Run jQuery code once jQuery loaded

Run jQuery code once jQuery loaded

I have jQuery loaded before the closing tag on my page.
<script
src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"
type="text/javascript"></script>
The majority of my js is loaded below this in
<script src="js/script.js" type="text/javascript"></script>
Which works well. There is some ad code that I need to run in the head
tag. This works fine usually however I want to add a responsive exception.
eg
var mob = 0;
if(jQuery(window).width() < 727) {
mob = 1;
}
if(mob) {console.log('run special mob related code');}
else {console.log('run special desktop related code');}
Because my code is in the I get the error ReferenceError: jQuery is not
defined. Is there a way around this? The ad code doesn't run unless it is
in the head, but I can't really move the jquery into the head as I have
this on multiple sites and may affect how other plugins run?