Smoke signals

The Sun goes down, another chilly night is coming. Wind stops just to see the day off, and while it welcomes the evening, it is as weak as a draft, bearly noticeable. Fires are lit in houses to make them warm and comfortable. Smoke rises straight up from a chimney, and one can see its greyish shadow quite clearly. Now you expect to see it rises straight up higher, but it's not about to happen. On the contrary, smoke sharply turns above the chimney, and it appears like crawling upon some invisible ceiling. It drifts further away like being pulled by some invisible force. So, you get a smoked air that makes you cough instead of fresh evening.
- Wasn't it enough with the cold night? Now this smoke makes me cough - you unwillingly comment this unexpected smoke motions. But only some minutes later, the situation turns a bit easier. Invisible ceiling rised a little, so the smoke is less felt. Further with time, that invisible ceiling that stops the smoke going up is still rising.

See how smoke crawls across that invisible ceiling...

 What stops the smoke going up? What is that invisible ceiling?

Sunset is the end time of heating from the Sun and the start of night cooling. During daytime, the changes of air temperature is driven by heating of the land surface. Naturally, the lowest air layer comes first to be cooled, while upper air layers come next. That cooling gradually weaks with the elevation from the ground, until it disappears at some significant height.
At the beginning of the described cooling, temperature lapse rate changes with height. Instead of temperature decreasing with height, the higher air is warmer. This is called the temperature inversion, the state opposite to normal. This makes that invisible ceiling that you noticed in this evening event. At heights with the inversion, it is impossible for warm air and thus smoke to rise further up. High temperature at this height surprises even the warm air from the smoke, since it cooles during its rise through the air. So, the smoke can't spread upwards, it stays below the invisible ceiling. This shape of the plume is called fanning.

This type of temperature change with height is usual during weather that is not just cloudy, while wind remains at low speed. Such event announces weather with no precipitation and temperature that falls down to its minimum value around sunrise.

I can see there, in the distance, a stack from a power plant. The smoke from the stack is not stuck at some height, but it rises freely. How come?

Power plant stacks are usually built to be much higher than house chimneys. The stacks might be even more than 100 meters high. The inversion height is often lower than the top of the stack, while temperature normally falls with elevation, so the smoke behaves regularly, freely rising up. But when the inversion rises above the stack, the smoke will feel the resistance in the inversion layer and get stuck, too. The smoke will spread wider than near the land surface, because the wind speed is usually higher at stack height than near the ground, where it is almost absent. The smoke will disperse and become more sparse, so it won't spoil anything that's on the ground surface.
If the smoke flows down the wind at night, while inversion is still below the stack height, you'll notice that smoke disperses everywhere, except downwards, below some height. The inversion really behaves like an invisible obstacle from both sides. So, this plume shape is called lofting.

I also noticed that sometimes, just after sunrise, it might happen that the smoke falls down to the ground. How's that?

At sunrise, the heating from the Sun restarts, warming up the ground surface first, the air above it, from lower to higher layers. The inversion starts to disappear from the bottom. Temperature lapse rates are changed again, restoring its desrease with the elevation, but only to some height where this heating didn't take effect yet. There is still the inversion above that height, so temperature rises from that height up again. This temperature change is called elevated temperature inversion, because the opposite temperature lapse rate from normal starts not at ground level, but from some height. The decribed plume shape is now called fumigation, which is quite logical, isn't it? The smoke rises up until it reaches the inversion layer, and it appears like hitting the invisible ceiling again and coming down to the ground. Luckily, such temperature changes with height can't last long and weaks rapidly with time, since a normal temperature lapse rate is being restored.

Intereting... but is there any possibility to have that invisible ceiling both below and above the smoke plume?

Of course. These situations are seldom, but they come up when the night cooling starts from the ground, while there still exists an elevated inversion. The top of the stack is between the two inversion layers, in the space where the temperature are normally lower with height, so the smoke disperses as much as possible, hitting not only the 'invisible ceiling', but also to an 'invisible table'. Obviously, this smoke plume is called trapping.

These are smoke signals, from upper row, from left to right: looping, fumigation, coning, lofting, fanning, trapping...

But how does the smoke spread at daytime, when there is no inversion?

The smoke usually spreads away from some axis that lies down the wind. The shape of the smoke plume is called coning. Normal temperature lapse rate allows free dispersion of the smoke. Its tilt depends on the wind direction and speed, making the smoke moves nearly horizontally at high wind speed, thinned by strong mixing of the smoke and a surrounding air.
But when the solar heating is quite intensive, a smoke (that might come from a fire) spreads irregularly all over, moving up and down. Due to this strong ground heating, the temperature is much higher just above the ground, decreasing with height more rapidly than normal. Such temperature distribution enables even stronger mixing the smoke and the surrounding air, bouncing the plume up and down. The shape of the plume (if there is any shape) is looping, looking like loops thrown away from some broken shoes.

So it turns out that we have smoke signals for weather description...

Exactly! You know, we, who observe the nature and live by its laws, sometimes are just like Indians that lived in some past times, so why we wouldn't use smoke signals, just like they did?