Clouds formed by aircraft (contrails) are the most easily visible human forcing of the climate system. Trapping energy in the Earth system, they contribute more than half of the total climate impact of aviation. This makes reducing contrails an important goal to achieve the UK's climate commitments. Theoretical considerations indicate two pathways for reducing contrails. First, improving engine design to emit fewer particulates may reduce contrail lifetimes and so their climate impact. Second, rerouting aircraft to avoid contrail forming regions. Assessing these pathways requires accurate models of contrail formation, evaluated at the level of an individual aircraft. This evaluation requires observations of contrails across their lifetime, coupled to details of the generating aircraft. Even where they are matched to specific aircraft, existing observations typically view a contrail once, (limiting their use for measuring contrail lifecycles) or cannot provide the detail on the contrail microphysical properties (such as ice crystal number or shape) necessary to assess the efficacy of different pathways to contrail reduction. Improving confidence in our contrail models urgently requires novel observations of contrail properties and lifecycles from individual aircraft. The impact of aircraft on clouds is not limited to contrails forming in clear air. Over half of contrails form embedded in existing clouds and the particulates emitted by aircraft can affect cloud formation several days after they were released. These effects produce a cooling, potentially large enough to offset all other warming effects of aviation, but are not represented in aircraft-level models used for planning contrail avoidance strategies. There are few observational constraints of these effects, targeted observations of the impact of individual aircraft on cloud microphysics are required to assess them and to improve future model simulations. To address these uncertainties and around contrail formation, persistance and climate impact as well as aerosol-cloud interactions, COBALT has three core components: 1. A measurement campaign in the southern UK, combining an array of ground-based cameras with a steerable cloud radar, to make high resolution observations of contrail formation from individual aircraft. Guided by aircraft transponder information, these observations will be focused on contrails and clouds modified by aircraft, characterising contrail formation and perturbed cloud properties within the first few hours of their lifecycle. 2. Counterpart satellite observations, using novel techniques to characterise contrail and cloud development from an hour to several days behind the aircraft. Building on techniques for studying natural cirrus, this will produce a complete characterisation of the contrail lifecycle, along with the first estimate of the aviation aerosol impact on existing cirrus clouds at a global scale. 3. The complete lifecycle characterisation will be combined with flight data from aircraft operators to produce a unique dataset designed specifically for the evaluation of aircraft-level models of contrail formation. An initial focus will be placed on evaluating aircraft-scale models, as these are currently being used to plan aircraft diversions. A comparison of climate model parametrisations of contrail formation will assess the ability of the parametrisations to reproduce the wide-area (>1000km2) contrail observations taken by the camera array. Led by an inter-disciplinary team of scientists and engineers, with partners in key international research centres and industry groups, COBALT will provide the tools necessary to evaluate our current models and ability to avoid contrails, guiding future modelling and operational trials of sustainable fuels and contrail avoidance.