Is there a way of displaying alerts?

Is there a way of displaying alerts?

I want to display an alert to the user after some basic bounds checking on an input field.

Something like:  "Hey you can't enter a negative value for age" or "Your age must be a number between 0-120".or "You left this field blank"

Is there a "jQuery Mobile way" of doing this?  I was thinking about just using a footer toolbar with a div that could be updated with an alert.

Looking through the documentation, I thought about using a dialog box, but that would be over kill because I don't want to display a whole page just a quick message.

So is there an official way of doing it?
            
Thanks.