On a cool but sunny morning I can see mist or smoke billowing from one of the side fences of our house. It looks like there’s a fire on the other side, but the mist is of course just made of water droplets.
Surprisingly many subtle physical effects need to come together in order to produce this phenomenon.
First the setting: the morning is cool but sunny, and all surfaces are wet because of copious rainfall the day before. The sun shines nearly straight onto the away-facing side of the fence. The video was taken in the direction of the sun while standing in the shadow of a neighbouring house.
You can see in the video that the mist moves quite rapidly upward. These updraughts are formed by the sun warming up the fence panels. Those panels are dark, absorb the sunlight, and warm up. It was about 5C outside. I do not know the temperature of the sun-facing side of the fence but I would have thought it is quite a few degrees warmer than the surrounding air temperature. The air touching the fence panel gets heated up conductively and becomes warmer than the surrounding air: it becomes positively buoyant and will accelerate upward.
But why is there a mist?
The fence panel was wet, like every surface that morning, following rainfall the day before. The relative humidity of the air was close to 100%, a common occurrence on such cool mornings. The air touching the fence panel is warmer than the surrounding air but it will also be close to 100% relative humidity because the fence panel was wet.
So we get warm, nearly saturated air moving upward into cool, nearly saturated air. These two air masses get mixed together. The temperature of this mixed air is close to the average of the two temperatures; its specific humidity is also close to the average of the two specific humidities.
Such a mixed air mass will end up being supersaturated because the specific humidity of saturated air increases nearly exponentially with temperature. That means that the saturated air that came from the warm fence has a much higher specific humidity than that of the cooler surrounding air. So on mixing the two air masses, the temperature gets somewhere in the middle between the two original temperatures but one part of the mixture has an exponentially larger specific humidity, which makes the average specific humidity of the two air masses higher than what you would expect at the average temperature.
(The nonlinear, quasi-exponential dependence of specific humidity at saturation with temperature is crucial here. It is a result of the Clausius–Clapeyron relation, which describes the thermodynamics of this temperature dependence. Ultimately, this quasi-exponential nonlinearity results from the fact that vapour density and vapour pressure are proportional for a given temperature, a rather pedestrian property of gases, better-known as Boyle’s law.)
The mixed air is supersaturated: there is more vapour (note: “vapour” is an invisible gas of water molecules) in the air than is needed for the air to be saturated. This surplus water vapour will condense into drops; there are plenty of condensation nuclei available on the fence for the drop to form in the updraught air.
Additionally, the cloud condensation nuclei likely have various soluble organic material in them. In that situation tiny drops can form even when the relative humidity is below 100%.
So the mist we are seeing in the video is a mixing cloud: it forms by mixing two nearly saturated air masses at different temperatures. Contrails are another example of mixing clouds: the exhaust air from the jet-engines is hot and humid (water is one of the main reaction products of burning fossil fuels) and gets mixed with the cold environmental air.
The final ingredient to the phenomenon is why these drops are visible; this is not some thick cloud, yet it is clearly visible on the video. The scattering of light from the drops is called Mie scattering: when the rays of sunlight hit a drop, the light gets scattered in all directions, although largely forward. That means that the light scattered from the drops is visible in the forward direction: hence I had to shoot the video looking more-or-less in the direction of the sun.
We have a similar fence on the other side of the garden. No doubt this fence also produces this mist under the same conditions, but I cannot see it. For that mist to be visible to me, the light would have to be scattered backwards to the camera but, as mentioned before, Mie scattering largely scatters light forward.
It is wonderful to realise there are so many subtle effects coming together to produce this phenomenon. Much complicated physics happens in the world around us, not just in laboratories.
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