1. Do people in suits and people in summer clothes feel the same heat?
Perceived temperature is a number that represents how hot or cold you feel on your body. It is thought to be determined by combining environmental conditions like air temperature, humidity, wind, and surrounding heat (such as sunlight) with human factors like activity level and amount of clothing [7]. Clothing is one of these human factors.
To show the effect of clothing, researchers looked at measurements from a Turkish office in winter. They calculated the Predicted Mean Vote (PMV), which estimates thermal comfort. People wearing suits (1.0 clo) scored about +0.3 to +0.4, while those in summer clothes (0.5 clo) scored about -0.3 to -0.5 [4]. A score close to 0 means "just right," plus signs mean feeling warm, and minus signs mean feeling cool.
When calculating the difference between suits and summer clothes across three measurements, the gap was always about 0.8. This shows that even in the same room, changing clothes can move your feeling to the opposite side of comfort. However, this result comes from one location and is a calculated value, not a direct answer from people. So far, we know clothing changes how you feel, but because wind, sunlight, and movement also affect it, there is no fixed conversion for "how many degrees equals one piece of clothing." Next, let us look at the unit used to count clothing warmth.
2. Clo: a unit that measures clothing warmth and can be added up
Clothing warmth is measured in a unit called "clo." One clo is defined as the amount of clothing that allows a person sitting quietly to feel comfortable in a room with an air temperature of 21°C, almost no wind, and humidity below 50%. This is roughly equivalent to a business suit. Being naked is 0 clo [1].
A useful feature is that you can add the clo values of individual items to get the total for your whole body. Reference values often cited include: T-shirt 0.09, long-sleeved shirt 0.12, thin sweater 0.20, thick sweater 0.35, jacket 0.35, and down jacket 0.55 [1].
In examples from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), shorts with a short-sleeved shirt equal 0.36 clo. Long pants with a long-sleeved shirt and T-shirt equal 0.61 clo. Adding a sweater brings it to 1.01 clo. Adding an outer coat and long underwear brings it to 1.30 clo [2]. Dividing the final value (1.30) by the initial value (0.36) gives about 3.6 times more insulation.
However, these values are estimates. The increase from 0.61 to 1.01 is 0.40, which differs slightly from the thin sweater (0.20) and thick sweater (0.35) listed earlier. Even within the same reference materials, numbers vary between tables and examples [1]. It is safer to view a single sweater as having a range of 0.2 to 0.4 clo. But why does the number increase as you layer clothes? Is the fabric itself warm?
3. Air trapped between layers is what makes clothes warm
Air has properties that make it poor at conducting heat. Therefore, when spaces form between your body and clothes, or between layers of clothes, heat trying to escape from your body finds it harder to get outside, making you feel warmer [3]. It is easy to understand if you think of clothes as "bags for storing air."
It is also thought that air remaining in the gaps between layered clothes helps with insulation. This connects to the addition method from the previous section. As you layer clothes, the clo value increases because there is more fabric. Additionally, air layers form in between, which is believed to help retain heat. Conversely, if you press clothes tightly together, the air layer becomes thinner, and insulating power decreases [3].
So far, this discussion has assumed windless indoor environments. If you go outside where there is wind, do clothes that store air still work the same way?
4. How wind and sunlight change perceived temperature apart from clothing
It feels colder when the wind is strong, but this effect depends on conditions like air temperature. There is no fixed number for "how much it drops every time the wind gets stronger" [6].
The Wind Chill index used by the US National Weather Service (NWS) is based on wind at face height and how heat escapes from skin. It applies when the air temperature is 50°F (about 10°C) or lower, and wind speed exceeds 3 mph (about 1.3 m/s). Importantly, it assumes there is no sunlight (like a clear night sky) and does not take clothing into account [5]. Because clothing is not a variable in this index, the numbers cannot be used to answer "how many degrees does clothing change."
The same source notes that strong sunlight can raise the wind chill temperature by 10 to 18°F. This equals about 5.6 to 10°C (since it is a difference, we multiply by 5/9), showing that just having or not having sunlight creates a significant difference in perceived temperature [5].
Also, this index was tested on adults, with warnings to apply it carefully for children, the elderly, or people with health concerns. Humidity has an effect of less than 1 degree, so it is not included in the formula [5]. Thus, clothing alone does not determine how you feel. How do we actually use clothing to reduce cold or heat in daily life?
5. Warm Biz and Cool Biz: setting room temperature and adjusting with clothes
In Japan, the Ministry of the Environment promotes "Warm Biz," asking people to set heated rooms to 20°C. In surveys on perceived comfort, more than 95% of people answered that 20°C was "comfortable" or "slightly cold but not a problem" [8]. The ministry also suggests using scarves and knee blankets to ease the cold.
In summer, there is "Cool Biz." This promotes clothing that allows people to be comfortable in offices even if air conditioning is set to 28°C [9].
One might argue that being able to manage at 20°C or 28°C was thanks to clever clothing choices. However, the Ministry of the Environment materials do not provide specific numbers for how much clothing changes comfort [8]. "Easing the cold with clothes" and "how many degrees that is worth" are different topics.
Perceived temperature is determined by combining air temperature, wind, sunlight, movement, and clothing. It is difficult to isolate just one variable and turn it into a number. How much clothing changes your comfort also varies with wind, sunlight, and how you move. You can try adding up your own clothing to find your clo value.
6. Try it yourself: add up the clo values of your clothes and check the air layer
Add up at home: Take the clothes you are wearing today and add their estimated values from Section 2. For example, a T-shirt (0.09) plus a long-sleeved shirt (0.12) plus a thin sweater (0.20) equals 0.41. Clo values can be added as estimates. To understand how air between clothes relates to warmth, review Section 3.
Check the air layer: Place a thick sweater on the floor. Measure its height when it is fluffy, then measure again when you press it lightly with your hand using a ruler. You will see that pressing makes the sweater thinner. As seen in Section 3, the air gaps in clothes help with insulation.
When going out: Walk around in the same clothes, comparing sunny spots and shaded areas, windy places and calm places. You will notice how conditions other than clothing change your feeling. On hot days, do not stay out for too long, drink water, and go with a family member or adult.
Official information: The Ministry of the Environment press releases [8] [9] are available in Japanese. The US National Weather Service wind chill materials [5] are in English but have a short summary of the conditions.