iPad Air unable to run IOS 8.3 update despite over 19GB iCloud storage available
iCloud storage has no correlation to a Device's available space to allow installation of an update.

To show how much space is free on the device:

 Settings &gt; General &gt; Usage &gt; Under 'Storage' &gt; Manage Storage


From here you can see what is Used vs Available, and you can even see what apps are using the most.

You can always update via iTunes by directly connecting the iPad to a PC/Mac, this will by pass any space issues on the device usually.