Heat Transfer in Nature

Hold one end of a metal spoon in a hot cup of soup for even a few seconds, and the end you are holding starts to feel warm too — even though it never actually touched the soup. Heat has travelled along the metal, from its hot end to its cooler end. This movement of heat from a hotter part of an object to a colder part, without the object itself moving anywhere, is called conduction.

What exactly is happening inside the spoon? Picture the particles that make up the metal as tightly packed neighbours. The particles at the hot end absorb heat and pass some of it on to their immediate neighbours, who pass it to the next, and so on down the spoon. Each particle stays roughly where it is; only the heat travels onward, neighbour to neighbour.

Materials that allow heat to travel through them easily this way are called good conductors of heat. Nearly all metals — copper, iron, aluminium — are good conductors, which is exactly why cooking vessels are made of metal: heat from the stove passes quickly through the metal base and into the food. Materials that resist this kind of heat flow are called poor conductors, or insulators of heat. Wood, glass, plastic, and ceramic are all poor conductors — which is why a cup made of clay or porcelain keeps tea hot for longer than it would stay hot in a thin metal tumbler, and why a wooden or plastic spoon handle never feels as hot as a metal one.

Even air is a surprisingly poor conductor of heat. Woollen clothing feels warm in winter not because wool itself generates any heat, but because its fibres trap countless tiny pockets of air next to the skin; since that trapped air barely conducts heat away from the body, warmth is held in for longer. The same idea explains why sleeping under two thin blankets usually feels warmer than one thick blanket of the same total thickness — the layer of air trapped between the two thin blankets adds its own extra insulation.

Light an incense stick and watch the thin trail of smoke it gives off: the smoke does not drift sideways or sink downward, it rises straight up. The gases released by the burning stick are hot, and hot air is lighter (less dense) than the cooler air around it, so it floats upward through the cooler air, carrying its heat along with it.

This is a completely different way of moving heat compared to conduction. Instead of heat hopping from particle to particle while the particles themselves stay in place, in convection, the heated particles of a liquid or a gas physically move from one place to another, carrying their heat with them as they go.

You can see the same idea at work inside a pot of water being heated from below. Water near the bottom of the pot gets hot first; it expands slightly, becomes lighter than the cooler water above it, and rises. Cooler water from the sides and top then sinks down to take its place near the hot bottom, gets heated in turn, and rises too. This continuous rising-and-sinking movement — called a convection current — keeps repeating until the whole pot of water is hot, which is exactly why constant stirring is rarely needed when boiling a pot of water on a stove.

Convection currents in air are also what make hot-air balloons float, and what carries warm air up and out through a chimney above a stove or fireplace, drawing cooler air in from below to replace it.

Anyone who has visited a beach may have noticed that the sand underfoot can feel scorching hot by afternoon, while the seawater nearby stays comfortably cool. By late evening, that same sand cools down quickly, while the sea still feels relatively warm. Soil and land, it turns out, heat up and cool down considerably faster than an equal-sized body of water exposed to the same sunlight.

During the day, the land near the coast heats up faster than the sea. The warm air sitting above this hot land becomes lighter and rises, in the same way that air above any heated surface rises. As this warm air rises and moves away, cooler air from above the sea flows in over the land to take its place — and this inward flow of cool air from sea to land is what is felt as a pleasant sea breeze on a hot afternoon.

At night, the situation reverses. The land, having heated quickly during the day, also cools down quickly after sunset, while the sea — slower to heat and slower to cool — stays comparatively warmer. Now it is the air above the sea that is warmer and rises, drawing cooler air from the land out towards the sea to replace it. This outward flow, from land to sea, is called a land breeze.

This daily back-and-forth is really just convection playing out on a much larger, geographic scale — driven entirely by how differently land and water respond to the same sunshine.

Stand a short distance from a campfire, without touching the flames or anything they are heating, and warmth can still be felt reaching the skin. This warmth has travelled directly, without needing air, water, or any solid material to carry it along the way. This way of transferring heat is called radiation, and it is the only one of the three heat-transfer processes that can travel through completely empty space — which is exactly how the Sun's heat crosses the vast emptiness between the Sun and Earth to reach us. (Light from the Sun takes a little over eight minutes to cross this roughly 150-million-kilometre gap, travelling at the speed of light — about 300,000 km every second.)

Every object, hot or not, constantly radiates some heat to its surroundings and absorbs some heat radiated by its surroundings in turn. A hot cup of tea left on a table slowly cools down over time largely because it keeps radiating heat outward to the cooler room around it.

How much heat a surface absorbs or reflects through radiation also depends on its colour. Dark-coloured surfaces absorb a large share of the heat radiation that falls on them, while light or white surfaces reflect most of it away. That is precisely why dark-coloured clothes feel more comfortable in winter — they absorb the Sun's radiated heat and help keep the wearer warm — while light-coloured clothes feel more comfortable in summer, since they reflect away much of that same heat instead of absorbing it.

The Sun does far more than just warm the skin — its heat is also the engine that drives the entire water cycle. Heat radiated by the Sun constantly warms the surface of oceans, rivers, and lakes, turning some of that liquid water into invisible water vapour that rises into the air. This process is called evaporation. Plants add to this too: water drawn up by their roots escapes into the air as vapour from their leaves, a related process called transpiration.

As this water vapour rises higher into the atmosphere, it reaches cooler air and condenses back into tiny droplets of liquid water, which cluster together to form clouds — a process called condensation. Eventually, these droplets grow heavy enough to fall back to the ground as rain, snow, or hail, in a process called precipitation.

Not all of this water runs straight into rivers and lakes. Some of it slowly seeps down into the ground, a process known as infiltration. How quickly this happens depends on the gaps between the particles of the ground it passes through — water seeps through loosely packed gravel quickly, through sand more slowly, and through tightly packed clay slowest of all, since clay's particles leave the smallest, most closely spaced gaps for water to pass through. Water that seeps down far enough collects in the pore spaces of underground rock and soil layers, forming what is called groundwater; the underground layers capable of storing this water are called aquifers, and this is the water that wells and borewells draw up from below the surface.

This constant cycle — water rising as vapour, falling back as rain, and seeping into the ground, only to evaporate and begin again — keeps replenishing rivers, lakes, and underground water supplies alike. Because growing cities and reduced open ground have slowed down how much rainwater can naturally infiltrate the soil, many places now deliberately dig rainwater harvesting pits to help rain soak into the ground and recharge the aquifers beneath, rather than running off wasted over paved, concrete surfaces.