Convection is an important mechanism for heat transfer in the atmosphere and ocean. Because of its low density, warm air or water rises, creating vertical currents that carry heat upward. The convection cycle is complete when the cooler air or water is replaced by the rising warm air or water. Convection works by areas of a liquid or gas heating or cooling greater than their surroundings, causing differences in temperature. These temperature differences then cause the areas to move as the hotter, less dense areas rise, and the cooler, denser areas sink. Convection currents are heat-driven cycles that occur in the air, ocean, and mantle. They are caused by a difference in temperature, often due to a differing proximity to a heat source. Convection is the transfer of heat energy through the movement of fluids, like when hot air rises and cools down. Radiation, on the other hand, is the transfer of heat energy through electromagnetic waves, which do not require any particles or medium to travel, allowing it to transfer through the vacuum of space. Convection within the atmosphere can often be observed in our weather. As the sun heats the Earth's surface, the air above it heats up and rises. If conditions allow, this air can continue to rise, cooling as it does so, forming Cumulus clouds. Ocean currents can be caused by wind, density differences in water masses caused by temperature and salinity variations, gravity, and events such as earthquakes or storms. Currents are cohesive streams of seawater that circulate through the ocean. When the mantle convects, heat is transferred through the mantle by physically moving hot rocks. Mantle convection is the result of heat transfer from the core to the base of the lower mantle. The primary contributors to heat in the core are the decay of radioactive elements, leftover heat from planetary formation, and heat released as the liquid outer core solidifies near its boundary with the inner core. The flow of heat from Earth's interior to the surface comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of Earth. Energy from the Sun heats the Earth unevenly. As a result, convection currents develop in the atmosphere and ocean. These redistribute heat in the atmosphere and oceans. The Sun generates energy, which is transferred through space to the Earth's atmosphere and surface.