One thing I have always been wondering, is what rule governs the ending of masculine adjectives which has the damn н before the ending.

Синий
Мобильный
Летний
Длинный

How do I know when to use what? The other letters (-ий: К Ч Ш Щ -ый: Т П В Л etc) are at least consistent so they are easy to memorize

Anyoneый?