I've tried using a small fan to cool my Pi 3, the fan itself is only about four times the bigger than the main Broadcom chip. I have found that alone, at best has knocked 10°C off the CPU temperature. If your interested in that, you can buy the case and fan here. The fan plugs into the GPIO, so its powered by the Pi. Therefore, be sure to use a decent power supply because the CPU under load and a fan connected is going to require a lot of current to run.
As well as temperature you need to think about exposure to UV, electronics and UV are not the greatest combination. So the Pi is going to need to be protected by a UV proof enclosure along with the camera.
The main Broadcom chip is not the only consideration with regards to cooling, I have heard the Ethernet/USB controller can get hot under load. As well as the power delivery circuitry on the Pi. You can get a heatsink set that will cover anything that as the potential to get really hot, for example this set.
Whatever enclosure the Pi is you need to think about airflow, heatsinks will not work effectively unless there is a current of air passing over them. Also given you are leaving the Pi outside you might want to consider some weather proofing.
Be aware that the Pi is designed to run without active cooling, it will only start to thermal throttle at 85°C. However given the Pi could be under a lot of load with the whole camera set-up, I do recommend cooling.