With a reading of -40, I'm fairly certain* he's not using Kelvins.

[*] Sometimes negative temperatures are used to represent temperatures ABOVE infinity. That's when high-energy quantum states are MORE populated than low-energy quantum states... it's useful for things like making a laser. But I still think it's unreasonable to believe that the temperature outside is -40 kelvin.

What do you mean by 'above infinity'?

In a nutshell, if you put a negative temperature object A in contact with any object B with a positive temperature, energy will flow from A -> B, regardless of how high B's temperature is. This is the opposite of everyday positive temperatures, where energy will flow from the high-temperature object to the low-temperature one. So in a sense you could say that the temperature of A is "above infinity".

Negative temperatures are effectively hotter than any positive temperature, as I understand it.

If he's using Kelvins, I sense a Nobel.

I sense a broken thermometer

Funny you should say that - the reason I said "way past -40" is because all the thermometers I can find don't go past -40.

Even the digital ones just say "-40.0C" at about 6pm, and won't show anything else until 17 hours later when the sun comes back, and they creep up a few degrees.

Someone else just mentioned it, but I used to live in northern BC and our "red liquid" thermometers recorded temps down to about -60C or so. Coldest I can recall as a kid was -54C. We still walked to school.

Awesome, I'll have to keep a lookout for one.

I rode my bike in yesterday at around -45C. The snow drifts were too big today, so I walked for 35 minutes instead.

I'm curious, do you use mountain bike tires to get more traction in the snow, or are you able to use the small, thin street tires so long as the snow is sufficiently slushy/salted/shovelled?

I ride a mountain bike, though my tires are only normal width. I did put studs in my tires to help with icy conditions.[1] So far I've had a good winter of riding.

I don't think anyone would ever ride a road bike in the winter, it would not be possible with the snow the way it is (deep, often drifting, rarely cleared)

Tons of people up here ride "fat tire" winter bikes with up to 4.5 inch tires. They ride the downhill trails right through winter.

[1] http://www.youtube.com/watch?v=GI-fR2tXR_8 (I used a cordless drill and just drove them through - took 30 minutes all done)

For mercury thermometers, it's because mercury freezes at -38C (-37F). Alcohol (red liquid) thermometers should work to -70C (-94F).

It's not clear why your thermometers don't register below -40, unless it's a legacy of the mercury thermometer limitation baked into the product (or frozen in, so to speak).