The Weather Watchman app is a side project I developed and self-funded its development. It ran on iTunes and Google Play for around 3 years. I have unpublished it for the present time while I update it. The small movie below shows how it worked, using affiliate ads to generate revenue.
The App works by performing a reverse lookup for historical weather conditions via a weather database, based on users preferred time to travel in the year. I have also started a blog to accompany the App at www.weatherwatchman.com.