The thermometer should be read and reset at the same time each day. Contrary to popular belief, the air does not absorb heat directly from the sun. In fact, on a clear day, much of the sun's energy passes through the air unscathed. Upon reaching the surface, the sun's radiant energy is absorbed, providing energy to heat air and soil, and to evaporate water. Bare soil absorbs more heat than soil covered by grass or a ground cover.
Temperatures fluctuate widely in the air and soil region closest to the surface.
Temperatures at the grass surface are likely to be colder than the surface temperatures of dry soil because the grass and thatch insulate the surface from heat flowing upward from the soil at night. You can take advantage of this soil heating to protect fruit trees in bloom on a cold night. The soil absorbs the sun's energy during the day and radiates it back at night, increasing slightly the air temperature around the fruit tree. And a bare soil will radiate back more heat than a soil with a plant cover. Soil temperature plays an important role in how seeds germinate when planted.