Using a particle accelerator, he demonstrated that cosmic rays colliding with molecules in the atmosphere can, in fact, cause gaseous water vapor to condense into cloud-forming droplets.

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While high-end evaluation methods like randomized control trials (RCTs) are the surest means of determining what interventions in fact cause certain outcomes, they are also time-consuming and costly and beyond the means of many, if not most, mainstream nonprofits.

FORBES: An Interview with Paul Carttar, Director of the Social Innovation Fund