When temperatures start to rise, one question inevitably follows: when is the next UK heatwave?
Unfortunately, there isn’t a fixed heatwave calendar. Heatwaves develop as a result of particular weather patterns, and forecasts become less certain the further into the future they look.
That means the next UK heatwave could be weeks away, months away or even arrive the following year.
What we do know is that periods of extreme heat are an increasingly important consideration in the UK. So, as well as keeping an eye on the weather forecast, it makes sense to think about how prepared our homes are for hotter conditions.
In this guide, we’ll look at when UK heatwaves tend to happen, what officially counts as a heatwave and the steps you can take to keep your home cooler when temperatures rise.
When is the next UK heatwave?
There is no way of giving a reliable date for the next UK heatwave far in advance.
Weather forecasts are most useful over shorter time periods, while longer-range forecasts generally describe broader trends rather than telling us exactly what the temperature will be on a particular day.
If you’re wondering whether a heatwave is currently on the way, the best place to check is the latest Met Office forecast and any active weather warnings.
A period of warm weather doesn’t automatically mean a heatwave is coming, either. Temperatures need to remain above specific regional thresholds for several consecutive days before the conditions officially qualify.
When do heatwaves usually happen in the UK?
UK heatwaves are most commonly associated with the summer months, particularly June, July and August, when the sun is strongest and average temperatures are highest.
However, unusually warm spells can also occur outside the traditional summer period.
The UK can experience very warm weather during late spring, while high temperatures can sometimes continue into September. This means it is possible to experience heatwave-level temperatures earlier or later than you might expect.
Exactly when hot weather develops depends on wider atmospheric conditions.
Periods of high pressure can bring settled weather, clear skies and strong sunshine, while the movement of warmer air from continental Europe or further south can also contribute to particularly high UK temperatures.
So, while summer is the most obvious time to expect a heatwave, there isn’t a particular week or month when one is guaranteed.
What counts as a heatwave in the UK?
A few hot days don’t necessarily constitute an official heatwave.
The Met Office defines a UK heatwave as a period when a location records at least three consecutive days where the daily maximum temperature meets or exceeds its heatwave temperature threshold.
Importantly, the threshold isn’t the same everywhere.
Different parts of the UK have different heatwave temperature thresholds because what is considered unusually hot varies according to the local climate.
Current thresholds range from around 25°C in many northern and western areas to 28°C in parts of southeast England.
This also explains why one part of the country can officially be experiencing a heatwave while another isn’t.
And one exceptionally hot afternoon?
Technically, that’s just a very hot day.
Are UK heatwaves becoming more common?
The UK’s climate is getting warmer.
According to the Met Office, rising global temperatures are making extreme heat events more frequent, longer-lasting and more intense. UK climate projections also indicate that hotter, drier summers are likely to become increasingly common during the 21st century.
That doesn’t mean every summer will contain a major heatwave.
UK weather will remain variable, and cooler or wetter summers will still occur. But the overall direction of travel means preparing buildings for periods of high temperatures is becoming increasingly important.
Historically, improving a home’s thermal performance has largely focused on one question:
How do we keep heat inside during winter?
Increasingly, we need to ask another:
How do we stop homes becoming uncomfortably hot during summer?
Why do some UK homes get so hot during a heatwave?
A home can gradually accumulate heat throughout a hot day.
Sunlight passing through windows is one of the biggest contributors. Solar energy enters through the glass and warms floors, furniture and other surfaces inside the property.
Once that heat is inside, getting rid of it can become difficult, particularly when temperatures remain high overnight.
Certain properties may be more susceptible to overheating, including:
- top-floor flats
- homes with large south-, east- or west-facing windows
- properties with limited shading
- homes with poor opportunities for cross-ventilation
- properties in densely built-up urban areas
- homes with restricted window openings
- very airtight homes without appropriate ventilation or overheating measures
Official hot-weather guidance similarly highlights factors such as large unshaded windows, limited ventilation and highly airtight or energy-efficient homes where overheating hasn’t been properly considered.
Electrical appliances, cooking and even the people inside the property generate additional heat.
Combine all of these factors during several consecutive hot days and indoor temperatures can quickly become uncomfortable.
Does insulation keep your house cool during a heatwave?
This is where things get slightly more complicated.
We normally talk about insulation in the context of winter because its most obvious benefit is reducing heat loss from a building.
But insulation works by slowing heat transfer through the building fabric.
During summer, this can also help slow the transfer of external heat through elements such as walls and roofs.
However, insulation alone won’t prevent overheating.
A large proportion of summer heat can enter through windows as solar gain or be generated inside the home through cooking, appliances and occupants.
Once that heat is inside, a highly insulated and airtight building may also lose it more slowly if there isn’t sufficient ventilation.
That doesn’t mean insulation is the problem.
It means summer comfort depends on several elements of a property working together, including:
insulation + shading + ventilation + glazing + airtightness + occupant behaviour
A well-designed home should be able to reduce unwanted heat gains during the day and then release excess heat when outdoor temperatures fall.
How to keep your house cool during a heatwave
You don’t necessarily need air conditioning to make a noticeable difference to indoor temperatures.
Some of the most effective measures involve preventing unwanted heat from entering the property in the first place.
Close curtains and blinds before rooms get hot
If strong sunlight is shining through a window, close the curtains or blinds before the room has already heated up.
Windows facing east may receive intense morning sunlight, while south- and west-facing windows can experience significant solar gain later in the day.
