Soil is stated as having an R-value of about 0.65-R per centimeter (0.08-R per 1 inch), or 0.25-R per 1 inch. Variations in R-value of soil may be attributed to different soil moisture levels, with lower R values as moisture level increases. The most superficial layer of earth typically is less dense and contains the root systems of many different plants, thereby acting more like thermal insulation, meaning, it reduces the rate of temperature flowing through it. Approximately 50% of the heat from the Sun is absorbed at the surface. Consequently, the temperature at the surface may vary considerably according to the day / night cycle, according to weather and particularly according to season. Underground, these temperature changes are blunted and delayed, termed thermal lag. The thermal properties of earth therefore mean that in Winter the temperature below the surface will be higher than the surface air temperature, and conversely in Summer the earth temperature will be lower than the surface air temperature. Indeed, at a deep enough point underground, the temperature remains constant year round, and this temperature is approximately the mean of Summer and Winter temperatures. Sources vary in their stated values for this deep earth constant temperature (also termed amplitude correction factor). Reported values include , , , for dry soil, and for wet soil. Below this level the temperature increases on average every due to heat rising from the interior of the Earth. Diurnal temperature changes between maximum and minimum temperatures can be modelled as a wave, as can seasonal temperature changes (see diagram). In architecture, the relationship between the maximum fluctuations of external temperature compared to internal temperature is termed amplitude dampening (or temperature amplitude factor). Phase shifting is the time take for the minimum external temperature to reach the interior. Partially covering a building with earth adds to the thermal mass of the structure. Combined with insulation, this results in both amplitude dampening and phase shifting. In other terms, earth sheltered structures receive both a degree of cooling in Summer and heating in Winter. This reduces need for other measures of heating and cooling, saving energy. A potential disadvantage of a thermally massive building in cooler climates is that after a prolonged period of cold, when the external temperature increases again, the structures internal temperature tends to lag behind and take longer to warm up (assuming no other form of heating). The reduction of