Sir Ridesalot writes:

On Wednesday, January 10, 2018 at 1:25:58 PM UTC-5, wrote:
On Saturday, January 6, 2018 at 11:28:40 PM UTC-7, Sir Ridesalot wrote:
Just wondering if when it's known that there can be black ice on
the roads if anyone here wears a helmet or other protection (such
as elbow pads) that they'd not normally wear when the roads are
clear. Do you?

Cheers

as a snow country biker, does SR use studded tires /

No, no studs for me. I drop; the air pressure a fair bit if I think
I'll be on black ice. Early in the season I take the bike to an
outdoor ice rimnks after dark when the rink is closed (It's a
temporary one they build forthe winter in a park) and practice riding
the bike on that ice.

Btw, contrary to many who believe that black ice is always thin,
that's not always true and also, black ice can form very quickly even
if the air temperature is above freezing but the moisture is in shade
and thus quite a bit cooler.

Thick, transparent ice can certainly form on the roadway, the only
question is whether such blatant stuff merits the name "black ice".

On the second point I think you're correct in general, but, to be frank,
mistaken in detail. The surface of a puddle trying to freeze can lose
heat through two mechanisms: convection by nearby air, and radiation.
If the local air temperature is above freezing convection cannot cool
anything to below freezing.

Radiation can cool the puddle to, roughly, the blackbody temperature of
whatever is "visible" from it. I remember being asked to show this as
homework -- we were to assume a (very cold) stratospheric blackbody
temperature, and show that ice could form even if the air temperature
was above freezing.

The catch is that shade is exactly the wrong condition for this to
happen, what's needed is a view of a clear nightime sky.

The blackbody assumption (absorption/emission spectrum depending
smoothly on temperature) isn't quite right, because the atmosphere
contains "greenhouse" gases with interesting spectra in the infrared.
By far the most important of these is water, so if you want to freeze
water at air temperatures above freezing, choose a clear night in a
desert.

I believe this phenomenon was actually used to produce ice in
pre-industrial India.


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