thermodynamics - How does frost form above freezing temperature?

This is a follow-up to the best answer linked below:

Why does ice form on bridges even if the temperature is above freezing?

I've witnessed that phenomenon, recently. The temperature was above zero (probably around 4-5 degrees celsius), and yet I had a thin layer of ice on my motorcycle seat and, more annoyingly, on the road (slippery!)

• the motorcycle was outside


• after the sunset


• it was a clear evening (could see the stars), so there was definitely heat lost to the sky by radiation.


• it wasn't raining. Not sure about humidity though

I still don't get how that water could freeze despite the 5° ambient temperature. More specifically, could anyone explain to me in simple terms (I'm not a physicist):

1. why did the water from the air freeze on my seat rather than in the air?



2. if heat was lost due to evaporation, how did water evaporate at 5°?


3. what provided the energy for the water to evaporate?


4. am I missing something?

Answer

The (long-term) temperature of an object depends on the heat transfer between it and all of the environment.

Air isn't a great conductor of heat. So if there is little air movement, the radiation environment may dominate the heat transfer. A cold calm day may feel quite balmy under full sunlight.

On a cold evening, the sky may have a radiation temperature of -40. Your motorcycle is radiating some energy, but very little is returning to upward-facing surfaces. Without a breeze to increase air convection or structures around to increase thermal radiation, it's quite possible for the vehicle surfaces to cool below zero. The air is adding heat to it at that temperature, just not quickly enough to counter the radiative losses.

Water evaporation isn't necessary.