The original note, as written at the time
A ride sharing service where discounts are given on rainy days. This idea is to encourage more ride sharing to offset weather based reasons for owning personal cars. This can also encourage more and more people to use these services.
Ongoing development
A ride-sharing service that gives discounts on rainy days, to encourage sharing instead of driving a personal car, and to pull more people onto the service.
The pricing is the entire product. Ride-hailing already charges more when it rains, because demand rises and the algorithm is doing what algorithms do. Reversing the sign is a genuinely different proposition, and it says something about why the service exists.
The behaviour being targeted is specific. A person who would normally take a boda or a bus takes a car in the rain, because the rain is the moment when comfort becomes worth money. That is a narrow, predictable, weather-triggered decision, and a targeted discount can beat it — which a general discount never could.
The economics can work if the discount is funded by matching rather than by the driver. Rain concentrates demand in time and place, so pooled trips are easier to fill in the rain than in the dry, which means the same vehicle serves more people per hour. The discount is paid for by the pooling.
The design question is whether the discount applies to solo trips. It should not. The point is not to make cars cheaper in the rain; it is to make sharing cars cheaper in the rain, and a discount on solo travel would simply subsidise the thing being discouraged.
There is a fairness wrinkle worth naming: the people most likely to change behaviour for a discount are those for whom the discount matters, which means the scheme is regressive in who it persuades.
Price the weather, and price it towards the shared vehicle.