Why can one outfit feel comfortable at 35°C while another feels unbearably hot?
Why can the same shirt feel comfortable in the morning but uncomfortable in the afternoon?
And why can a light breeze suddenly make a hot environment feel much more manageable?
The answer is not simply temperature.
What we experience as thermal comfort is the result of a complex interaction between the human body, clothing and the surrounding environment.
Air temperature matters, but so do humidity, air movement, radiation, clothing, metabolic activity and the body's ability to regulate its temperature.
For summer clothing, understanding thermal comfort is particularly important because clothing sits directly at the boundary between the body and its environment.
What Is Thermal Comfort?
Thermal comfort refers to a person's subjective state of feeling thermally satisfied with their environment.
It is important to distinguish thermal comfort from body temperature.
Your internal body temperature can remain within a relatively stable range while you experience different levels of thermal comfort.
For example, you might feel:
- Comfortable
- Slightly warm
- Hot
- Sweaty
- Cool
- Chilly
without necessarily experiencing a dangerous change in core temperature.
Thermal comfort is therefore both physiological and perceptual.
Your body is constantly regulating heat while your brain interprets the resulting conditions as comfortable or uncomfortable.
The Human Body as a Thermal System
The body continuously produces heat through metabolism.
At the same time, it exchanges heat with the surrounding environment.
A simplified representation is:
Metabolic heat + environmental heat gain → heat loss + stored heat
The body attempts to maintain thermal balance.
When heat production or environmental heat gain exceeds heat loss, body heat begins to accumulate.
When heat loss exceeds heat production, the body becomes cooler.
Clothing influences this balance by modifying the pathways through which heat and moisture move between the body and the environment.
The Six Major Factors Affecting Thermal Comfort
Thermal comfort is influenced by several interacting variables.
A useful framework is:
- Air temperature
- Mean radiant temperature
- Humidity
- Air velocity
- Metabolic activity
- Clothing insulation
These variables are commonly considered in thermal comfort research and building science.
For clothing, however, the last three can become particularly important because garments directly affect the body's immediate microclimate.
1. Air Temperature
Air temperature is the most obvious environmental variable.
When surrounding air is cooler than your skin, the body can potentially lose heat to the environment through convection.
When air temperature rises toward skin temperature, this pathway becomes less effective.
If the air becomes hotter than the skin, convection can instead contribute to heat gain.
This explains why simply saying "the temperature is 35°C" does not fully describe how an environment will feel.
The same air temperature can produce very different experiences depending on humidity, radiation, wind and clothing.
2. Mean Radiant Temperature
Your body does not exchange heat only with the air.
It also exchanges thermal radiation with surrounding surfaces.
These include:
- Buildings
- Roads
- Walls
- Vehicles
- The sky
- Trees
- Other surfaces
- Direct sunlight
This is particularly relevant outdoors.
Standing under direct sunlight can feel significantly hotter than standing in shade even when the air temperature is essentially the same.
The difference comes partly from radiant heat exchange.
This is why summer clothing has to consider both:
Airflow
and
Solar exposure.
A garment can facilitate ventilation while still providing useful physical coverage from direct sunlight.
3. Humidity
Humidity determines how much water vapor is already present in the air.
This matters because the human body relies heavily on sweat evaporation for cooling.
When humidity is relatively low, sweat can evaporate more readily.
When humidity is high, evaporation becomes increasingly difficult.
The result can be a familiar summer experience:
You sweat heavily, but you do not feel much cooler.
This is because sweating and evaporative cooling are not the same thing.
Sweat must evaporate to remove substantial heat from the skin.
4. Air Velocity
Air movement is another major component of thermal comfort.
Moving air can influence:
- Convective heat transfer
- Sweat evaporation
- The boundary layer around the skin
- Moisture accumulation inside clothing
A breeze can therefore make a warm environment feel more comfortable.
But airflow is not universally cooling.
When the surrounding air becomes extremely hot, increased air movement can reduce or reverse convective heat loss.
The cooling effect of airflow therefore depends on the relationship between:
Air temperature + skin temperature + humidity + air velocity.
5. Metabolic Activity
Your body produces more heat when you are physically active.
Walking, exercising, climbing stairs, carrying loads and working outdoors all increase metabolic heat production.
This changes thermal comfort.
An outfit that feels comfortable while sitting in an air-conditioned office may feel very different during a 30-minute walk outdoors.
Physical activity also increases sweating, making moisture management more important.
