Why Does a Winter Sunny Spot Feel Much Warmer Than the Air?
― The Low Winter Sun Delivers Nearly Twice the Heat Your Body Makes Itself
The forecast says 10 degrees. Yet stand in a sunny, windless spot and you want to take your jacket off. This warmth is not your imagination. Sunlight passes through the air almost without heating it, and warms your body directly. And because the winter sun is low, it hits the front of a standing body more squarely than the summer sun does.
It is a clear lunch break in November. Walking in the shade of a building, your fingertips feel numb with cold.
But the moment you sit on a bench in the sun, your back slowly warms up. A cat by the window squints happily in a patch of sunlight.
Sun and shade are only a few steps apart. The air temperature should be almost the same. So why is the difference so large?
There are only two reasons a sunny spot is warm
Air absorbs almost none of the sun's light. Light passes through the cold air and turns into heat only when it hits your clothes or skin.
The summer sun is almost straight overhead, so it hits a standing body only at a slant. The winter sun is low, so your body can catch its light on the front.
When these two combine, a winter sunny spot delivers from outside an amount of heat close to, or greater than, what your body makes itself. Let's look at them in turn.
Air temperature is deliberately measured in the shade
The temperature in a weather forecast is the temperature of air measured where the sun does not reach. At a weather station, the thermometer sits inside a shielded, well-ventilated housing, about 1.5 metres above the ground.
The sun is avoided on purpose because a thermometer in direct sunlight would warm up itself. Air absorbs hardly any light, so even in the sun the air temperature barely rises. In other words, "10 degrees" never counted the heat you receive when standing in the sun.
On a clear day, a surface facing the sun receives about 800 watts of sunlight per square metre. Most of this light reaches the ground without warming the air.
The winter sun works on your body because it is low
In Tokyo, the sun's height at noon is about 78 degrees near the summer solstice, about 38 degrees in early November, and about 31 degrees near the winter solstice. The winter sun travels across a much lower sky than the summer sun.
Look at Figure 1. The more squarely light strikes a surface, the more energy it delivers. For a "horizontal surface" like the ground, the high summer sun works best. But for a "vertical surface" like a standing body, it is the other way round. The low winter sun strikes it more squarely.
Take noon in early November, with the sun 38 degrees high. About 630 watts per square metre reach the front of a standing body. Multiply by the area of an adult seen from the front and by the share of light the clothes absorb, and the heat the body receives is about 180 watts.
A seated adult at rest makes about 100 watts of heat. So in a winter sunny spot, nearly twice that amount arrives from outside. It feels as if there were another person's worth of body heat added to your own. Do the same calculation for noon at the summer solstice, and the heat reaching the front of the body drops to about 46 watts. This is why the summer sun burns your head and shoulders but barely strikes your front.
Koharu biyori (Indian summer weather) means a warm, calm, clear day in late autumn to early winter. "Koharu" ("little spring") was another name for the tenth month of the old calendar (around November today). Light wind is a condition for this warmth, because wind quickly carries away the warmed air at the body's surface. This is also why a sheltered "sun trap," such as the lee of a building or inside a wall, feels especially warm in the same sunlight.
White clothes reflect much of the sunlight, while dark clothes soak up much of it. This is why your back warms up faster in a dark coat in a winter sunny spot. Cats and birds also fluff up their fur or feathers in the sun, which is thought to increase both the surface that catches light and the layer of air that holds heat.
Summary
Air temperature is the temperature of air measured in the shade, and it does not count the heat your body receives in the sun. Sunlight passes through air almost without warming it, and becomes heat only when it hits your body. On top of that, the low winter sun strikes the front of a standing body squarely. A windless winter sunny spot feels much warmer than the temperature figure because these two effects combine.
A winter sunny spot is a place warmed by light, not by air.
The low sun reaches you, standing, more strongly than it reaches the ground.
The way a campfire's heat reaches only your face is the same "heat carried by light" mechanism. For details, see Why does a campfire burn your face but leave your back cold?. The reason the sun's height changes with the seasons is covered in Why are there seasons?, and the amount of sunlight reaching the whole Earth is covered in How much energy does the sunlight reaching Earth carry?.
- On a clear day, prepare two identical thermometers. Put one in the shade and the other on a black cloth in the sun.
- Wait about 10 minutes, then compare the two readings. The thermometer on the black cloth in the sun should read much higher than the one in the shade.
- Next, fan the thermometer in the sun with a paper fan. If the reading drops, you can see that the wind is carrying heat away.
What the thermometer in the sun shows is not the air temperature but "the temperature of an object that has received light." This experiment also lets you feel why weather stations keep their thermometers in the shade.
For those who want to know more ― Terms, formulas and links to textbooksFrom middle-school science to university courses, with each level clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics and earth science
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity-level courses not taught in high school (heat transfer engineering, meteorology, building environmental engineering)
- ResearchNot yet settled even at university level; researchers are studying it now
Middle schoolTerms: this phenomenon has a name
- Radiation: heat carried directly as light (electromagnetic waves), without any material such as air in between. The sun's heat and a campfire's heat both arrive this way.
- Solar radiation (insolation): the energy of light reaching the ground from the sun. It is expressed in watts per square metre.
- Meridian altitude of the sun: the angle above the horizon when the sun is due south (its highest point of the day). It is high in summer and low in winter.
Middle schoolHigh schoolChecking with a formula: how much heat does the body receive in a winter sunny spot?
