1. Incense Smoke Moves to the Bottom in a Beaker
Incense smoke usually rises, so it is easy to think the particles are light. However, educational materials from Tochigi Prefecture describe a 4th-grade science experiment. They state that if incense smoke is collected quietly in a beaker, the smoke, which is heavier than air, moves down the walls to the bottom. [6] This suggests that unheated smoke particles are heavier than air. Since the contents of the particles vary (soot, ash, water droplets), it is safe to say "many smoke particles" are heavier. If the particles are heavy, something else must be carrying them up.
2. Heated Air Lifts the Smoke
When air is heated, it becomes lighter. At the same pressure, air expands as temperature rises. Because the volume increases for the same weight, the density (weight per cubic meter) decreases. Air that is lighter than the surrounding air experiences buoyancy. [11] This is the same principle that allows hot air balloons to float. [11]
Buoyancy is explained by Archimedes' principle. An object receives a buoyant force equal to the weight of the fluid (liquid or gas) it displaces. [2] This applies to air as well. A mass of heated air is lighter than the same volume of surrounding air, so the weight of the displaced air wins, causing it to float.
How much lighter does it get? Dry air weighs about 1.29 kg per cubic meter at 0°C. [3] At the same pressure, volume is proportional to absolute temperature. Calculations show that at 20°C it is about 1.20 kg, and at 100°C it is about 0.95 kg. Since 0.95 divided by 1.20 is approximately 0.79, 100°C air is about 20% lighter than 20°C air. These are calculated values from the 0°C figure, not numbers directly from the source. Encyclopedias also list air near the ground being heated, becoming light, and rising as one cause of updrafts. [4]
3. Why Heavy Particles Do Not Fall
Smoke particles are carried without falling because they are extremely small. Cigarette smoke particles are said to have a diameter of 1.0 micrometer (1/1000 of a millimeter) or less, which is much smaller than pollen at about 30 micrometers. [7] The size of particles from wood or incense may differ, so this is a rough guide. Because the particles are so small, they can be carried by air currents. [7] They can be carried upward by air currents.
4. Fire Smoke Rises at 3 to 5 Meters per Second
How fast does smoke rise during a fire? According to disaster prevention learning materials from the Tokyo Fire Department, the speed of rising in vertical spaces like staircases is about 3 to 5 meters per second. [5] The same materials explain that smoke comes from burned things, is warm, and is lighter than air, so it rises quickly toward ceilings and upper floors. [5] Here, "lighter" means it is mixed with hot air, not that the particles themselves are light. Therefore, during evacuation, people are advised to stay low. [5]
5. Why Rising Smoke Spreads and Disappears
In fluid dynamics, a mass of fluid that is lighter than its surroundings and rises without external push is called a "buoyant plume." A plume widens as it rises by entraining surrounding fluid at its edges. [10] In a column of cigarette smoke, the change from smooth flow (laminar) to turbulent flow is clearly visible. [10] Note that this source is an English encyclopedia and not a primary research paper.
The height at which smoke stops is not fixed; it depends on the surrounding air and room conditions. As smoke mixes with the surrounding air, its warmth dilutes. If the temperature difference with the surroundings becomes small, the density difference also becomes small, and buoyancy weakens. This explanation is derived from the previous section's idea that warm air is light.
6. Chimneys and Atria Borrow the Rising Force
According to the World Encyclopedia, chimneys have two purposes. First, to guide smoke to a high place and disperse it, reducing impact on the ground. Second, to use the density difference between smoke and surrounding gas to create ventilation force, or the force that draws air in. [8]
This is also used in building ventilation. Technical materials from Osaka Prefecture call the phenomenon where heated air becomes light and rises the "chimney effect." They state that if an atrium is used as a wind path, ventilation can be expected from temperature differences even without wind. [9] The force lifting the smoke is not the smoke itself, but the lightness of the heated air. People have used this force for drawing in air for burning and for building ventilation.
7. Observing the Column of Incense Smoke
Observe incense smoke with an adult. Because fire is used, avoid areas near cloth or paper, observe in a windless room with the windows closed, and have water ready. Observe for a short time to avoid inhaling smoke, then extinguish the fire and ventilate the room. [5] [6]
The base of the smoke column rises smoothly. A little higher up, you may see a point where it begins to waver and become turbulent. Look at the distance from the incense stand where turbulence begins and compare it with nearby objects. It changes with room drafts or people's movements. The height where turbulence begins is not always the same. Recording data can help investigate if it changes with seasons or room temperature. Among the materials used in this article, the Tokyo Fire Department's disaster prevention learning materials and the Tochigi Prefecture incense experiment materials are available in Japanese. [5] [6]