On most nights & particularly so when skies are clear & there is little or no wind, the lowest temperatures are found on the ground. At weather stations, the ground minimum temperature is measured by a thermometer which just touches the grass tips (as shown)
The ground itself loses heat to a greater degree than the air above it. Last night, this site had a ground minimum temperature of minus 12.8, whereas the minimum temperature in the Stevenson screen was minus 9.6. Occasionally, I have recorded a difference of as much as 7 degrees between the two.