Mr. Brown's Science Labs

Every joule of energy that drives weather, ocean circulation, and photosynthesis on this planet arrives as electromagnetic radiation. Unlike conduction and convection, radiation requires no medium; it crosses 150 million kilometers of vacuum in roughly eight minutes and then must be dealt with by whatever surface it strikes. The portion of solar radiation that reaches a horizontal surface on Earth is called insolation, a contraction of incoming solar radiation. The Sun radiates across a broad band of the electromagnetic spectrum, but because its photosphere is near 5,800 K, the greatest intensity is emitted at short wavelengths clustered in and around the visible band, roughly 400 to 700 nanometers. Locate that band on page 14 of the reference tables and notice how narrow it is compared with the infrared region to its right.

When that shortwave energy encounters a material surface, only three outcomes are possible, and together they must account for one hundred percent of the incident energy. The radiation may be reflected, bouncing off with its wavelength essentially unchanged. It may be transmitted, passing through a translucent or transparent medium such as clear water or atmospheric gases. Or it may be absorbed, in which case the electromagnetic energy is converted into the internal kinetic energy of the atoms and molecules of the material, and the temperature of the surface rises. Because energy is conserved, a surface that reflects a great deal cannot also absorb a great deal.

The fraction of incident radiation reflected by a surface is its albedo, reported as a decimal or a percentage. Fresh snow can return more than 0.80 of the insolation striking it, which is why a snow-covered field in February can remain frozen through a bright afternoon while the bare asphalt of the parking lot beside it, with an albedo near 0.10, becomes uncomfortably warm to the touch. Albedo is governed by several observable properties: darker pigments absorb more strongly than light ones; rough, irregular textures trap radiation in repeated internal reflections while smooth polished surfaces reject it; wet soil is measurably darker and lower in albedo than the same soil dry; and a surface struck at a low angle of incidence reflects a far larger share than the same surface struck from directly overhead.

Absorption alone, however, does not determine how hot a surface becomes. That depends additionally on specific heat, the quantity of energy required to raise one gram of a substance by one Celsius degree. Water absorbs most of the visible light that reaches it, yet its temperature climbs slowly, because water has an unusually high specific heat, transmits energy to depth, circulates that energy by convection, and expends a substantial share on evaporation. Dry sand, by contrast, has a low specific heat and is opaque, so the absorbed energy is concentrated within the uppermost few millimeters. Two surfaces can absorb identical amounts of energy and still arrive at very different temperatures.

Absorption is only half of the story. Every object with a temperature above absolute zero continuously emits electromagnetic radiation, and the character of that emission is governed by temperature alone. As an object grows hotter, the total energy it radiates increases sharply and the wavelength at which it radiates most intensely grows shorter. The Sun, at roughly 5,800 K, emits its peak energy in the visible band. Earth's surface, averaging near 288 K, radiates its peak energy in the far infrared, close to 10,000 nanometers — a wavelength no human eye can detect but that a thermal camera renders in vivid false color. This is the essential asymmetry of the planetary energy budget: Earth absorbs shortwave radiation and emits longwave radiation.

A second principle follows from the physics of emission and is frequently tested: a surface that is an efficient absorber at a given wavelength is an equally efficient emitter at that wavelength. Dark asphalt does not merely take in more insolation than concrete; it also radiates that stored energy away more efficiently once the Sun sets. This explains a familiar observation. Two hours after sunset, a blacktop driveway may still feel warm underfoot while the grass beside it has already cooled, because the driveway both absorbed more energy during the day and holds a greater quantity of energy to release.

The atmosphere responds selectively to these two wavelength ranges. Nitrogen and oxygen, which together compose roughly 99 percent of dry air, are largely transparent to incoming shortwave radiation and allow it to pass to the surface. Carbon dioxide, water vapor, and methane, however, absorb strongly in the infrared. They intercept a substantial portion of the longwave energy radiated upward by the surface and re-emit it in all directions, including downward. Raising the concentration of these gases raises the fraction of outgoing longwave energy that is intercepted, and the surface must warm until outgoing energy again balances incoming energy.

Human construction alters this budget at the local scale as decisively as emissions alter it globally. Urban development replaces vegetated ground, which has a moderate albedo and cools itself by transpiration, with asphalt, dark roofing membrane, and masonry — materials of low albedo, high heat capacity, and no evaporative cooling. Tall buildings form vertical canyons in which radiation reflected from one wall is absorbed by the next, so energy that would have escaped a flat landscape is instead trapped. The result is the urban heat island: a city center may run 1 to 3 Celsius degrees warmer than the surrounding countryside by day and as much as 5 to 7 degrees warmer on a calm summer night, when stored energy is being re-radiated and there is no wind to disperse it. The consequences are measurable in emergency room admissions during heat waves, in peak electrical demand for air conditioning, and in the temperature of stormwater running off pavement into local streams. Mitigation strategies attack the same variables the physics identifies: raise albedo with reflective roofing, restore evaporative cooling with tree canopy and green roofs, and reduce heat storage with lighter, more porous paving materials.