Let's estimate the heat received by an adult standing and facing the sun at noon in early November in Tokyo (sun height 38 degrees).
| In symbols | P ＝ a × S × A × cos h |
| In words | Heat received by the body ＝ absorbed fraction × sunlight reaching a surface facing the sun head-on × area of the body's front × cosine of the sun's height |
| Where it comes from | Conservation of light energy. When a beam of the same width strikes a tilted surface, it spreads thinly over a wider area. On a vertical surface, the strength reaching it falls by the factor of the cosine (cos) of the sun's height |
| P | Heat received by the body (unit: watts) |
| a | Fraction of light absorbed by clothes and skin (0 to 1) |
| S | Strength of sunlight reaching a surface facing the sun head-on (watts per square metre) |
| A | Area of the body seen from the front (square metres) |
| h | Height of the sun (angle above the horizon) |
| Sunlight S reaching a surface facing the sun head-on on a clear day | About 800 watts per square metre |
| Meridian altitude h in Tokyo in early November, and its cosine | About 38 degrees. cos 38 degrees is about 0.788 |
| Area A of an adult body seen from the front | About 0.4 square metres (approx.) |
| Fraction a of light absorbed by dark clothes | About 0.7 (approx.) |
| Heat made by an adult at rest | About 100 watts |
| Sunlight reaching a vertical surface (per square metre) | 800 × 0.788 ≒ 630 watts |
| Sunlight reaching the whole front of the body | 630 × 0.4 ＝ 252 watts |
| Heat P the body absorbs from that | 252 × 0.7 ≒ 176 watts |
| Ratio to the heat the body makes itself | 176 ÷ 100 ≒ 1.8 times |
| For comparison: noon at the summer solstice (height 78 degrees, cos is about 0.208) | 800 × 0.208 ≒ 166 watts |
| Combined coefficient for area and absorbed fraction | 0.4 × 0.7 ＝ 0.28 |
| Heat absorbed by the whole front of the body at the summer solstice | 166 × 0.28 ≒ 46 watts |
In a winter sunny spot, the body receives about 176 watts, about 1.8 times the heat it makes itself. That is like adding another person's worth of body heat. With the same sunlight strength, at noon on the summer solstice the front of the body receives only about 46 watts. A difference of nearly four times between seasons arises from the sun's height alone.
High schoolHigh school+Why air is not warmed by sunlight
High schoolSunlight is mostly visible light and near-infrared. Nitrogen and oxygen, the main components of air, absorb almost none of light at these wavelengths. So sunlight passes through the atmosphere and becomes heat only when it hits the ground or a body. Air is warmed mainly by touching the warmed ground from below.
High school+How warm a person feels depends not only on air temperature but also on radiation from surrounding walls and sky, solar radiation, wind speed and humidity. In architecture and meteorology, indices such as "mean radiant temperature," which sums the radiation from all surroundings into a single temperature, and "operative temperature," which combines it with air temperature, are used. A winter sunny spot is a place where the mean radiant temperature is far higher than the air temperature.
UniversityRadiative heat transfer and solar radiation calculations
Solar radiation at the ground is divided into "direct radiation," which arrives straight from the sun, and "diffuse radiation," which arrives after being scattered in the sky. Direct radiation on a tilted surface is found by multiplying by the cosine of the angle of incidence (Lambert's cosine law). The solar radiation received by a human body is calculated from a "projected area factor," found from the body's orientation and the sun's direction. Heat leaving the body is treated as the sum of radiation, described by the Stefan–Boltzmann law, and convective heat transfer, which depends strongly on wind speed.
📖 Derivations and further reading: Solar radiation (Wikipedia, in Japanese) ／ Lambert's cosine law (Wikipedia, in Japanese)
ResearchWhat is still not clearly understood
- A way to express "warmth" as one number. Many indices of perceived warmth, combining air temperature, solar radiation, wind, humidity and clothing, have been proposed, including international ones. Which one best matches how people feel in an outdoor sunny spot is still being compared.
- Black clothes versus white: which is really warmer? Black clothes absorb more light, but the heat is produced at the clothing's surface. With wind, that heat can be carried away before it reaches the skin, and the difference is reported to become small under some conditions.
- How animals decide to sunbathe. Lizards and butterflies are said to adjust the sunlight they receive by changing body orientation or wing angle. What cues they use to decide when and at what angle to bask is being studied species by species.
In other words, this article too is "an explanation within what is known today." The areas and absorbed fractions used here are approximations, and they can vary by several tens of percent with build, clothing colour and posture.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "How heat travels," "Sun's meridian altitude and the seasons" | Radiation, sun height in summer and winter |
| High school | Physics: "Heat and energy"; Earth science: "Heat balance of the atmosphere" | Why sunlight passes through air |
| High school+ | Heat exchange between environment and human body (advanced) | Mean radiant temperature, operative temperature |
| University | Heat transfer engineering, meteorology, building environmental engineering | Direct and diffuse radiation, cosine law, radiation and convection |
| Research | Outdoor thermal comfort, animal thermoregulation | Perceived-warmth indices, clothing colour, animal basking |
| ― | Links to daily life | Basking in a winter sunny spot, choosing a sun trap, coat colour |
- Koharu biyori (Wikipedia, in Japanese)
- Solar radiation (Wikipedia, in Japanese)
- Japan Meteorological Agency (気象庁), Guide to Meteorological Observation (気象観測の手引き) (air temperature observation methods)
- National Astronomical Observatory of Japan (国立天文台), ed., Chronological Scientific Tables (理科年表), Maruzen Publishing (solar radiation, meridian altitude of the sun)
- Architectural Institute of Japan (日本建築学会), ed., Teaching Material for Building Environmental Engineering: Environment Volume (建築環境工学用教材 環境編), Maruzen Publishing (mean radiant temperature, operative temperature)
*This article is a general-audience science explainer. The figures given are rough estimates meant to help you understand the mechanism. The heat a body receives varies greatly with build, clothing, posture and the state of the sky.