It will thicken. By late morning, the river will be invisible; the clouds will lower, and the sound of the birds will be lost in the soft, moist silence.
It will thicken again. By lunchtime, the world will be about five metres wide around you; by early afternoon, about one metre. When you reach your arm out, your hand disappears and becomes numb. It's about at this point you realise that this isn't an illusion.
By three o'clock, all you can see is mist, and both your arms are numb.
By five, the mist is invading your eyeballs; slowly it separates you from yourself and what you see becomes dark; and you forget what seeing is. There is nothing left but your mind and torso.
By six, you are just a word - "I am" - in your mind.
By seven, you are not.