Temperature
Temperature (C)
FIGURE 2.1. Hypothetical profiles of maximum and minimum temperature above
and below soil surface on a clear, calm day.
The fact that the temperature maximum occurs after the time of maximum solar energy input is sigtllficant. This type of lag is typical of any
system with storage and resistance to flow. Temperatures measured close
to the exchange surface have less time lag and a larger amplitude than
those farther from the surface. The principles involved can be illustrated
by considering heat losses to a cold tile floor when you place your bare
foot on it. The floor feels coldest (maximum heat flux to the floor) when
your foot just comes in contact with it, but the floor reaches maximum
temperature at a later time when heat flux is much lower.
The amplitude of the diurnal temperature wave becomes smaller with
increasing distance from the exchange surface. For the soil, this is because
heat is stored in each succeeding layer so less heat is passed on to the
next layer. At depths greater than 50 cm or so, the diurnal temperature
fluctuation in the soil is hardly measurable (Fig. 2.1).
The diurnal temperature wave penetrates much farther in the atmosphere than in the soil because heat transfer in the atmosphere is by eddy