FIAT scientific formula reveals the perfect weather for roof-down driving in Britain

• FIAT has partnered with Dr Rob Thompson, Meteorology Researcher at the University of Reading, to develop the La Dolce Vita Convertible Driving Index (CDI).
• The scientific model scores roof-down driving conditions based on temperature, sunshine and wind, with any rainfall automatically reducing the score to zero.
• Evaluation of 15 years of weather data found that almost 25% of the year (88 days) in Britain has decent convertible-driving conditions, with 34 deemed good, 22 great and 32 superb.
• Analysis found the optimal roof-down conditions occur at around 24°C, with 75% sunshine and a light-to-gentle breeze of 4-12mph.
• Late summer and early autumn can often provide some of Britain’s best convertible-driving weather, so there are still plenty of top-down days ahead in 2026.
• September typically delivers around 12 days rated good or better for roof-down driving, including four superb days.
• With more than half of Britain's leading car brands no longer offering a convertible, FIAT continues to champion open-air motoring with the Fiat 500 Convertible.
• Fiat 500 Convertible represents one of the most accessible routes into open-air motoring, with a choice of hybrid or fully electric powertrains and prices starting from £21,995. The fully electric version benefits from the Government's Electric Car Grant, making it available from just £23,245.

A scientist has found the formula for the perfect weather for roof-down driving, with results suggesting that there is still plenty of enjoyable convertible-driving days to come this year.

FIAT has partnered with Dr Rob Thompson, Meteorology Researcher at the University of Reading, to create the La Dolce Vita Convertible Driving Index (CDI), a scientific model designed to identify the ideal conditions for open-air motoring in the UK. With more than half of Britain's leading car brands no longer offering a convertible, FIAT continues to champion open-air motoring with the Fiat 500 Convertible, a model that continues an open-air tradition that started with the brand’s first car, the Fiat 3 ½ HP,  in 1899.

Using established thermal comfort principles, plus 15 years of Met Office weather data from locations across Great Britain, if rain is not forecast, the index combines a temperature score, sunshine score and wind conditions to identify the weather most likely to deliver an enjoyable roof-down driving experience. Dr Thompson has created a supporting equation: CDI = Tempscore + SunScore + Windscore.

The research found that the ideal roof-down driving occurs when weather conditions create an effective cabin comfort temperature of around 21°C, typically achieved when outside temperatures reach around 24°C with 60 per cent humidity, around 75% sunshine, cloudy intervals and a light breeze of around five miles per hour.

Crucially, the model's temperature score is based on how hot it actually feels inside an open-top car, factoring in humidity and the cooling effect of the wind, rather than simply the air temperature reported in a weather forecast.

According to Dr Thompson’s research, Britain experiences conditions favourable for convertible driving for almost 25% of the year (88 days), with 34 days deemed good, 22 great and 32 superb.

While many motorists associate roof-down driving with the hottest days of the year, the analysis suggests some of the best convertible-driving conditions can occur after peak summer. Early autumn often provides an ideal combination of warmth, sunshine and comfort, with September typically delivering around 12 days rated good or better for roof-down driving.

To illustrate how the index works, Dr Thompson applied the La Dolce Vita CDI to the forecast for a typical early September day at the University of Reading's Whiteknights campus. Based on a forecast of 24°C and light 10mph winds, the CDI score is 64, placing it in the good category, despite cloudy skies. Slightly warmer temperatures or a little more sunshine, would move it into the great category, highlighting how typical late-summer weather can provide excellent roof-down driving conditions.

Regional variation was significant. Heathrow recorded the strongest results, averaging 11.2 superb days a year, compared with Tiree in Scotland's Western Isles, which averages just one every five years – evidence that drivers in south-east Britain get considerably more chances for ideal open-air motoring than those further north and west.

Dr Rob Thompson, Meteorology Fellow at the University of Reading, said: “We wanted to move beyond the common assumption that hotter automatically means better for roof-down driving. Using long-term weather data and meteorological analysis, we developed a model that evaluates the combination of temperature, sunshine, rainfall and wind conditions that contribute to driver comfort.

“What emerged was a more nuanced picture, with the best scores typically occurring when warm temperatures, sunshine and gentle breezes coincide, rather than during the most extreme summer heat. The research also highlights clear regional differences, with south-eastern parts of Britain typically enjoying far more high-scoring roof-down driving days than western and northern areas.”

