The conclusions of the full-scale driving rain gauge comparison
test are listed here (and were partially presented in [Högberg et al. 1999]):
- Driving rain intensities on façade positions P4 and
P5 are considered to be identical (figures
4.6b). Temporal fluctuations
between these positions were not investigated, because the test
did not include measurements with identical driving rain
gauges at P4, P5 and P7, which are located adjacent
to each other. We suggest such an experiment for a further
- The monthly driving rain amounts of the CTH, DTU and TUE-II
gauges deviate within 30% from each other (table
4.1). Gauge TUE-I registers approximately half
of the monthly driving rain amount.
On much smaller time bases, such as 10-min intervals, gauge
responses can deviate significantly (figures
4.1-4.4). This applies
especially to small time bases of the used tipping-bucket
driving rain gauge (CTH): during a 10 min period it tips only
once at a driving rain intensity of 0.18 mm h. Therefore
we suggest that for short-time intervals one should apply a
continuous measuring principle instead of the tipping-bucket
- The effect of size and shape of the catchment
area cannot clearly be deduced from the experiments. A comparison
of the CTH gauge (0.032 m) and the TUE-I gauge (0.5
m) does not give a straightforward conclusion, because of
the difference in measuring principle of the collected rain flux,
as pointed out in the previous item. However, the results of
gauges TUE-I and TUE-II suggest that for large
catchment areas (e.g. 0.5 m) a wiper is necessary.
Another difficulty in interpreting the effect of the size of the
catchment area results from a comparison between the TUE-I,
TUE-Ib, TUE-II and CTH gauges. While the CTH and
TUE-II gauges (two very different catchment areas and
measurement principles) yield comparable results (at least over
long periods of time), the TUE-I and TUE-Ib gauges
(same measurement principle but very different catchment areas)
register about 47% and 32% of the driving rain amount of
the TUE-II gauge, respectively. One would expect that the
TUE-Ib gauge would suffer less from raindrop evaporation
than the TUE-I gauge because of its smaller catchment area,
and consequently measure higher driving rain intensities. An
explanation may be the fact that the drainage path length of the
TUE-Ib gauge is much larger than the CTH gauge.
- Teflon coating for a smooth, hydrophobic collector surface is on itself
not completely satisfactory. This has also been concluded from
laboratory tests at the CTH. Teflon gets dirty, like other
finishes (e.g. perspex). A wiper can serve to keep the
surface clean, and to improve coagulation and dripping-down of
- Readings of the DTU gauge (compared to the TUE-II readings)
are sensitive to driving rain intensity (figures
driving rain intensities are overestimated by the DTU gauge.
The scatter in the DTU/TUE-II correlations is larger than
in the TUE-I/TUE-II correlations.
This is probably due to the noise caused by the wind
acting on the freely suspended collector. The applied signal-processing
(see section 3.4.6)
was kept simple and can perhaps be improved.
- Given every aspect of the driving rain comparison test, one concludes
that the TUE-II gauge registers driving rain intensities well. It has
a good resolution for shorter time intervals (e.g. 10-min periods). Its
wiper keeps the surface clean and forces impinged raindrops to coagulate
and drip down (hence less evaporation). Moreover, it is not sensitive
The comparison test gives the following design rules for driving rain gauges:
- a large catchment area (0.5 m) is useful for a better
resolution for the water flux gauge,
- one should prevent drops from remaining stuck onto the collector surface.
A rotating wiper is a simple solution for this problem,
- the water flux gauge, which measures the water flux coming from the
collector, should be adapted to the desired time basis and the
catchment area of the collector. For a resolution of mm h in
min, a water flux gauge with a collector of
should be able to detect
When designing a driving rain gauge it is important to have a fairly
good estimate of . As an example of full-scale driving rain measurements,
one can consider the distribution of 5-min driving rain intensities measured at
the Main Building in the next chapter (figure 5.15).
Maximum values of at the Main Building are listed in
tables 5.5 and 5.6,
- a smooth surface (e.g. of perspex or teflon) will enable easy cleaning of the driving rain
gauge. A rotating wiper also helps in this respect.
Some problems were not solved by the full-scale experiments:
- the influence of shape and size of the collector on the
reading of the driving rain gauge is still unclear,
- the possibility of splashing and the effects of protruding rims
and other projections were not investigated,
- the influence of the raindrop spectrum on the readings of the gauges
was not investigated due to lack of sufficient data.
© 2002 Fabien J.R. van Mook
Published as issue 69 in the Bouwstenen series of the Faculty of Architecture, Planning and Building of the Eindhoven University of Technology.