This creates a fundamental design challenge for summer clothing:
The garment must function while the body is generating additional heat and moisture.
6. Clothing
Clothing is the variable most directly controlled by the wearer.
A garment creates a microenvironment around the body.
Its thermal behavior depends on factors including:
- Fabric thickness
- Fabric structure
- Air permeability
- Water-vapor transmission
- Moisture absorption
- Garment fit
- Garment openings
- Layering
- Surface properties
- Color
- Coverage
This means that two garments made from different fabrics can produce very different thermal experiences even when they weigh approximately the same.
The Clothing Microclimate
Between your skin and your clothing exists a small region of air and moisture.
This is called the clothing microclimate.
It is affected by:
Body → heat → clothing → surrounding environment
and
Body → sweat/water vapor → clothing → surrounding air
The conditions inside this microclimate can determine whether clothing feels comfortable or oppressive.
A garment that traps warm, humid air close to the skin can increase discomfort.
A garment that allows appropriate air and moisture exchange can support better thermal regulation.
Why Air Permeability Matters
Air permeability describes how readily air can pass through a textile under a specified pressure difference.
For summer clothing, higher air permeability can facilitate air exchange between the body and surrounding environment.
This can support:
- Ventilation
- Convective heat transfer
- Moisture removal
- Evaporative cooling
However, air permeability is only one part of thermal comfort.
A very open fabric may provide excellent airflow but insufficient protection from sunlight or other environmental factors.
Good summer clothing therefore requires a balance between ventilation and protection.
Thermal Resistance vs Evaporative Resistance
Two important concepts in clothing science are:
Thermal resistance
Thermal resistance describes how strongly a clothing system resists heat transfer.
Higher insulation generally reduces heat exchange.
This can be useful in cold environments.
In hot environments, excessive insulation can make it more difficult for body heat to escape.
Evaporative resistance
Evaporative resistance describes how strongly a clothing system restricts the movement of water vapor.
If evaporative resistance is high, sweat evaporation can become more difficult.
For hot-weather clothing, reducing unnecessary resistance to heat and moisture transfer can improve thermal comfort.
Why "Lightweight" Does Not Automatically Mean Cool
One of the most common assumptions in summer fashion is:
Lower weight = cooler garment.
Weight alone cannot determine thermal performance.
A lightweight fabric can still feel hot if it:
- Has low air permeability
- Restricts moisture transport
- Fits tightly
- Traps humid air
- Absorbs significant solar radiation
- Has poor garment ventilation
Conversely, a slightly heavier garment may feel comfortable if its structure provides effective ventilation and moisture exchange.
The relevant question is not simply:
How much does it weigh?
It is:
How does the entire garment interact with heat, air and moisture?
Fabric Structure and Thermal Comfort
Textile construction has a major influence on thermal behavior.
Important characteristics include:
Porosity
The size and distribution of spaces within a fabric can influence air and moisture movement.
Yarn structure
Yarn diameter, twist and construction influence the resulting fabric architecture.
Fabric density
More densely constructed fabrics can restrict air movement.
Thickness
Thicker materials generally provide greater thermal resistance.
Surface structure
The textile surface can influence contact, moisture spreading and heat exchange.
Finishing
Chemical and mechanical finishes can modify properties such as moisture behavior and air permeability.
Thermal comfort is therefore fundamentally connected to textile engineering.
Garment Fit Changes Thermal Comfort
Fabric is only half of the equation.
The way a garment sits on the body also matters.
A tight garment reduces the amount of air between the skin and textile.
A relaxed garment can create a larger clothing microclimate.
Movement can further change this space.
Walking, arm movement and body motion can cause air to enter and exit through:
- Sleeves
- Collar
- Hem
- Side openings
- Other garment gaps
This can create what is sometimes described as a bellows effect, where movement contributes to air exchange.
The Role of Clothing Openings
Garment openings are often overlooked in discussions about fabric.
Yet openings can strongly influence ventilation.
Examples include:
- Open collars
- Vented backs
- Relaxed sleeves
- Side openings
- Open hems
- Mesh panels
- Layer gaps
These features can provide pathways for warm, humid air to escape and surrounding air to enter.
In other words, garment architecture can be as important as fabric selection.
Thermal Comfort Is Not the Same as Cooling
This distinction matters.
A person can experience improved thermal comfort without a major change in body temperature.
For example, increasing air movement can make a warm environment feel more comfortable by improving heat and moisture exchange at the skin.
