Detect viewport orientation, if orientation is Portrait display alert message advising user of instructions

Question

I am building a website specifically for mobile devices. There is one page in particular which is best viewed in landscape mode.

Is there a way to detect if the user visiting that page is viewing it in Portrait mode and if so, display a message informing the user that the page is best viewed in landscape mode? If the user is already viewing it in landscape mode then no message will appear.

So basically, I want the site to detect the viewport orientation, if orientation is Portrait, then display an alert message advising the user that this page is best viewed in Landscape mode.