The reason I do not think it was carb ice is that the weather was below
freezing, which means that the air was probably relatively dry and it
may even have been too cold for carb ice to form (the air would have to
have liquid water precipitate out of it to cause carb ice, meaning that
the air would have to be warmed in the carburetor, not cooled, and
somehow also exceed 100% humidity in the process).

The textbooks will tell you that water can exist in liquid form
down to minus 20 degrees Celsius, or about minus 3 degrees F. We have
encountered carb icing at these temperatures here in Western Canada, in
clear air. Carb ice can also form from dissolved water in the gasolne,
and all gas has at least a small amount of water in it. It can't be
seen unless the temp gets really low, where it finally clumps up and
forms snowflakes in the fuel.

I put a carb temp guage in my 182..at low power settings the carb temp
is near ambient air temp. It's only when you start sucking a lot of air
through the carb that the temps drop.

The 172's Lycoming will ice up easily enough after startup, while
idling, as will many other engines. There is a massive pressure drop as
the air squeezes past the throttle plate at idle, therefore a large
temperature drop, and ice WILL form there. Seen it many times since I
began flying in 1973. Unless the ambient temperature is very high, very
low, or the air is very dry, or that carb is plenty warm, carb ice can
form. It can catch the unwary when they fly in climates different from
what they're used to.

Dan