The performance and efficiency of cars are affected by the weather, and this effect can be greater for electric vehicles as vehicle batteries are particularly affected by ambient temperature.

It is likely that in the colder winter months the hybrid battery cannot achieve the same levels of performance, which results in the petrol engine being used more of the time and higher fuel consumption. In addition, higher winds and wet winter weather will also hurt fuel consumption, so this would explain why your Yaris Hybrid works best in warmer weather.