Environmental Engineering Reference
In-Depth Information
snow produces only about 10 mm of water. Where snow is formed in very cold, dry air
the moisture equivalent is even smaller, and it may take as much as a metre of snow to
produce 10 mm of water. In high mountain and polar regions where temperatures are low
throughout the year, the majority of precipitation falls as snow. Even so, because of the
low temperatures preventing the atmosphere holding much moisture, many of these areas
are, in fact, quite arid. There are no adequate records to provide accurate values, but it
seems likely that on a world basis no more than 1 per cent of the total annual
precipitation occurs as snow.
Table 5.2 Water input during a snowstorm in
California
Intensity (mm hr
-1
)
Period
Duration (hr)
Depth (mm)
Heaviest clock hour
1
5
5
Heaviest three-hour period
3
11
3·7
Heaviest six-hour period
6
17
2·8
Heaviest twelve-hour period
12
26
2·2
Heaviest twenty-four-hour period
24
38
1·6
Total storm
42
49
1·2
Source
: After Miller (1977).
Snowfall usually starts in the atmosphere as tiny ice crystals produced at temperatures
well below freezing. As the crystals fall they tend to aggregate, particularly where there is
sufficient moisture in the air to bind the crystals together. This mainly occurs where
temperatures are close to freezing point, and in these conditions large snowflakes may be
formed (Plate 5.1). At lower temperatures, moisture is lacking and the crystals do not
aggregate (Figure 5.1).
As with rain, the fall velocity of the snowflakes depends on size and, all else being
equal, large flakes fall more rapidly than small ones, with maximum speeds of about
ACID RAIN
human impact
Climatology has a number of misleading terms. We have already mentioned that the
greenhouse effect
of the atmosphere should really be called the 'enhanced greenhouse
effect' because it supplements the natural processes operating. Similarly the term
acid
rain
is used to indicate precipitation which is more acidic than normal - but even pure
rainfall is acidic, with a pH of about 5·5. Neutral water would have a pH of 7.
