The color of an object can impact how much heat it absorbs from sunlight. Specifically, there is a common belief that white objects attract more heat than darker colors. In this article, we’ll explore whether this claim is true by looking at the science behind how color affects heat absorption. We’ll also discuss some real-world examples of how white vs. darker colors impact heating in things like clothing, cars, and buildings. Read on to learn the answer to the question: Does white really attract more heat?
How Color Affects Heat Absorption
To understand if white objects attract more heat, we first need to understand some basics about heat and color.
Heat comes in the form of infrared radiation from sunlight. When this radiation hits an object, some of it gets absorbed while the rest gets reflected. Darker colors absorb more of the sun’s infrared rays while lighter colors reflect more of this radiation.
This concept is known as albedo – the ratio of radiation reflected by a surface to the total radiation falling on it. White has a higher albedo than black, meaning it reflects more sunlight while black absorbs more.
For example, fresh snow has an albedo of about 0.9, meaning it reflects 90% of sunlight. Coal, on the other hand, has an albedo of about 0.04, only reflecting 4% of sunlight while absorbing the vast majority.
So in theory, darker colors like black should absorb more heat from sunlight while lighter colors like white should stay cooler by reflecting heat. But is this true in practice?
Real-World Examples
While dark colors logically should absorb more heat, there are some real-world examples where white objects get hotter than darker ones:
Clothing
Wearing black clothing in the sun will absorb more heat than white clothing. However, loose-fitting white clothing allows more sunlight to pass through the fabric and reach the skin versus darker loose fabrics. So in practice, white clothing does not keep people cooler than looser black clothing that provides more shade.
Cars
Similarly, one might assume white cars stay cooler in the sun than black ones. But studies show that interior car temperatures when parked in the sun only differ by about 5-10 degrees F. This relatively minor difference occurs because most heat enters through the windows, not the exterior color. And when driving, the cabins of all colored cars equalize to similar temperatures.
Buildings
White roofs and facades reflect more sunlight and theoretically should be much cooler. However, factors like insulation, ventilation, and heat production from people/appliances inside have larger impacts on building temperature. One study found that white roofs only reduced indoor temperatures by 0.3-0.8 degrees F compared to traditional dark roofs.
So in practice for cars and buildings, exterior color has a smaller influence on interior temperatures than one may assume. The key reasons why white does not affect temperature as much as expected include:
– Sunlight enters through windows, warming interiors regardless of exterior color.
– Insulation, ventilation rates, and internal heat generation outweigh exterior reflection in determining interior temperatures.
– White materials can get dirty over time, reducing their reflectivity.
When Does White Make a Difference?
Based on the above examples, white does not always equal a cooler temperature in the real world. However, painting surfaces white does make a measurable difference in these cases:
Outer Space
In space, white spacecraft stay cooler than dark ones since their only source of heat is sunlight. Without atmosphere for conduction/convection, radiation from the sun is the only heat source. So white spacecraft like the Apollo missions absorbed less heat.
Extreme Temperatures
In extremely hot conditions like deserts, white clothing provides more cooling power since infrared radiation from the sun is more intense. Bedouins and Tuaregs wear white robes to stay cooler in desert heat.
Roofs in Cooler Climates
In northern cooler climates, white roofs make a bigger difference since less heat enters through windows. White roofs reflect summer sunlight in these regions, keeping interiors cooler by around 1 degree C or more.
Low Emissivity Building Materials
Special “cool” building materials with low emissivity can take advantage of white’s reflectivity. These materials use white coatings that reflect sunlight while also emitting less of the building’s internal heat, providing passive cooling.
So while white doesn’t make a dramatic cooling difference for everyday objects, it does provide notable cooling in certain extreme radiative heat scenarios.
Does White Attract or Retain More Heat At Night?
Another factor to consider is nighttime cooling. While white may reflect more sunlight during the day, does it also retain more nighttime ground radiation and get hotter overnight?
The answer is no – white actually cools faster at night because it emits heat more efficiently. How quickly an object cools depends on its “emissivity” – the relative ability of its surface to radiate energy.
White has a higher emissivity than most colors other than black. At night, high emissivity allows an object to emit energy as heat radiation faster, causing faster cooling. An exception is that black has slightly higher emissivity than white, allowing it to cool quickest at night.
But in general, white releases daytime heat faster at night than most other lighter colors. This makes sense when considering Kirchhoff’s law of thermal radiation, which states that good absorbers of radiation are also good emitters. Since white absorbs less radiation during the day, it can also emit less at night.
Factors Besides Color That Impact Heating
While color has some effect on heat gain/loss, a surface’s material properties play a bigger role:
Thermal Mass
Objects with high thermal mass (e.g. stone, concrete) absorb and hold on to more heat than low thermal mass materials (e.g. wood, cloth). High thermal mass surfaces change temperature slowly and can store daytime heat to release at night.
Conductivity
Materials like metals conduct heat quickly, while insulators like wood or plastic do not. High conductivity allows efficient heat transfer into or out of an object.
Texture
Rough, textured surfaces absorb and emit radiation differently than smooth surfaces. Roughness can enhance radiation effects since it creates more surface area.
Geometry
The shape and orientation of an object impacts how much radiation it absorbs. E.g. A perpendicular or curved surface absorbs less direct radiation than a flat, angled surface.
So in many cases, tweaking these other properties has a larger effect on temperature than just changing color.
Conclusion
In summary, while darker colors theoretically absorb more radiative heat from sunlight, in practice white only makes a significant cooling difference in certain situations:
– White objects reflect the most heat in space where radiation is the only heat source.
– White clothing provides superior cooling in extremely hot desert climates.
– White roofs in cooler northern regions can reduce indoor temperatures a few degrees.
– Specialized white building materials with high reflectivity and low emissivity provide passive cooling.
For everyday objects like cars and buildings, exterior color has a smaller influence on interior temperatures than insulation, windows, ventilation, and internal heat generation.
And white does not retain more nighttime ground radiation – its high reflectivity during the day allows it to also emit surface heat efficiently at night for faster cooling.
So while white can provide some cooling advantages through reflectivity in the right contexts, for many everyday applications its color alone does not make a major impact on temperature compared to other material factors. The notion that white always attracts or retains more heat is a common misconception.
