Environmental Engineering Reference
In-Depth Information
If temperatures at the ground are below freezing point, the drifting cloud droplets
freeze on the vegetation to form rime. Although this may occasionally produce
Plate 5.3
Rime accumulation in the Westerwald, Germany.
The direction of air movement can be determined, as rime
will grow into the wind when supercooled droplets freeze on
contact with the frozen surface. Photo: Peter Smithson.
spectacular scenes (Plate 5.3), its hydrological and vegetational impact is small.
MEASUREMENT OF PRECIPITATION
RAIN
It is easy to measure rainfall. Any watertight container sited well away from buildings
and trees will act as a rain gauge. How much it collects will depend not only upon the
amount of rain and the strength of the wind, but also on the gauge diameter and its height
above the ground (Table 5.3). Because of this, rain gauges in the United Kingdom have a
standard diameter of 12ยท7 cm and are set a fixed distance of 30 cm above the ground.
Unfortunately the standard varies from country to country, so that in Canada the diameter
is 9 cm at a height of 30 cm above the ground, while in the United States the gauges are
20 cm wide and 78 cm high (Figure 5.2). Comparisons of rainfall totals between
countries are therefore more difficult than might be expected.
