Summer Freezing of Wells

During the course of a year, the temperature of the air fluctuates more than the temperature of the soil just at the ground surface. In addition to having a lesser seasonal variation, the ground normally has a higher mean annual temperature than the air. The reason is that snow cover insulates the ground and thereby inhibits heat loss from the soil in winter. During summer, when the snow is gone, the flow of energy from the air to the colder soil below is comparatively efficient, especially if there is not a moss layer.

During the year the soil temperature right at the ground surface typically varies by 35° to 90°F (20° to 50°C). The variation is least in wetter terrains. Below the ground surface, the seasonal variation in temperature diminishes rapidly with depth. Approximately every 20 feet the variation reduces by a factor of 10.

Consequently, at a depth of 20 feet the ground temperature fluctuate seasonally only a few degrees, and at 40 feet the fluctuation is only a few tenths of a degree. At 60 feet the variation is hard to measure.

Seasonal fluctuations in temperature propagate downward at a rate of about 5 feet per month. Thus, the coldest temperature at a depth of 10 feet may occur in March, 2 months after the month of coldest average air temperature. At 30 foot depth, the coldest temperature is reached in midsummer.

One result of the lag in ground temperature at depth can be the shock of having one's well freeze up any time during the year. This happens to wells driven through permafrost with temperature enough below freezing that the presence of the well is unable to maintain the soil around the well unfrozen. The wave of low temperature creeping down each year may be just enough to cause the soil around the pipe to freeze up.