As drivers look to make the most of Britain's perfect roof-down days, the Fiat 500 Convertible offers one of the most accessible routes into open-air motoring. All variants of the all-electric Fiat 500e are eligible for the UK Government's £3,750 Electric Car Grant, reducing the starting price of the 500e Convertible to £23,245 on-the-road. Meanwhile, hybrid versions of the Fiat 500 Convertible are available from £21,995 on-the-road, making iconic Italian style and open-top driving more attainable than ever.

Kris Cholmondeley, managing director, FIAT UK, said: “The Fiat 500 Convertible is a great example of how FIAT brings joy to driving through style, colour and open-air fun. That’s why we were fascinated to explore what science says creates the perfect roof-down driving experience.

"While every driver has their own favourite conditions, the research shows there are more opportunities for enjoyable top-down motoring in the UK than many people might expect.

"At a time when so few manufacturers offer convertible models, we’re proud to give drivers the chance to enjoy those special open-air moments behind the wheel of the Fiat 500 Convertible.”

To find out more about the Fiat 500 Convertible, visit HERE.

3^rd September 2026

ENDS

NOTES TO EDITORS

The La Dolce Vita Convertible Driving Index (CDI)

The La Dolce Vita Convertible Driving Index (CDI) was developed by Dr Rob Thompson, Meteorology Research Fellow at the University of Reading, to identify the weather conditions most likely to deliver an enjoyable roof-down driving experience.

The CDI was applied to 15 years of Met Office weather station data from locations across Great Britain.

How the CDI works

If rain is forecast:

CDI = 0

If rain is not forecast:

CDI = TempScore + WindScore + SunScore

The CDI combines:

• Temperature Score (0-60)
• Sunshine Score (0-30)
• Wind Score (0-10)

CDI Ratings

• Wet Day: 0
• Poor: 1-20
• Fair: 21-50
• Good: 51-70
• Great: 71-85
• Superb: 86-100

Key Findings

• Britain experiences approximately 88 Good, Great or Superb roof-down driving days each year.
• Approximately 32 days achieve the highest "Superb" classification.
• The highest-scoring conditions occur at around 24°C, with 75% sunshine and a light-to-gentle breeze of 4-12mph.
• South-eastern regions typically record the highest number of favourable roof-down driving days.
• Late summer and early autumn can often outperform periods of extreme summer heat for roof-down driving comfort.
• The CDI's temperature model is based on thermal comfort rather than air temperature alone, taking humidity into account to estimate perceived comfort in an open-top vehicle.
• The research found conditions equivalent to a cabin comfort temperature of around 21°C produced the highest scores.

La Dolce Vita CDI Temperature Scoring

For this we use the Maximum and Relative humidity in the day to estimate the effective cabin temperature, corrected for humidity, in the wind from the motion – this is almost always lower than the air temperature - as this is the biggest factor for comfort in the car, with the optimal being set to 21°C

If humidity isn’t available, it can be estimated from the minimum temperature:

As for the ideal timing, mid to later afternoon, spanning into early evening in the summer, avoiding the most burning sun.

La Dolce Vita CDI Wind Scoring

Ideally, we want a light breeze, enough to reduce the pollution and pollen, but not enough to affect driving, but strong winds rapidly reduce the enjoyment, so negative scores hurt otherwise good days.

La Dolce Vita CDI Sunshine Scoring

Overcast isn’t what we imagine for a top-down drive, but too much sun leads to sun burn, plus a gentle cloudscape adds to the view, the ideal is about 75% clear.

Seasonal Analysis

By looking at the full 15 years we can work out how often the different categories are achieved.

Looking at the frequency of somewhere reaching good, great or superb conditions and then applying a 15-day rolling average (because 15 years makes noisy data) we find that most summer days are good somewhere in Britain from mid-Jun to early-September. The top-down driving season starts in Early April, with the odd great day happening in April. The arrival of Autumn sees a slight dip in the chance of that perfect driving day occurring during September, but the first half of October still retains even the occasional superb day.

Methodology

Analysis was conducted using 15 years of Met Office weather station data (2011-2025) from locations across Great Britain.

Ref 26-31

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