Capitalizing words only applies in a handful of circumstances:
1) The first word of a sentence.
2) The first word of a sentence in quotes, as with "This example is appropriate."
3) A proper noun (Jody, Empire State Building, Germany).
4) A person's title (Dr. Thugnor, Mr. Frotnip, Sir Henry Aith, or simply Doctor).
5) Compass points when they refer to regions ("I'm coming down from the North").
There are more rules that can be found at the related link below. However, to answer this question, "atmosphere" is not capitalized if it refers to air, but if you were referring to the name of a band, then you would capitalize it:
"The atmosphere was thick with dust."
"I bought the latest album from Atmosphere."
"I read Atmospheres Of Planets the other day."
Yes, "PC" should be capitalized in a sentence.
No, "secondary students" should not be capitalized in the middle of a sentence unless it is a proper noun or at the beginning of a sentence.
No, the word "fifteen" should not be capitalized in a date within a sentence unless it is the first word of the sentence.
Since "transition process" is made up of common nouns, it should generally not be capitalized in sentences. It should only be capitalized when it starts a sentence or appears in a proper name or title.
Since 'felon' is a common noun, it is only capitalized if it is at the start of a sentence.
No. The separate levels of the atmosphere are not capitalized.
No, except at the beginning of a sentence because it is n ot a proper n ou n.
Yes, "PC" should be capitalized in a sentence.
No, the word forecast should not be capitalized in a sentence.
Yes it should be capitalized.
No, "secondary students" should not be capitalized in the middle of a sentence unless it is a proper noun or at the beginning of a sentence.
No, the word "fifteen" should not be capitalized in a date within a sentence unless it is the first word of the sentence.
Since "transition process" is made up of common nouns, it should generally not be capitalized in sentences. It should only be capitalized when it starts a sentence or appears in a proper name or title.
Have should be capitalized if it is the beginning of a sentence. Summer should not be capitalized.
Since 'felon' is a common noun, it is only capitalized if it is at the start of a sentence.
No, "methamphetamine" should not be capitalized unless it is at the beginning of a sentence.
No it shouldn't be capitalized.