How does water evaporate if it doesn't boil?

When the sun is out after a rain, I can see what appears to be steam rising off a wooden bridge nearby. I'm pretty sure this is water turning into a gas.

However, I thought water had to reach 100 degrees C to be able to turn into a gas.

Is there an edge case, for small amounts of water perhaps, that allows it to evaporate?

Evaporation is a different process to boiling. The first is a surface effect that can happen at any temperature, while the latter is a bulk transformation that only happens when the conditions are correct.

Technically the water is not turning into a gas, but random movement of the surface molecules allows some of them enough energy to escape from the surface into the air. The rate at which they leave the surface depends on a number of factors - for instance the temperature of both air and water, the humidity of the air, and the size of the surface exposed. When the bridge is 'steaming': the wood is marginally warmer than the air (due to the sun shine), the air is very humid (it has just been raining) and the water is spread out to expose a very large surface area. In fact, since the air is cooler and almost saturated with water, the molecules of water are almost immediately condensing into micro-droplets in the air - which is why you can see them.

BTW - As water vapour is a gas, it is completely transparent. If you can see it then it is steam, which consists of tiny water droplets (basically water vapour that has condensed). Consider a kettle boiling - the white plume only occurs a short distance above the spout. Below that it is water vapour, above it has cooled into steam. Steam disappears after a while, as it has evaporated once again.

• I would add that even ice evaporates with the same process, in this case called sublimation. That is how we have no frost freezers and refrigerators. May 27 '11 at 5:57
• ""Below that it is steam, above it has cooled into vapour."" Is this really the meaning of steam vs. vapour? May 27 '11 at 8:32
• Dear @Peter and @Georg. Unfortunately, it seems that Peter got vapor and steam mixed up in his answer (v1), see e.g., wikipedia en.wikipedia.org/wiki/Water_vapor and en.wikipedia.org/wiki/Steam May 27 '11 at 9:57
• I don't the the rate of evaporation depends upon the temperature of the overlying air, i.e. it is a function of the thermodynamic processes withing the liquid (or solid, like in ann's example). Water vapor is also going the other way, from the air onto the surface, and this is affected by the thermo conditions of the air, i.e. humidity doesn't prevent evaporation, it competes against it. May 27 '11 at 16:45
• @Qmechanic - I just had a look at the Wiki articles. All I can say is that those definitions are exactly the opposite of what I learnt at school. I'll have to be more careful of terminology in future :-) May 30 '11 at 23:03

For every temperature, there is some amount of water vapor that can exist as gas mixed in with the air. This is called the saturation pressure of water at that temperature. The relative humidity is the amount of water vapor pressure, expressed as a percentage of the saturation pressure. As you increase the temperature, the saturation pressure increases.

Steam is water in its gaseous phase.

You can't see water vapor, you can't see steam, but you can see mist, which is liquid water droplets suspended in the air.

When you boil water on the stove, you get steam. This then cools when it comes into contact with the air, increasing the relative humidity above 100%, so the water vapor condenses into mist.

If the relative humidity is bigger than 100%, water vapor will condense from the air, becoming dew and/or mist. If the relative humidity is less than 100%, water will evaporate into the air, becoming water vapor.

If the wooden bridge is warmer than the surrounding air, and the relative humidity is around 100%, then water will evaporate off of the wooden bridge, turning into water vapor (the relative humidity is lower right next to the bridge, because the bridge is warmer). When the air containing this water vapor rises and cools, water condenses out of it, turning into the mist that you see.

Here is a graph of the saturation pressure (from this website). Note that at 100°C, the pressure is $\approx10^5$ Pa $=1000\,$hPa, which is roughly atmospheric pressure. This means that at 100°C, you can have pure water vapor at atmospheric pressure. This is why water boils at 100°C at sea level—a bubble of steam can form below the surface of the water. At higher altitudes, the boiling point can be substantially lower.

• What exactly did you mean by above 100%? That sounds technically incorrect. Apr 1 '19 at 8:15
• @ΕГИІИО Consider a room with a max occupancy of 100 people containing 100 people. To allow 10 more to enter, you could either: a) remove 10 people, then let the new 10 in, or b) let the 10 in and allow the 110 to shove 10 random people out. The latter is what happens here. Puddles can still evaporate at 100% humidity, as long as some of the existing vapor condenses to balance it out. Jun 13 '19 at 14:43

Below "boiling point" (not always 100C), water can exist in both gas and liquid phase, and has a temperature-dependent vapour pressure, which represents a point of equilibrium between liquid water wanting to evaporate and water vapour wanting to condense. When liquid water meets dry air, it is not in equilibrium; water molecules evaporate off the surface until the amount of water in the air creates enough vapour pressure to achieve equilibrium.

When water is heated to a temperature of 100C, the vapour pressure equals that of sea-level air pressure. Since the air pressure can no longer overcome the vapour pressure of the water, the water boils.

At higher elevations, air pressure is lower; as water is heated, its vapour pressure overcomes ambient air pressure at a lower temperature i.e. the boiling point is lower.

Vice-versa for higher pressures.

As for the steam rising off the bridge, that is actually water vapor condensing. Very close to the wet surfaces, the air is saturated with water vapor, which is transparent. It is also less dense than dry air, so it rises. As it rises away from what is likely a warm surface, it cools, As it cools, it condenses, but it is also mixing with more drier air, so it evaporates again and disappears.

• I think this is actually the most correct answer both in terms of the physics and the terminology in use. Mar 1 '18 at 17:24

Steam rising from a warm bridge is vaprization of water. Boiling water is vaporization of water. Getting cooled down by a breeze after a sweaty workout is vaporization of water. All result in the same phase change with the same latent heat of vaporization of 540 cal./gram, which is a very powerful cooling effect.

Boiling water is a subset of vaporization of water, wherein the heating of the water is fast enough that vaporization is forced to occur very rapidly AND there is enough water such that the vaporization occurs under water.

• ""Boiling water is a subset of vaporization of water, wherein the heating of the water is fast enough that vaporization is forced to occur very rapidly AND there is enough water such that the vaporization occurs under water. "" This definition can be improved, very much so. :=( May 27 '11 at 20:15
• @Georg: If it can be improved, then do so. May 27 '11 at 20:55