FYI, the iOS Weather app does have this now. If you click on a day in the 10-day forecast, you get a detail view with hour-by-hour forecast data for many metrics. One of them is pressure (although I don't normally look at barometric pressure so maybe this different than what you need).
Edit: You can also click on the "pressure" widget and it brings up the same hour-by-hour detail view.
Edit: You can also click on the "pressure" widget and it brings up the same hour-by-hour detail view.