The Nitpicky English Major's Guide to Grammar :: i, t, and s

It's raining today.  It's miserable.  The sun is hiding its face from us all.

We use that funny little combination of i, t, and s every day.  It's short and easy to say and write.  But in writing, it gets mixed up constantly.  It also gives me a headache--okay, maybe not, but if I see i-t-s (referring to either it's or its, depending on the context) used incorrectly, I have a hard time concentrating on anything else.  How nerdy is that?  I once didn't buy a t-shirt that I otherwise loved because someone used the wrong i-t-s.  I am that far gone into the land of nitpicky English degree holders.

Anyway, I could go in depth and talk about all the different times to use and not to use an apostrophe, but I'll save that for another day and break it apart into contractions, possessives, and the like.  For today, I'm just going to talk about i-t-s, and when you need an apostrophe and when you don't.  It's very simple, really.  There is one governing principle: if you could replace i-t-s with "it is," then you need an apostrophe.  Here's your example:

It's horrible outside today.

It is horrible outside today.

Both of those make sense, right?  Think of the apostrophe as a replacement for the i in "is."

Now, if you're using i-t-s to talk about something belonging to it, then you forget the apostrophe.  For example, if you were saying that your dog has a tail and keeps chasing it, you would write this:

My dog keeps chasing its tail.

You can make sure you don't need that apostrophe if you replace "its" with "it is," like so:

My dog keeps chasing it is tail.

That doesn't make sense, does it?  No, it makes no sense at all, so you can see here that you don't need that apostrophe.  

Let's recap: if you could replace the i-t-s with "it is," you need an apostrophe.  If you can't use "it is" instead, then you do not need and should not use an apostrophe.  It's that easy!