the strength and movement of high pressure, incoming air masses, cloud cover, soil moisture, wind, humidity, and nighttime temperatures. We compare the expected conditions with the local climate, while official alerting authorities apply their own weather and health criteria.
Confidence is usually higher when several model runs show the same persistent pattern. Small changes in cloud, wind direction, or the position of the high can still shift the hottest area. The forecast becomes more specific as the event approaches and new observations arrive.
Look beyond a single maximum value. The hourly temperature, overnight low, humidity, feels like value, wind, UV index, and air quality together give a better picture of exposure.
Check Heat Conditions with Airpult
Our forecast page shows modelled air temperature and feels like temperature, the hourly temperature curve, each local day’s high and low, humidity, wind, and the current UV index. Air quality appears in a separate card where data and account access are available.
Use the hourly curve and overnight low to see whether the forecast offers relief. Airpult does not assign a heatwave label or replace official heat alerts and health advice. Use Explore to check current conditions and the forecast for a location.
