Sillage
French, for the wake a vessel leaves behind it. You are a chimney, and your perfume travels in your own rising air.
The most beautiful word in perfumery describes a measurable physical phenomenon. A human body is warmer than the room, so a column of air rises continuously off the skin, forming a boundary layer a few millimetres to twenty centimetres thick that moves upward at roughly a fifth of a metre per second. Everything volatile leaving your skin enters that column. Your fragrance does not radiate outward from you; it is carried up and off you by your own heat, and released at head height. This essay is about the physics of the trace a person leaves in a room, and about why you are the last person in that room to smell it.
The word comes from ships. Sillage is the wake, the disturbed water a vessel leaves behind it, from sillon, a furrow, the line a plough opens in a field. Both images are agricultural or nautical and both are about the same thing: a mark left in a medium by something that has moved through it and gone.
Perfumery borrowed it, and it is the rare piece of trade vocabulary that improves on the ordinary word. Projection is a marketing term about force. Throw is about distance. Sillage is about afterwards. It names the specific and slightly haunted experience of walking into a lift somebody has just left, or a room where a person you know was standing an hour ago, and knowing exactly who it was.
What almost nobody who uses the word realises is that it describes a well-studied fluid dynamics problem, and that the mechanism is not the one most people assume.
You are a chimney.
Skin sits at roughly thirty-three degrees. A room sits at twenty or twenty-four. That difference means the thin layer of air touching you is constantly being warmed, and warm air is less dense than the air around it, so it rises. It is replaced from below, warmed, and rises in turn.
The result is a continuous convective updraft that clings to the body and streams upward past it, gathering as it goes, until it detaches above the head as a plume. Researchers in indoor air quality call it the human thermal plume, and they study it seriously, because it governs what you breathe.
It begins as a thin, orderly film near the ankles, a few millimetres thick. It thickens as it climbs, gathers speed, becomes turbulent somewhere around the torso, and by the shoulders it is a column up to about twenty centimetres thick moving at its maximum speed of roughly 0.2 to 0.3 metres per second. That is slow enough that you never feel it and fast enough to move a molecule from your wrist to above your head in a handful of seconds.
That third figure is the one worth pausing on. Because the plume sweeps upward along the body, it collects whatever is on the floor and on the skin and delivers it to the breathing zone, and measurements have found particle concentrations there several times higher than in the surrounding room. In calm indoor conditions, most of the air you inhale has just travelled up your own body. You are breathing yourself.
Your fragrance is not radiating outward from your wrist. It is being carried up your body by your own heat and released at the height of your face.
Five things this explains.
Why pulse points work, and not for the reason you were told. The standard explanation is that wrists and neck are warm, so they volatilise more. True, but secondary. The real advantage is that they sit inside a strong, fast part of the boundary layer, so material applied there is picked up and lifted efficiently. A perfume on the ankle is not wasted; it simply takes the long route.
Why scent rises to meet other people. Everything you wear is delivered upward and outward at roughly head height, which is where other people's noses are. The system is almost embarrassingly well designed for social transmission and it was not designed at all.
Why you smell it less than everyone else. Two effects compound. Olfactory adaptation switches off a constant stimulus within minutes. And the plume carries your fragrance past your own face and away, rather than holding it in front of you. You are simultaneously the person most exposed to it and the person least able to detect it, which is precisely why people over apply.
Why loose clothing changes things. Studies of thermal plumes find that clothing, particularly loose clothing, disrupts and weakens the plume. Fabric interrupts the boundary layer, traps warm air, and slows the updraft. A fragrance sprayed on skin under a loose garment behaves very differently from the same fragrance on bare skin in a warm room, and this is a real variable rather than folklore.
Why a hot day is not simply louder. More heat means a stronger plume and faster evaporation, so more material is released sooner. That is not the same as smelling better. A composition built to unfold over eight hours can be forced through in three, and what arrives is unbalanced, because the top and heart are being dispatched at a speed the base cannot match.
The wake proper.
Everything above describes a person standing still. Start walking and the plume stops being a column and becomes a trail, which is where the word finally earns itself.
A moving body drags its boundary layer behind it and sheds it into a wake, in the same way a ship does. The faster you move, the more the plume is stripped off and left behind rather than carried with you, which means the trace you leave is densest at the speed at which people actually walk and dissipates faster at either extreme. Stand still and you build a column around yourself. Walk, and you lay down a line.
