ON THE PLOT
Frost: When, what and how to deal with it
W
hen, on average,
do you experience your first
frost? When do you expect
your last? And how variable
are occurrences of frost from
season to season on your
plot?
To predict when the frosts can
be expected, weather
scientists consider five factors:
K Altitude ~ in general the
higher the altitude the lower
the temperature and hence a K Latitude ~ the further
greater likelihood of frost
from the equator the lower
occurrence;
the temperatures;
K Distance from sea ~ on
the assumption that the
further a location is from the
moderating influence of the
ocean the more likely it is that
freezing temperatures would
be reached;
K A topographic valley index
~ a location surrounded by
higher altitudes experiences
already display relatively low
o
values (below 6 C), would
also be more prone to frost
formation.
Based on these factors and
using data from 216 climate
stations in southern Africa,
meteorologists inform us that
on average first frost occurs in
May over most of the interior
of South Africa.
In Gauteng 20% of the
province, on average, will
cold air drainage resulting
from radiative cooling on the
upper slopes, causing the
denser cold air to "sink" into
the valleys, with the intensity
of valley frost occurrence
depending on the steepness
of the surrounding topography;
K General minimum
temperature conditions ~ ie,
those areas which in terms of
daily minimum temperatures
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