Critical thermal gradients for the onset of convection in small‐diameter wells are computed by means of an expression given by Krige (1939) and are shown graphically for water of several salinities and for glycerol. A series of field and laboratory tests in wells with diameters of 4.8 cm and 10.2 cm showed that water columns are unstable at temperature gradients as low as 0.003 °C/m. Magnitudes of thermal oscillations at higher gradients were as large as 0.49 °C, and apparent sizes of convection cells ranged up to 210 cm in height in a 10.2 cm cylinder and to 48 cm in wells 4.8 cm in diameter. Comparison of temperatures in adjacent wells showed temperature differences as great as 3 °C when thermal gradients were greater than the theoretical critical gradients. It is concluded that temperatures in the upper zone of deep wells may depart widely from ambient temperatures in the formations penetrated.