The last lesson said the troposphere is heated from below; this one follows the heat on its travels. Air warms and cools by a handful of named processes (radiation, conduction, convection, advection, turbulence and latent heat), and wherever local cooling or warming makes one place differ from another, these processes move heat between them. Their nightly failure to keep the surface warm produces the inversion, the upside-down layer that ends the lesson.
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
The influence of the wind on the cooling and warming of the air near the surface is:
The cooling and warming of the surface of the Earth by radiation works as follows:
After a cloudless, calm night over land, the surface air is 8 °C colder than the air at 1,000 ft. The inversion type and the reason for its formation are:
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Radiation is energy travelling as electromagnetic waves, needing no material to carry it: it is how the sun's heat crosses empty space. Solar radiation reaching the Earth is shortwave, and the atmosphere filters it on the way down: ozone absorbs most of the ultraviolet high up, clouds and air scatter and reflect a share back to space, but the clear air absorbs remarkably little — most of what arrives passes through and is absorbed by the ground. The warmed Earth answers with terrestrial radiation: longwave, infrared, radiated upward day and night. And unlike the incoming sunshine, outgoing terrestrial radiation is absorbed by components of the atmosphere (water vapour and carbon dioxide above all), which warm and radiate part of it straight back down. Clouds do the same job wholesale: a cloud layer absorbs the ground's radiation and returns much of it, which is why an overcast night stays mild and a clear one turns cold.
Conduction — heat passed by direct molecular contact. Air conducts poorly, so conduction matters only in the shallow film touching the surface; but that is exactly where the atmosphere is warmed on a sunny day and chilled at night, so its role in the cooling and warming of the lowest air is decisive.
Convection — heat carried upward by rising air. It occurs wherever air is heated from below until it becomes buoyant: over sun-warmed ground on a summer afternoon, and over any warm surface with colder air above, such as a winter sea under polar air.
Advection — heat carried horizontally by moving air: a warm wind arriving over a cold landscape, or tropical air riding poleward ahead of a front. It occurs wherever the wind blows air from one kind of surface to a different one.
Turbulence — the stirring of the lower atmosphere by wind blowing over rough ground, which mixes surface-warmed or surface-cooled air through a deep layer far faster than conduction ever could.
Latent heat — heat travelling hidden inside water: evaporation at the surface stores heat in the vapour, and condensation aloft releases it again, so every cloud is a delivery of surface heat to altitude. The thermodynamics module weighs this one properly.
The surface itself runs on a radiation budget. By day insolation exceeds what the ground radiates away and the surface warms; by night the radiation continues with no sun to balance it and the surface cools: land doing both quickly, water slowly. The air above follows the surface's lead by conduction in the contact film and by turbulent mixing above it, warming from below by day and chilling from below at night.
Two spectators change the script:
Clouds — they blunt both halves of the cycle: by day they intercept sunshine and the surface warms less, by night they return the ground's radiation and it cools less; cloudy weather means small temperature swings.
Wind — it decides how deep the surface's influence reaches: a strong wind stirs the warming or cooling through a thick layer, so the surface itself never gets far from the air's mean; a calm night leaves all the cooling concentrated in a shallow skin of air that can chill drastically.
An inversion is a layer in which temperature rises with height: the normal lapse turned upside down. Its defining property follows at once: with warm air already sitting on top of cold, an inversion is extremely stable and shuts down vertical motion. Rising air meets warmer surroundings, loses its buoyancy and stops. An isothermal layer, temperature constant with height, acts the same way in gentler form: still stable, still a brake on vertical motion, just not a wall. Beneath an inversion the air is smooth and whatever it carries stays: haze, smoke and moisture accumulate under the lid, visibility suffers, and where the wind changes sharply across the inversion top there can be a marked shear.
Ground inversion — the nocturnal radiation inversion of a clear, calm night: the surface radiates itself cold and chills the lowest air, while the air a few hundred feet up never hears about it. The same profile forms by advection when air flows over a cold surface and is cooled from below.
Subsidence inversion — sinking air in an anticyclone is compressed and warms on the way down, laying a warm layer over the cooler air below: an inversion aloft, the trademark of settled high-pressure weather.
Frontal inversion — at a front, warm air rides up over the cold wedge beneath it, so a climb through the frontal surface passes from cold into warm: an inversion marking the boundary.
Inversion above the friction layer — turbulent mixing stirs the friction layer toward one uniform state and carries the surface cooling upward through it; at the top of the mixed layer the stirring stops, and a step to the warmer, unmixed air above appears.
Valley inversion — on mountain nights, air cooled on the slopes drains downhill (the katabatic flow the wind module returns to) and pools on the valley floor, ponding cold air under warm: valleys hold their inversions longer and deeper than open country.
The six transport processes and the five inversion types are the recall backbone; the cloud and wind effects on the surface cycle and the stability consequences of an inversion carry the understanding marks. The overview, following the learning objectives:
Explain how local cooling and warming transfer heat, and describe radiation, solar and terrestrial, with the atmosphere's filtering and absorption
Explain conduction, convection, advection, turbulence and latent-heat transfer, and where each occurs
Explain the surface's radiative cooling and warming, the air's response, and the influence of clouds and wind
Describe the development, types and stability characteristics of inversions and isothermal layers