Variations in the nature of the ground cover and slope angle and azimuth within 50-200 meters of a borehole (well) collar can cause variations in gradient in the upper part of the hole. Some examples that cause surface temperature changes are the Earth's average air temperature warming, woods being clear-cut or growing, and pavement laid around a well. Most often the effect is to change the gradients no more than 20- 30°C/km at 20-50 meters depth, and only less than 5°C/km at 100-150 meters depth. The effects are rarely larger than this. The extrapolated surface temperatures from deeper than 150 meters is usually within 2°C of actual surface temperatures.