Most people will be aware that even their regional weather forecast does
not do justice to the whole of the area. The local climate reflects the
infl uence of the topography (hills and valleys), altitude and lakes and
seas that modifi es the general infl uence of the atmospheric conditions.
These are subject to the moderating influence of the body of water. Water has a large heat capacity compared with other materials and
this modifies the temperature of the surroundings.
The climate of the area is affected by altitude; there is a fall in
temperature with increase in height above sea level of
nearly 1°C for each 100 m. The frequency of snow is an
obvious manifestation of the effect. In the southwest of
England there are typically only 5 days of snow falling at
sea level each year whereas there are 8 days at 300 m. At
higher altitudes the effect is more dramatic; in Scotland
there are nearer 35 days per year at sea level, 38 days at
300 m, but 60 days at 600 m.
The colder conditions at higher altitudes have a direct
effect on the growing season. On the southwest coast of
England there are nearly 365 growing days per year, but
this decreases by 9 days for each 30 m above sea level. In
northern England and Scotland there are only about 250
growing days which are reduced by 5 days per 30 m rise,
i.e. to just 200 days at 300 m (1000 feet) above sea level in
|Figure 2.9 Effect of aspect;
note the difference between the
north-facing slope (right) with
snow still lying after it has melted
on the south-facing slope (left).
The presence of slopes modifi es climate by its aspect and
its effect on air drainage. Aspect
is the combination of the
slope and the direction that it faces. North-facing slopes
offer plants less sunlight than a south-facing one. This
is dramatically illustrated when observing the snow on
opposite sides of an east– west valley (or roofs in a street),
when the north facing sides are left white long after the
snow has melted on the other side (see Figure 2.9); much more radiation is intercepted by the surface on the south facing slope.
Closer examination reveals considerable differences in the growth of the
plants in these situations and it is quite likely that different species grow
better in one situation compared with the other. Plants on such slopes
experience not only different levels of light and heat, but also different
water regimes; south-facing slopes can be less favourable for some plants
because they are too dry.
Cold air tends to fall, because it is denser than warm air, and collects at
the bottom of slopes such as in valleys. Frost pockets
occur where cold
air collects; plants in such areas are more likely to experience frosts than
those on similar land around them. This is why orchards, where blossom
is vulnerable to frost damage, are established on the slopes away from
the valley floor. Cold air can also collect in hollows on the way down
slopes. It can also develop as a result of barriers, such as walls and solid
fences, placed across the slope (see Figure 2.10).
|Figure 2.10 The creation of frost pockets : (a) natural
hollows on the sides of valleys. (b) effect of solid barriers
preventing the drainage of cold air.
Permeable barriers, such as trees making up shelterbelts, are less of
a problem as the cold air is able to leak through. Frost susceptible
plants grown where there is good air drainage
may well experience a
considerably longer growing season. Gardens on slopes can be modifi ed
to advantage by having a low-permeable hedge above (a woodland is
even better) and a very permeable one on the lower boundary.
The features of the immediate surroundings of the plant can further
modify the local climate to create the precise conditions experienced
by the plant. This is known as its microclimate
. The significant factors
that affect plants include proximity to a body of water or other heat
stores, shelter or exposure, shade, altitude, aspect and air drainage. The
modifications for improvement, such as barriers reducing the effect of
wind, or making worse, such as barriers causing frost pockets, can be
natural or artificial. The microclimate can vary over very small distances.
Gardeners will be familiar with the differences across their garden from
the cool, shady areas to the hot, sunny positions and the implications this
has in terms of the choice of plants and their management.
Growers improve the microclimate of plants when they establish
windbreaks, darken the soil, wrap tender plants in straw, etc. More
elaborate attempts involve the use of fleece, cold frames, cloches,
polytunnels, glasshouses and conservatories. Automatically controlled,
fully equipped greenhouses with irrigation, heating, ventilation fans,
supplementary lighting and carbon dioxide are extreme examples of an
attempt to create the ideal microclimate for plants.
are materials such as water and brickwork, which collect
heat energy and then release it to the immediate environment that would
otherwise experience more severe drops in temperature. Gardeners
can make good use of brick walls to extend the growing season and to
grow plants that would otherwise be vulnerable to low temperatures.
Water can also be used to prevent frost damage when sprayed on to fruit
trees. It protects the blossom because of its latent heat
; the energy that
has to be removed from the water at 0°C to turn it to ice. This effect is
considerable, and until the water on the surface has frozen, the plant
tissues below are protected from freezing.
|Figure 2.11 The effect of windbreaks : (a) solid barriers ?
tend to create eddies to windward and, more extensively,
to leeward, (b) a permeable barrier tends to filter the air
and reduce its speed without setting up eddies.
that reduces the effect of wind comes in many different
forms. Plants that are grown in groups, or stands, experience different
conditions from those that stand alone. As well as the self-sheltering
from the effect of wind, the grouped plants also tend to retain a
moister atmosphere, which can be an advantage but can also create
conditions conducive to pest and disease attack. Walls, fences,
hedges and the introduction of shelterbelts also moderate winds, but
there are some important differences in the effect they have. The
reduction in flow downwind depends on the height of the barrier
although there is a smaller but signifi cant effect on the windward side
(see Figure 2.11).
The diagram also shows how turbulence can be created in the lee of the
barrier, which can lead to plants being damaged by down forces. Solid
materials such as brick and wooden fences create the most turbulence.
In contrast, hedges and meshes fi lter the wind; the best effect comes
from those with equal gap to solid presented to the wind. However, the
introduction of a shelterbelt can bring problems if it holds back cold air
to create a frost pocket.
reduces the radiation that the plant and its surroundings receive.
This tends to produce a cooler, moister environment in which some
species thrive (see ecology). This should be taken into account
when selecting plants for different positions outside in gardens. The
grower will deliberately introduce shading on propagation units or
on greenhouses in summer to prevent plants being exposed to high
temperatures and to reduce water losses.
The horticulturist is always confronted with choices; plants can be
selected to fit the microclimate or attempts can be made to change the
microclimate to suit the plant that is desired.
Not only is there an interest in weather forecasting in order to plan
operations such as cultivations, planting, frost protection, etc., but also
for predicting pest and disease attacks, many of which are linked to
factors such as temperature and humidity. Examples of outbreaks and
methods of predicting them, such as critical periods
that are used to
predict potato blight.