Movement of heat and weather systems

Heat energy moves from warmer areas (i.e. those at a higher temperature) into cooler areas (i.e. those at a lower temperature) and there are three types of energy movement involved. Radiation energy moves efficiently through air (or a vacuum), but not through water or solids. Heat is transferred from the Earth's surface to the lower layers by conduction. As soil surfaces warm up in the spring, temperatures in the lower layers lag behind, but this is reversed in the autumn as the surface cools and heat is conducted upwards from the warmer lower layers. At about one metre down the soil temperature tends to be the same all the year round (about 10°C in lowland Britain).

Heat generated at the Earth's surface is also available for redistribution into the atmosphere. However, air is a poor conductor of heat (which explains its usefulness in materials used for insulation such as polystyrene foam, glass fibre and wool). It means that, initially, only the air immediately in contact with the warmed surface gains energy. Although the warming of the air layers above would occur only very slowly by conduction, it is the process of convection that warms the atmosphere above. As fluids are warmed they expand, take up more room and become lighter. Warmed air at the surface becomes less dense than that above, so air begins to circulate with the lighter air rising, and the cooler denser air falling to take its place; just as with a convector heater warming up a room. This circulation of air is referred to as wind.

In contrast, the water in seas and lakes is warmed at the surface making it less dense which tends to keep it near the surface. The lower layers gain heat very slowly by conduction and generally depend on gaining heat from the surface by turbulence. Large-scale water currents are created by the effect of tides and the winds blowing over them.

On a global scale, the differences in temperature at the Earth's surface lead to our major weather systems. Convection currents occur across the world in response to the position of the hotter and colder areas and the influence of the Earth's spin (the Coriolis Effect). These global air movements, known as the trade winds, set in motion the sea currents, follow the same path but are modified as they are deflected by the continental land masses (see Figure 2.5).

Weather and climate

Weather is the manifestation of the state of the atmosphere. Plant growth and horticultural operations are affected by weather; the influence of rain and sunshine are very familiar, but other factors such as frost, wind, and humidity have important effects. It is not surprising that growers usually have a keen interest in the weather and often seek to modify its effect on their plants. Whilst most people depend on public weather forecasting, some growers are prepared to pay for extra information and others believe in making their own forecasts, especially if their locality tends

Angle Incidence Earth

Figure 2.4 Effect of angle of incidence on heating at the Earth's surface. A higher proportion of the incoming radiation is reflected as the angle of incidence increases. Note also that a higher proportion of the incoming radiation is absorbed or reflected back as it travels longer through the atmosphere in the higher latitudes.

Figure 2.4 Effect of angle of incidence on heating at the Earth's surface. A higher proportion of the incoming radiation is reflected as the angle of incidence increases. Note also that a higher proportion of the incoming radiation is absorbed or reflected back as it travels longer through the atmosphere in the higher latitudes.

World Wind Circulation System
Figure 2.5 Global sea and wind movements. Warmer and colder water currents set in motion by the wind circulation around the world.

to have different weather from the rest of the forecast area. Weather forecasting is well covered in the literature and only its component parts are considered here.

Climate can be thought of as a description of the weather experienced by an area over a long period of time. More accurately, it is the long-term state of the atmosphere. Usually the descriptions apply to large areas dominated by atmosphere systems (global, countrywide or regional), but local climate reflects the influence of the topography (hills and valleys), altitude and large bodies of water (lakes and seas).

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