convection: one construct, many systems

In fluid physics, convection is the transport of heat by the bulk motion of a fluid: when a layer is heated from below, warmer and less dense fluid rises while cooler fluid sinks, and once the destabilising buoyancy overcomes the damping of viscosity and thermal diffusion the layer breaks into organised overturning cells. Rayleigh's 1916 On convection currents in a horizontal layer of fluid, when the higher temperature is on the under side set out the stability analysis that fixed the critical threshold, now written as the Rayleigh number, at which conduction gives way to convection. The referent is a buoyancy-driven flow instability, characterised by that dimensionless threshold and the resulting cell pattern, and it is the same construct that organises thunderstorms and that stirs the Earth's mantle.[1]

Unit

The Rayleigh number at which a heated fluid layer begins to overturn

Recovery means

Not applicable

Measured by

Characterised by the critical Rayleigh number, the Nusselt number (convective versus conductive heat transport), and the observed cell geometry

In the atmosphere the same instability lifts warm, moist surface air that is lighter than its surroundings, so parcels rise, cool, and condense their water vapour, releasing latent heat that drives the updraught further and builds cumulus and thunderstorm clouds. Emanuel's Atmospheric Convection treats the parcel buoyancy, conditional instability, and convective available potential energy that decide whether and how violently the air overturns. It is the identical buoyancy-driven overturning as in a heated laboratory layer, realised in a moist compressible atmosphere, and measured by instability indices and convective energy rather than a laboratory Rayleigh number.[2]

Unit

The convective available potential energy (CAPE) stored in an unstable air column

Recovery means

Not applicable

Measured by

Assessed by CAPE, convective inhibition, lapse-rate instability indices, and radar or satellite observation of convective cloud

In the solid Earth the same construct operates on rock that, though solid, creeps like an extremely viscous fluid over millions of years: heat from the core and from radioactive decay makes deep mantle rock buoyant enough to rise while cold slabs sink, and this slow overturning is the engine that moves tectonic plates. Turcotte and Schubert's Geodynamics develops the mantle Rayleigh number, boundary-layer theory, and the very high viscosities that set the pace of this flow. It is the same buoyancy-driven convection as in a fluid layer or a storm, here acting on creeping rock over geological time and measured by seismic imaging of up- and down-wellings rather than by watching a fluid move.[3]

Unit

The mantle Rayleigh number and the overturn time of a convecting rock layer

Recovery means

Not applicable

Measured by

Inferred from the mantle Rayleigh number, seismic tomography of up- and down-wellings, surface heat flow, and plate velocities