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When the Sun shines bright, say on the beach or when you ski in mountains, you cannot see what's on the screen of your mobile phone. You cannot read or write SMS. You cannot even dial a number because you cannot see anything on the screen. The reflection is just to high.
Mobile phones should be equipped with a special screen mode suitable for high sun activity. Eg, I noticed that B&W screens of GPS navigators can be read in such conditions. A special switch would change the mode of the screen.