Local climate

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.

Coastal areas
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.

Altitude
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 northern England.

Topography
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.

Air drainage
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).

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.

Microclimate
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.

Heat stores 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.

Shelter 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.

Shade 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.

Plant selection
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.

Forecasting
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.