The C

EXAMPLE

Figure 1 shows the typical diurnal course of C
n 2 measured with the LAS for a sunny day with some scattered clouds. The two transition periods at sunrise and sunset are clearly noticeable by the distinct drops of C
n 2 . During these transition periods the atmospheric stability changes from unstable (day-time period) to stable (night-time period) or vice versa.

ALIGNMENT

It is important that the transmitter and receiver have stable, secure mountings and that the transmitted infrared beam is optimally aligned with the receiver. To simplify the procedure the LAS and X-LAS are supplied with integrated pan and tilt mechanisms and pre-aligned sighting telescopes which can be removed after installation. The transmitter power is adjustable for different path lengths and the receiver incorporates a signal strength meter for fine tuning. The controls are visible through windows in the rear covers.
n 2 data obtained with the LAS can be used to derive the surface flux of sensible heat. The fluxes derived from the C
n 2 data shown in Figure 1 are plotted in Figure 2 together with net radiation measurements. The sensible heat fluxes were calculated by the WINLAS software using additional air temperature and wind speed data. Again it can be clearly seen that during the day clouds pass the measurement site (net radiation drops). Once the net radiation and soil heat flux densities are known the evaporation rate can be derived indirectly from the LAS data. Finally, the daily average evaporation can be estimated.