Towards the end of the match, after the air drop.has landed, a timer will start to tick in the background.
side note, if you look at one of the 4 corners of the map you will notice some red arrows that show where the wind is blowing
The storm is going to start shortly and the map is gonna start getting colored red from the corner where the red zone started developing. Simply walk in it or wait until it comes to you and the screen is gonna start appearing a little fuzzy and change color. The storm will not kill you right away but slowly deplete your health if you stay in it long enough (a minute or two). You cannot heal up once radiations start affecting you, so make sure you do this close to an exit spot.