Official hot-weather advice recommends using curtains and blinds to reduce the amount of heat entering sun-facing rooms.
External shading, such as shutters or awnings, can be particularly effective because it blocks solar radiation before it passes through the glass.
Be strategic about opening windows
One of the first things many of us do during hot weather is open every window in the house.
But that isn’t always the best option.
If the temperature outdoors is substantially higher than the temperature inside your home, opening windows can allow more hot air into the property.
Instead, use windows strategically.
Ventilate during the early morning, evening and overnight when outdoor temperatures are cooler. During the hottest part of the day, keeping sun-facing windows closed and shaded may help prevent additional heat from entering.
Where possible, opening windows on opposite sides of the property when conditions are cooler can create cross-ventilation, helping warmer indoor air escape.
Stop creating unnecessary heat indoors
Your home doesn’t only gain heat from outside.
Many everyday activities create heat internally, including:
- using the oven
- cooking on the hob
- running electrical equipment
- leaving unnecessary lights switched on
- using computers and televisions for long periods
- running certain household appliances
During a heatwave, try moving heat-generating activities to cooler parts of the day.
If you’re planning to use the oven, for example, cooking later in the evening may be preferable to having it running during the hottest part of the afternoon.
Use cooler nights to release heat
Night-time ventilation can play an important role in cooling a property after a hot day.
When temperatures outside fall below the indoor temperature, opening windows can help release accumulated heat and introduce cooler air.
The process can be particularly valuable after several consecutive hot days, when parts of the building may have gradually absorbed heat.
Remember to consider security when leaving windows open overnight.
Reduce solar gain
If overheating is a regular problem rather than something that happens once every few years, it may be worth looking beyond temporary measures.
Long-term options can include:
- external shutters
- awnings
- solar-control glazing
- external blinds
- strategically planted trees
- pergolas and other shading
- appropriate window design
Stopping sunlight reaching the glass can often be more effective than trying to remove the resulting heat once it has entered the building.
Can external wall insulation help during hot weather?
External wall insulation is primarily installed to reduce heat loss and improve the thermal efficiency of solid-wall properties.
However, improving the thermal performance of the external walls can also slow heat transfer through the building envelope during warmer weather.
This can contribute towards a more stable indoor environment, but it shouldn’t be viewed as a standalone solution to summer overheating.
If a property receives large amounts of direct sunlight through its windows, for example, external wall insulation won’t stop that solar energy from entering through the glass.
Similarly, if heat can’t escape from the building overnight, indoor temperatures may continue to rise over several days.
For that reason, anyone carrying out a broader energy-efficiency retrofit should consider both winter performance and summer overheating.
Should you use a fan during a heatwave?
Fans don’t actually lower the temperature of a room.
Instead, moving air across the skin can make us feel cooler by helping sweat evaporate.
They can therefore be useful for personal comfort, particularly when temperatures aren’t extreme.
But there’s an important distinction between making yourself feel cooler and cooling the building itself.
A fan won’t remove the heat stored inside walls, floors or furniture.
For longer-term comfort, reducing solar gain and using cooler outdoor air at the right times can be just as important.
What about air conditioning?
Air conditioning can provide effective cooling, but it comes with trade-offs.
Systems require electricity to operate and may significantly increase household energy consumption during hot periods.
For UK homes that only experience uncomfortable temperatures occasionally, passive measures such as shading, ventilation and reducing solar gain may make more sense as a first step.
Properties with persistent overheating problems may require a more detailed assessment to identify the causes before deciding on the most appropriate solution.
How can you prepare your home for future heatwaves?
The best time to think about overheating isn’t necessarily when you’re already sitting in a 30°C bedroom.
If hot weather regularly causes problems in your property, use the cooler months to work out where the heat is coming from.
Look at:
Your windows
Which windows receive the strongest sunlight?
Could blinds, curtains, shutters, awnings or other shading reduce solar gain?
Your ventilation
Can cooler air flow through the property effectively?
Are there opportunities for cross-ventilation?
Your insulation
Are your walls and roof properly insulated?
Improving the building fabric can help create a more thermally stable home throughout the year when combined with appropriate ventilation and shading.
Your loft
Upper floors can become particularly uncomfortable because heat rises within the home and the roof is exposed to strong sunlight.
Appropriate roof or loft insulation is an important part of a home’s overall thermal performance.
Your surroundings
Trees, vegetation and external shading can all help reduce direct solar exposure.
Even relatively simple changes to the outside of a property can sometimes have a noticeable impact on the amount of summer sun reaching windows and walls.
When will the next heatwave happen?
Unfortunately, there’s no date you can circle on the calendar.
The next UK heatwave will depend on weather patterns developing at the time, which is why shorter-range forecasts from the Met Office are much more useful than trying to predict extreme heat months in advance.
What we can say is that high temperatures and extreme heat are something UK homes will increasingly need to be designed and adapted to cope with. The Met Office expects warmer summers and more frequent extreme heat as the climate continues to warm.
So, rather than only asking “When is the next heatwave?”, there may be a more useful question:
“Will my home be ready when it arrives?”
Improving insulation, controlling solar gain, introducing effective shading and making sensible use of ventilation can all contribute to a home that’s more comfortable throughout the year, whether it’s freezing outside or the UK is experiencing its next spell of extreme heat.
For current temperatures, weather warnings and heatwave forecasts, always check the latest information from the Met Office.
