Brown Bear with Lunch, Lake Clark National Park, Alaska
What color is the coolest for clothing? What color(s) provide the most warmth? What color fabric dries the most rapidly?
Colors change heat transfer by modifying radiation heat transfer. A key to understanding radiation balance is the difference in characteristics between the visible and infrared ranges. Shiny objects (aluminum, polished metal, white) reflect most of the incident visible light. Dark colors absorb most of the visible light. If you wish to absorb more radiation when in the sunlight, dark colors are the answer and they are always warmer in the daytime.
Generally when the sun is shining, the problem is getting hot, not getting cold. Even in the winter, hikers have clothing optimized for night, shaded north faces, and bad weather. Staying warm is mostly an issue when the sun is not shining on us. The sun emits radiation predominantly in the visible range (i.e., the range our eyes have evolved to see) thus our eyes are a good measure of absorbance and reflectance in the daylight. Darker colors are by definition better at absorbing the sun's rays than light ones. The same is not true of the infrared wavelengths. The earth, most campfires, and animals, emit radiation predominantly in the infrared wavelength. Did you know that your body emits radiation all the time? The wavelength of radiation emitted by all objects is related to their temperature and the amount of radiation emitted is proportional to absolute temperature raised to the fourth power.
A portion of the heat we lose and gain is by net inflow or net loss of radiation. For example we feel warmer when standing in the sun than when standing in the shade. The difference is the increase in incoming radiation from the sun. Infrared radiation, the radiation we emit and that can cause a portion of our heat loss, is invisible and thus we don't intuitively understand it.
Some pigments have different "colors" depending upon the wavelength of light. Particularly, most paints and dyes are black in the infrared range. The exception is metallic (aluminum) paints and space blankets, which have low IR emissivity (link to table of emissivity).
A few examples may help. Aluminized fabrics (space blankets, some special silnylon) has low absorption and emission in the visible and infrared ranges. It is poor at emission and absorption in both bands. During the daytime aluminized fabric or painted surface reflects most incoming radiation from the sun, causing it to be cooler than most materials. During the night it emits very little infrared radiation causing it to be warmer than most coverings. When the sun is shining (or standing by a fire) the aluminized coating keeps one cooler than other surfaces. Except for standing by the fire, this situation is close to optimal for cold weather, one does not overheat in the sun and stays warmer in the shade or at night.
White pigments also reflect most visible light (but, in general, not quite as much as aluminized coatings), But . . . . . white pigments are "black" (have high emissivity) in the infrared wavelengths. During the daylight white reflects more of the incoming radiation from the sun and also emits infrared radiation when it gets hot. The loss of infrared radiation causes the white surface to be cooler than an aluminum surface when in the sunlight. This in the reason it is best to paint the roof white when living in a hot, sunny, climate. Other cool options besides white are being investigated in regards to cool roofs. (http://www-library.lbl.gov/docs/LBNL/504/51/PDF/LBNL-50451.pdf) or search for "cool roof" on the internet.
Black is the warmest color when in the sun. The disadvantage is that this is the time when one is usually too warm already. A major advantage of black and other dark colors is that they dry most rapidly in the sun. In the dark most colors dry at about the same rate as most pigments are "black" (have high emissivity and absorbtivity) in the infrared wavelengths.
To stay warm at night a tarp should be aluminized. It is likely that the driest tarp or rainfly would have an aluminized coating on the top and any other color on the bottom. This would cause the tarp to absorb IR from the ground and your body, keeping the fabric warmer. The aluminum top would prevent escape of IR from the top, also keeping the fabric itself warmer. Warmer fabric temperatures, relative to surroundings, are very important for reducing condensation. So one would stay slightly warmer with aluminum on both sides but have less condensation with aluminum only on the exterior surface. A prefect combination might be a reversible tarp with white on one side and aluminum on the other. For shade during the day put the white side up. Little sunlight is absorbed and little of that is reemitted beneath the tarp. For the night put the aluminum side up, minimizing IR loss to the sky while keeping the fabric temperature warm to minimize condensation.
The US military is working on (www.opticorp.com) IR coatings that reduce IR emissivity while maintaining ordinary colors. These materials may eventually work their way into outdoor equipment (clothing and tents). These coatings would provide the benefits of the aluminized coatings without the poor aesthetics.
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Copyright 2008 John Walton