# How does frost form above freezing temperature?

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?
• My feeling is that any answer here will be a guess, because there could be any number of reasons. That said: 1. The seat could be colder than the air, 2. and 3. physics.stackexchange.com/questions/10470/… – kd88 Apr 28 '16 at 14:27
• The seat was very likely at the same temperature or warmer than the air, provided that I had used the motorcycle about 2 hours before, and the temperature didn't drop lower than zero. Or maybe occasionally because of the wind, then... – aspyct Apr 28 '16 at 15:06

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.

• How does the presence of clouds modify the sky radiation temperature? I thinking specifically of full cloud cover. – NeutronStar Apr 28 '16 at 16:46
• 1) the clouds are warm and radiate themselves, 2) they are often associated with higher humidity which means the entire atmosphere is more opaque (to IR). This means you won't get super-low temperatures that can be "seen" on dry nights. – BowlOfRed Apr 28 '16 at 16:51
• Go get one of those IR thermometers and point it at the sky on a clear night. It can be below -60 if the air is very dry. Most of what it can detect is the IR radiation of water vapor. Dry night, little radiation. – BowlOfRed Apr 28 '16 at 17:02
• It was indeed low wind conditions these evenings. I should have an IR thermometer lying around, I'll try that! – aspyct Apr 28 '16 at 18:36

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.

• This was in the evening, after a rather warm day. I don't think the motorcycle would have had a chance to drop below 0. Wait, ice is evaporating? Sounds crazy and interesting at the same time. – aspyct Apr 28 '16 at 14:59
• teacher.pas.rochester.edu/phy121/lecturenotes/Chapter17/…. First Law of Thermodynamics: heat always flows from a hot body to a colder one. In short, there's no way a motorbike could cool to 0 C, unless it was in contact with something even colder. And yes, ice does evaporate slowly, even at below 0 C. – Gert Apr 28 '16 at 15:40
• Yeah, but that's the point: the motorcycle didn't have a chance to drop below zero, because the air temperature didn't either. – aspyct Apr 28 '16 at 16:01
• @aspyct, air doesn't heat or cool as rapidly as other objects. Your motorcycle can drop below zero before the air temperature does. – BowlOfRed Apr 28 '16 at 16:19

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.

• It was indeed feathery every time, and there was no tree or anything else above, only sky. So that's quite a lot of heat lost. Is there some research showing how much can be lost through this? – aspyct Apr 28 '16 at 18:33