Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 23)

Section 6. Installation
6.1 Location and Orientation
The PWS100 measures environmental variables and is designed to be located
in harsh weather conditions. However there are a few considerations to take
into account if accurate and representative data from a site are to be obtained.
The descriptions in this section are not exhaustive. Please refer to
meteorological publications for further information on the
locating of weather instruments. NOTE
The PWS100 should be sited in a position representative of local weather
conditions and not of a specific microclimate (unless the analysis of
microclimate weather is being sought).
To give non-microclimatic measurements the PWS100 should be sited away
from possible physical obstructions that could affect the fall of precipitation.
The PWS100 should also be positioned away from sources of heat, electrical
interference and in such a position as to not have direct light on the sensor
lenses.
Whenever possible, the PWS100 should be located away from windbreaks.
Several zones have been identified upwind and downwind of a windbreak in
which the airflow is unrepresentative of the general speed and direction. Eddies
are generated in the lee of the windbreak and air is displaced upwind of it. The
height and depth of these affected zones varies with the height and to some
extent the density of the obstacle.
Generally, a structure disturbs the airflow in an upwind direction for a distance
of about twice the height of the structure, and in a downwind direction for a
distance of about six times the height. The airflow is also affected to a vertical
distance of about twice the height of the structure. Ideally, therefore, the
PWS100 should be located outside this zone of influence in order to obtain
representative values for the region (see Figure 6-1).
FIGURE 6-1. Effect of structure on air flow
6-1