Physics Asked by aspyct on March 16, 2021
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!)
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):
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.
Correct answer by BowlOfRed on March 16, 2021
Ice, at least at atmospheric pressure, cannot form above the melting point of water (0 Celsius).
The phenomenon of water freezing on objects like the ground, parked cars, motorbikes etc, is due to thermal inertia. On a long, cold spell these objects will cool down to below 0 Celsius. But when the ambient air temperature rises above 0 Celsius, the actual temperature of the objects will not immediately follow because heat transfer takes time.
As long as the object's temperature (which is now slowly rising) remains below 0 Celsius, water, moisture from the air etc will freeze on the object, despite the ambient air's temperature being slightly above 0 Celsius.
As regards water evaporating at low temperatures, even ice has a so-called vapour pressure and will slowly evaporate, as long as the air around it isn't saturated with moisture. This principle is even commercially exploited in a process called freeze-drying. So both ice and cold water will slowly evaporate.
Answered by Gert on March 16, 2021
If it was a clear sky then your motorbike most probably lost heat by radiative cooling and the ground usually cools more rapidly than the air. If you have placed your motorbike under a tree you would have probably found that there was no frost on your motorbike.
You would have noticed the opposite of radiative cooling, radiative heating, when the Sun was shining on you.
All bodies radiate infra red radiation (heat) and the hotter they are the more radiation they emit.
Your motorbike radiated more upwards then the sky above you radiated downwards (the Sun had gone) so the motor bike lost heat and its temperature dropped.
When the temperature of the air goes below the dew point the air is supersaturated with water vapour and some of it condenses out into dew (liquid) or if it is cold enough into frost (solid).
If the frost on your motor bike was feathery then the motor bike was already below 0$^circ$C when the dew point was reached and the water vapour condensed and almost immediately turned into a solid - frost.
If the frost was made up of more globular particles then that meant that dew (liquid) formed on your motor bike which then turned into frost when the motor bike cooled below 0$^circ$C.
Answered by Farcher on March 16, 2021
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