Similarly, shade can reduce radiant heat exposure without changing the air temperature.
Thermal comfort is therefore about the interaction between the body and its environment, not simply about lowering temperature.
Why Humid Heat Feels Different
Consider two environments at 35°C.
35°C + relatively dry air
Sweat may evaporate efficiently.
Air movement can support further evaporation.
The body may be able to dissipate heat relatively effectively.
35°C + high humidity
Sweat evaporation becomes more difficult.
Clothing can become damp.
The skin can remain wet.
The clothing microclimate can become warm and humid.
The result is often substantially greater discomfort.
This demonstrates why temperature alone is insufficient to describe summer conditions.
Solar Radiation Changes the Equation
Outdoor thermal comfort is strongly affected by sunlight.
Direct solar radiation can add heat to the body and clothing.
The amount of radiant heat absorbed depends on factors such as:
- Solar intensity
- Exposure duration
- Garment color
- Surface properties
- Fabric structure
- Coverage
- Shade
This creates a design tension:
More coverage can reduce direct solar exposure, but more material can also affect ventilation and evaporative heat loss.
The solution is not necessarily less clothing.
It is better-designed clothing.
The Summer Clothing Design Problem
A summer garment has to manage several competing requirements.
It should ideally:
- Allow appropriate airflow
- Avoid unnecessary insulation
- Support moisture evaporation
- Provide useful sun coverage
- Remain comfortable during movement
- Avoid excessive cling when damp
- Maintain an appropriate clothing microclimate
- Fit the intended environment
This is particularly challenging for jackets.
A conventional jacket is generally designed to create a thermal barrier.
A summer jacket has a different objective.
It needs to provide protection without unnecessarily isolating the body from the environment.
Thermal Comfort and the Urban Summer
Urban environments introduce additional challenges.
Cities can combine:
- High air temperatures
- Concrete and asphalt surfaces
- Reflected solar radiation
- Traffic
- Limited shade
- High humidity
- Crowded pedestrian areas
- Long commutes
- Heat generated by buildings and vehicles
A person moving through a city therefore experiences constantly changing thermal conditions.
One moment may involve direct sun.
The next may involve shade.
Then an air-conditioned building.
Then a hot road or crowded street.
Summer clothing has to function across these transitions.
Thermal Comfort During Travel
Travel adds another layer of complexity.
A person may move between:
Airport → aircraft → taxi → outdoor street → hotel → restaurant
Each environment can have different:
- Temperatures
- Air velocities
- Humidity levels
- Radiant conditions
- Activity levels
This is one reason versatile summer layers can be useful.
The ideal travel garment is not necessarily the garment with the lowest possible insulation.
It is the garment that can adapt to changing environmental conditions without becoming unnecessarily hot outdoors.
A Systems Approach to Summer Clothing
Thermal comfort is best understood as a system.
A useful model is:
BODY
Metabolism
Sweat
Skin temperature
Movement
↓
CLOTHING
Fabric
Air permeability
Moisture transport
Fit
Openings
Coverage
↓
ENVIRONMENT
Temperature
Humidity
Air movement
Solar radiation
Radiant temperature
↓
THERMAL COMFORT
The final experience emerges from the interaction between all three.
This is why isolated fabric claims rarely tell the complete story.
Airlayer Perspective: Clothing as a Thermal Interface
At Airlayer, we approach summer outerwear through this systems perspective.
The purpose of a summer jacket is not simply to add another layer of fabric.
It is to create a controlled interface between the body and a demanding outdoor environment.
That means considering:
- Air movement
- Moisture
- Solar exposure
- Textile structure
- Garment volume
- Openings
- Movement
- Climate
The objective is to create clothing that works with the body's natural thermoregulation rather than unnecessarily obstructing it.
For hot climates, the future of outerwear may not be about making jackets warmer or heavier.
It may be about making them smarter about heat.
Final Takeaway
Thermal comfort is not determined by temperature alone.
It is the result of a continuous interaction between the human body, clothing and environment.
Air temperature, humidity, radiation and airflow influence how much heat the body can exchange with its surroundings. Physical activity changes heat production and sweat. Clothing then modifies the entire process through its fabric structure, fit, permeability, moisture behavior and coverage.
The key lesson is simple:
Comfort in summer is not just about wearing less. It is about managing heat, air, moisture and solar exposure intelligently.
As temperatures rise and climates become increasingly challenging, summer clothing has an opportunity to move beyond conventional fashion categories and become a more deliberate form of climate-responsive design.