This is the preview shown on the App Store for LuckGrib on iPhone.
The intent here is to show that LuckGrib does in fact run very nicely on an iPhone. The device shown is an iPhone SE, which is the smallest iPhone in the Apple lineup.
Showing LuckGrib on iPad makes for a much nicer demonstration, which is why there is a long iPad introduction but not one for iPhone. There are a few differences when you run LuckGrib on iPhone and iPad, but the two are very close. There is support for the same weather models on both iPhone and iPad, all the same draw styles, sidebar, timeline, and so on. One difference is that less text information is shown on iPhone, as the smaller screen makes it impossible to show as much information as on iPad.