What does the blue line in iPhone's Weather app indicates?
What does the blue line indicate in the 10 Day Forecast of the Weather app?
I really like this Weather app.
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It indicates a number of things.
The longer the line, the greater the difference between the high and low that day.
The relative position (towards the left or right) of the line compared to other days, the warmer (left) or colder (right).
All the lines in your example are blue because the weather those days is cold, but if a day has warmer temperatures the warmer part of the day will be yellow/red.