This month, the Aerospace Technology Institute (ATI) announced the launch of Project MIST, the latest funding initiative in its Non-CO2 portfolio. Led by Honeywell in collaboration with Boeing and the University of Reading, it aims to advance in-flight sensing capabilities to improve the accuracy of contrail forecasting. Adam Morton, the ATI’s Head of Technology – Sustainability & Strategy, considers the pressing need for such technology and the main challenges this presents.
In 2024, the ATI launched its Non-CO2 Programme in partnership with the Department for Business and Trade (DBT) and Innovate UK. In parallel, ATI also published a pioneering Non-CO2 Technologies Roadmap, setting out the technological advancements needed to address the climate impact of these emissions. The first three projects funded under this programme were announced in November 2025. This fourth project in the series aims to develop an aircraft-based humidity sensor.
The latest research indicates that contrails may account for around half of the climate warming associated with aviation. At the same time, only a small proportion of all flights, possibly 2-3%, are responsible for 80% of that contrail warming. This is because the vast majority of flights do not encounter Ice Super Saturated Regions (ISSRs), where all the necessary conditions for persistent contrails combine. Therefore, a key prerequisite for successfully avoiding these contrail-prone areas is that both humidity and temperature are known at cruise altitude.
The vast majority of civil aircraft today do not have on-board humidity sensing equipment. In contrast, most of the cruise-altitude humidity data used in weather models is collected using weather balloons, supplemented by data from a small research aircraft fleet and satellite sources. With more than 100,000 civil aviation flights per day on a global basis, aircraft-mounted sensors offer much better coverage than that from sparse weather-balloon measurements, particularly at cruise altitude. Meanwhile, satellites lack the necessary granularity to accurately match contrails with the height at which they were generated. It is for these reasons that the International Air Transport Association (IATA) concluded in its 2024 report, Aviation contrails and their climate effect , that it is not currently possible to accurately plot ISSR locations or their borders.
Whilst humidity sensor technologies are readily available for other applications, at cruising altitudes ambient temperatures can reach -50°C or lower. Here, the water vapour concentration might be several thousand times lower than at sea level, whilst still capable of forming persistent contrails. Very few sensors can operate at such low humidity levels whilst producing highly repeatable results, with minimal maintenance and infrequent recalibration. Highly sensitive sensors must also exhibit fast reaction times, given the aircraft’s high speed through ISSRs of varying size. Whilst all of these factors partly explain the need for better sensor technologies, other technical criteria, such as weight and power requirements, are at or near the levels required.
Other outstanding issues still remain before suitable sensor technologies can be routinely deployed on the thousands of aircraft necessary. Humidity sensors require air intakes that can protrude from the fuselage. Whilst the impact of the associated drag is small at the individual aircraft level, fleet effects need to be taken into account to appropriately minimise fuel-burn penalties. Even for lower-profile, less intrusive options, retrofitting may have certification issues that need to be minimised through careful sensor design and integration. To support real-time contrail mitigation, the resulting data will also need to be fed into meteorological and forecasting models promptly. Rapid data transfer is therefore likely to be incorporated into commercially attractive solutions.
By incorporating such advanced sensor technologies to produce ‘airborne laboratories’, the output not only supports Non-CO2 mitigation through contrail avoidance. Across thousands of commercial aircraft, there is also a genuine prospect of filling key gaps in upper-air data, leading to more accurate warnings of storms and other hazardous weather conditions. It is these spillover benefits for other users that makes delivery of advanced humidity sensor technology even more important.
Further information on the Non-CO₂ Programme, including how to submit a funding application, can be found here.
