The Nissan Path

So I notice a 2 foot hairline crack running across where my defrost blows. I noticed it after I got to work. Our windchill is around -20 deg Faren. I did not hit anything on the way in. No rocks etc. is this from the extreme cold and then the heat from my defrost?

It could be. Thermal expansion can cause windshields and even sunroofs to crack or shatter. Usually, it starts with a small chip in the glass which you might not notice, but not always.
Many years ago I worked in a Nissan dealer with an enclosed service write-up area. A late-80's Nissan 300ZX came in on a snowy day and had been parked with the windows closed inside the write-up area for around 20-minutes when all of the sudden the entire rear hatch window "popped" and disintegrated into thousands of tiny pieces! Sometimes, glass can develop large cracks suddenly not due to thermal expansion but simply due to stress on the glass caused by flex in the body. It happens!

My R51 has a tiny Star crack in the bottom right corner.
Barely noticable...

I hope it doesnt get worse in this weather...

You could always take it to a glass shop and have them fill it in.

I got my windshield replaced. I had zero deductible on my insurance.
I asked the tech why the crack. He said there was a tiny impact point - so the extreme weather changes is enough to cause the impact point to then turn into a crack.

He also recommended not to washing during the winter with car washes that us hot water. He said try to findbwarm water. He said you cant even see those tiny impact points so all it takes is extreme temperature change to cause the crack.

Also when he was replacing the windshield, he noticed some rust below the rubber above the passenger side. He fixed the rust issue, but said he cant warranty the windshield anymore.