Three words people use interchangeably and should not.
Projection
How far the scent reaches from the body at a given moment. A property of the composition and of your heat. High projection is not a virtue in itself; it is a decision about how much room you are taking.
Longevity
How long anything is still detectable on skin. Governed by the heavy end of the formula: the fixatives, musks and resins that evaporate slowly. Independent of projection. A quiet perfume can last all day.
Sillage
What remains where you are not. The only one of the three that is about other people and about the past tense. It is a function of projection, longevity, your movement and the room, and it is the one the word was invented for.
The distinction matters commercially, because the three are routinely conflated in reviews and a fragrance is frequently called weak when what is meant is that it has low projection and excellent longevity, which is often precisely what the perfumer intended.
How a perfumer actually controls it.
Sillage is not a setting. It is an outcome of decisions about volatility, and a perfumer manipulates it mainly through the weight distribution of the formula.
Small, light molecules leave fast and travel far. Citrus, aldehydes, light esters and green notes have low molecular weight and high vapour pressure, so they enter the plume immediately and disperse widely. They also disappear quickly. This is why a fragrance can announce itself across a room for twenty minutes and then vanish.
Heavy molecules stay close and stay long. Sesquiterpenes such as patchoulol, resins, and most musks are large and reluctant. They barely leave the skin, which is why a base can be worn for a day and be perceptible only at an arm's length. They are also what makes the trail persist after the top has gone, because what remains in a shed wake is whatever was still evaporating when it was shed.
The middle is where sillage is engineered. Materials of intermediate volatility, and in particular certain synthetics chosen for their diffusive power, are what let a composition be present in a room without being loud. Some molecules are simply better travellers than their weight predicts, and a perfumer selects for that property specifically.
Which means the shape of a perfume through time is also its shape through space. The drydown is not only what is left; it is what your wake is made of for the rest of the day.
An involuntary broadcast.
The physics has a social consequence that is worth stating without moralising. Because the plume delivers upward and outward continuously, wearing a fragrance is not a private act. It is a low-level transmission to everyone within a few metres, which they did not choose and cannot switch off, for as long as you are in the room.
Most of the time this is entirely benign and is one of the pleasures of being around other people. It is not benign in a hospital, on a long flight, in a small office, or near somebody with migraine, asthma or chemical sensitivity. And, as the companion essay to this one describes, it is not benign near a person with parosmia, for whom a warm gourmand base may not read as sweetness at all.
The current fashion for so-called skin scents is often described as a retreat or a failure of ambition. Read through the physics it looks more like a deliberate choice about how much of a room to occupy, and there is nothing timid about deciding that the people who smell you should be the ones who are close enough to have chosen to be.
You cannot smell your own wake. It is the one part of you that only ever exists for other people.
The part of you that stays in the room.
There is a reason this word survived the crossing from ships to perfumery when so much trade vocabulary did not, and it is not that it sounds elegant in French.
Sillage is the only term in the whole craft that is defined by absence. Projection and longevity describe a person who is present. Sillage describes what is there when they are not: a room they have left, a coat in a hallway, a passenger seat an hour later, a scarf found in a drawer four years after somebody died. It is the vocabulary of trace evidence, and it is the exact point at which perfume stops being an accessory and becomes a record.
We have spent this journal arguing that fragrance carries memory, and this is the mechanism by which that is physically true rather than poetically true. A person moves through a room; a boundary layer forms on their skin and is shed behind them; molecules remain in the air and settle on fabric; and someone arriving later receives, with perfect clarity and no possibility of doubt, information about a person who is not there.
That is not a metaphor for memory. It is one, running in air, at a fifth of a metre per second.
Figures are schematic. Plume behaviour varies considerably with posture, clothing, ventilation, room temperature and activity.
- The human thermal plume, boundary layer thickness, velocity and effect on the breathing zone: Liu et al., How human thermal plume influences near-human transport of respiratory droplets and airborne particles, a review, Environmental Chemistry Letters; publisher version.
- Plume effects on inhaled particulate concentration: A review of human thermal plume and its influence on inhalation exposure to particulate matter; Computational modelling of the thermal plume adjacent to the body.
- Related reading in this journal: Why You Stop Smelling Your Own Perfume, on adaptation, and What a Formula Actually Looks Like, on the weight distribution that produces a trail.