A temperature gradient is the gradual variance in temperature with distance. The slope of the gradient is consistent within a material. A gradient is established anytime two materials at different temperatures are in physical contact with each other. Units of measure of temperature gradients are degrees per unit distance, such as °F per inch or °C per meter.
Many temperature gradients exist naturally, while others are created. The largest temperature gradient on Earth is the Earth itself. The temperature of the Earth’s core is estimated at about 9,000°F (5,000°C); it is 6,650°F (3,700°C) at the boundary between the core and the mantle, while the crust temperature is about 200°F (93°C). Each layer has a temperature gradient of a different slope, depending on the heat conductivity of the layer.
No temperature gradient exists between the Earth and the sun because there is not an atmosphere between them. Heat capacity is the ability for a material to hold heat. A vacuum has zero heat capacity.
Convection destroys a thermal gradient. In heating a pot of sauce, the liquid closest to the burner becomes the hottest. When stirred, the hot liquid mixes with cooler liquid, the heat becomes evenly distributed, and the temperature gradient is nullified.
If left unstirred, convective heat transfer will cause warm liquid to rise and cold to fall, and some circulation will occur, although it will not be as effective as active stirring. Over time, the conduction forces transferring heat from the bottom will establish equilibrium with the convective forces causing the water to circulate. If the heat source is low, circulation will be slow, a steep temperature gradient may exist, and the sauce may be burned on the bottom. If the heat is high, the sauce will boil, heat transfer by convection will be high, and the temperature gradient will be near zero.
Insulation is used to retard heat transfer by putting material with low heat conductivity next to the heat source. The insulation helps maintain the thermal gradient between the insulated object and ambient conditions. Coffee will stay warmer in a foam cup than in an aluminum cup because the foam conducts heat less readily. Likewise, the coffee drinker may burn a few fingers picking up the aluminum cup because the thermal gradient is near zero and the temperature of the outside of the cup is nearly the same as the inside of the cup.
To be stable, a thermal gradient must have a constant heat source and an available heat sink. Maintaining constant gradients is seldom important, except when conducting chemical reactions. Many industrial processes require careful heat control. The living cell also must maintain careful heat controls for optimum performance. While scientists understand how the human body as a whole maintains a temperature gradient between its core and the outside world, the options available to individual cells are less clear.