ion is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection may also take place in soft solids or mixtures where particles can flow.This figure shows a calculation for thermal convection in the Earth's mantle. Colors closer to red are hot areas and colors closer to blue are in warm and cold areas. A hot, less-dense LOWER boundary layer sends plumes of hot material upwards, and likewise, cold material from the top moves downwards.Convective flow may be transient (such as when a multiphase mixture of OIL and water separates) or steady state (see Convection cell). The convection may be due to gravitational, electromagnetic or fictitious body forces. Heat transfer by NATURAL convection plays a role in the structure of Earth's atmosphere, its oceans, and its mantle. Discrete convective cells in the atmosphere can be identified by clouds, with stronger convection resulting in thunderstorms. Natural convection also plays a role in stellar physics. Convection is often categorised or described by the main effect causing the convective flow, e.g. Thermal convection.Thermal IMAGE of a NEWLY lit Ghillie kettle. The plume of hot air resulting from the convection current is visible.Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place.