Kanji pronunciation

I have a question concerning: 明日

When do you pronounce it あす and when あした and how do you know?

Thank you for clarification.

In fact, you never know. It's up to the reader to decide. If you want the reader to read it in a certain way, you just have to write it in kana. Same thing with words like　昨夜　昨日　一昨日　行った　and many many others.

Thank you Uco.

If it's any consolation, even though ashita and asu are both written as 明日, the majority of the time in modern communication, the pronunciation is going to be "ashita." "Asu" has a more literary/poetic feel to it, so it's not usually used in everyday emails, signs, etc.

Thank you @ ...

Well it would really have been a consolation :)  but I've come to this question by weather forecasts. Yes it's written in Kana あすの気温 for example. But as far as I can see it's quite commonly used in this field.
What I'd like to ask too is if it always means "tomorrow". My dictionary translates あす also as "near future" in distinction to 明日. So how does あすの気温 translates? Temperature tomorrow or temperature in near future (Whatever this might mean).

Actually, my paternal grandmother almost always said "asu" at home, while my maternal grandmother mostly said "ashita" at home. For formal business purposes, I have the impression that one would mostly use "asu", but then, you might say "ashita" to a colleague.

@ UCO How complicated! :)