Running in -20 wind chill temperatures

You will need an extra layer on your core. You need one layer with a collar on your neck over your ski mask.
You need your warmest ski mask and cover as much of your face as possible. Still wear your regular hat.

Wear your warmest socks. They need to be thicker than usual.

Wear two gloves (now that I'm older I'm going to experiment with warmer non running gloves).

Put a sock on your testicles (peeing on a long run can be dangerous in this weather).

Have fun ! Nothing like a long run on a cold day in January.

"Put on a hat and gloves and you'll make your own heat"

“Run on treadmill below 40.” Pussycat?

Been running to -20 windchill for 20 years. Have no fear if you have the gear. it is easier to run inside. Balaclava & wind proof hat along with ski googles will do your head face and neck. Wool shirt like Smartwool and a wind proof jacket. I use Cw-x insulator tights and CEP merino compression socks. Ski gloves and great traction like Yax tracks on your shoes.

Very thorough approach. This is quite helpful. Thank you.

-20 windchill isn't ideal but it's not THAT bad if you are dressed properly. I can wear shorts without any issue until it gets to like 10 degrees, though, so I'm pretty tolerant of the cold. My hands and ears are what get the coldest on me. Cold fingers on a run hurts and it sucks, so I usually wear a smaller pair of gloves under my typical running gloves when it's under 0.

the temperature isn't the problem but how is the surfaces? if it's icy I can't see you getting anywhere and risk falling and hurting yourself.

My best advice is to wait at least 30 minutes before you shower after you run. Your nether regions will appreciate this -- the sting of showering right after a very cold run is one of the worst pains.

When the weather is like that you should XC ski, if there is no xc ski piste go straight to the Sauna

Some county parks near Minneapolis will not allow scheduled events to proceed unless the wind chill warmer than 40 below.

So 20 below wind chill is safe if you wear the right clothing.

The clothing I use while running in any winter temps has become so good over the years. The musts I have for tights are they must have a wind breaker front panel (Nike or craft), I wear Saxx front panel jammers underneath the tights. On top depending I layer up with a moister wicking long sleeve shirt under a wind breaker front panel craft shirt. I then pull on a wind breaker outer layer that is a half zip. Full zippers leak air. For my face I have found silk baklavas work best for both protection and breathability. They also keep their shape better when frost/ice forms from your breath. Yesterday I used two because the windchill was -20. I place a stocking cap over (thickness depends on temps). Hands I use wool gloves with a leather outer shell. I found leather beats all for wind protection and keeping my fingers warm. Lastly don’t worry about pace. Eventually the temps will warm again and you can work on all the speed you want!

this is from years of running/experimenting in Minnesota winters because there were no treadmills to turn too. In the early 80s I ran and won the St. Paul Winter Carnival Marathon with a 2:32 when temperature (not windchill) was -24f.

It can be unsafe if you treat it like a normal run. Windchill that low increases frostbite risk fast.
Dress for the wind, not the temp. Cover all skin. Face, ears, hands matter most. Use a buff or mask. Mittens beat gloves. Wear wind blocking layers, not just warm ones.
Slow the pace. Shorten the route or add loops so you can bail early. Avoid exposed areas. Run with your buddy the whole time.
If anything goes numb or painful, stop. At that point it is not training, it is damage control.