Mr. Brown's Science Labs

Every transfer of energy in the Earth system obeys a single, non-negotiable rule: energy moves spontaneously from a region of higher temperature toward a region of lower temperature, and never the reverse. What differs from one situation to the next is not the direction of the flow but the mechanism that carries it. Earth scientists recognize three mechanisms, and distinguishing among them is the first step toward explaining nearly every process in the atmosphere, hydrosphere, and geosphere.

Conduction operates through direct contact. When two substances at different temperatures touch, the faster-vibrating particles of the warmer substance collide with the slower-vibrating particles of the cooler substance, and kinetic energy is handed off collision by collision. Because conduction depends on particles being close together and held in fixed positions, it is most efficient in solids — particularly in metals, whose loosely held electrons carry energy rapidly through the material. It is least efficient in gases, where particles are widely separated. This inefficiency is why still air trapped in insulation, in a wool coat, or in the pore spaces of freshly fallen snow is such an effective barrier to heat loss.

Convection operates only in fluids — liquids and gases — because it requires the material itself to move. When a portion of a fluid is heated, its particles spread apart, its density decreases, and the buoyant force lifts it upward. Cooler, denser fluid sinks to replace it, is heated in turn, and rises. The resulting circular circulation pattern is a convection cell. Convection cells drive the churning of Earth's mantle that displaces lithospheric plates, the rising columns of air that build thunderstorms, and the vertical exchange that stirs the world ocean.

Radiation is unique among the three because it requires no matter at all. Energy travels as electromagnetic waves that move through the vacuum of space at 3.0 × 10⁸ meters per second. Radiation is the only mechanism capable of delivering solar energy across the 1.5 × 10⁸ kilometers separating the Sun from Earth. Once that energy arrives, however, conduction and convection take over and redistribute it through the atmosphere, oceans, and crust.

Solar radiation striking Earth's surface is called insolation — a contraction of incoming solar radiation. The amount of insolation any location receives depends chiefly on the angle at which the rays strike the surface. Rays arriving near 90° concentrate their energy over a small area, while low-angle rays spread the identical amount of energy across a much larger area and pass through more atmosphere before arriving. This geometry alone explains why equatorial latitudes receive far more energy annually than polar latitudes, and why summer afternoons are warmer than winter mornings at the same location.

Once insolation reaches a surface, the fate of that energy depends on the surface itself. Dark-colored, rough surfaces absorb a high percentage of the radiation striking them; light-colored, smooth, and shiny surfaces reflect much of it. The fraction reflected is called albedo. A surface that is a good absorber of radiation is also a good radiator: after sunset, dark asphalt releases its stored energy far more rapidly than the surrounding vegetation does.

A second control is specific heat — the quantity of energy required to raise one gram of a substance by one Celsius degree. The Earth Science Reference Tables list liquid water at 4.18 joules per gram per Celsius degree, roughly four to five times the value for basalt, granite, or dry air. Water therefore warms slowly, cools slowly, and moderates the climate of coastal locations, while land surfaces warm and cool rapidly. The resulting temperature contrast between land and sea produces the daily reversal of sea breezes and land breezes along the shorelines of Long Island.

Human activity now alters these controls directly. Replacing vegetated ground with asphalt and dark roofing lowers albedo and raises specific-heat storage in a city's building materials, producing the urban heat island effect: nighttime air temperatures in dense urban cores routinely run 2–5 Celsius degrees warmer than in surrounding suburbs. Municipalities have begun mitigating this by installing reflective “cool roofs” and expanding tree canopy, both of which raise surface albedo and restore evaporative cooling.