Environmental Engineering Reference
In-Depth Information
Plate 5.2
Effects of windblow through a hedge following a
heavy fall of snow. The snow accumulates where air
movement is least and is blown away in windier locations.
Photo: Peter Smithson.
100 cm s
−1
. As we all know, however, snowflakes vary considerably in shape and that too
may influence fall speeds. Moreover the density of snowflakes is very low; large flakes
often have a density of as little as 100 kg m
−3
(compared with approximately 1000 kg m
−3
for raindrops). Consequently, for their size, snowflakes are light and they are readily
blown by the wind. For this reason the distribution of snowfall during a storm is greatly
influenced by surface wind conditions, and even after reaching the ground the snow may
be redistributed to form deep drifts and snow-free areas (Plate 5.2).
Another important feature of moisture inputs in the form of snow is that it is often
many weeks or, in polar areas, even years before the water is actually released. Thus in
mountain areas, winter snowfall may survive into the spring and so represents a
temporary store of water which is released only by melting. In Greenland and Antarctica
snow accumulates for centuries, moving with imperceptible slowness in the ice sheets
and glaciers before melting, perhaps thousands of years later. Unlike rainfall, therefore,
snow is not always an immediate input to the hydrological system.
HAIL
The word 'hail' can strike fear into the heart of farmers in many parts of the world.
Damage to crops can be severe, though normally the devastation is local. Hailstorms
usually produce a swath of stones as the parent cloud moves across the country. Because
