Everyone notices the temperature of the air because the human skin is covered by sensory cells which are sensitive to heat and cold. These cells are very precise and human skin is sensitive to very small rises and falls in temperature.
Temperature controls the humidity (the amount of moisture or water vapor in the air) because hot air is often dry and cool air often damp. Temperature also controls the speed of photosynthesis of plants; every 10°C rise in air temperature doubles the rate of photosynthesis provided other factors such as moisture and nutrients are not limited. When temperatures reach above 25°C, the increase in photosynthesis slows down and above 35°C, the plant becomes too hot and the rate decreases rapidly. The temperature of the air affects the temperature of the soil because air passes into the crevices and pores in the soil.
MEASUREMENT OF TEMPERATURE
Temperature used to be measured in Fahrenheit degrees but since all measurements were metricised is now measured in degrees Celsius; commonly written °C. Water freezes at a temperature of 0°C and boils at 100°C. Normal temperatures are measured with a mercury thermo- meter and the maximum and minimum temperatures during a 24 hour period are measured using a special Maximum – Minimum thermometer. This type of thermometer is also used to record the temperatures in tobacco barns during curing.
Figures 1 and 2: Shows the inside and the outside of a basic Stevenson’s screen
Source: wicklowweather Source: weathershop
Figures 3 and 4: Shows a minimum and maximum thermometer and a normal thermometer.
Source: commons.wikimedia Source: klockit
The soil acts as a blanket for plant roots and young seedlings protecting them from sudden changes in air temperature. At a depth of 300mm in the soil day-to-day temperature variations which occur in the air, do not occur in the soil. The only variation is seasonal with the soil at this depth warming up in the summer and cooling down in the winter. At 150mm deep, slight daily variations in the temperature do occur but not as great as in the air. There is a delay between a fall or rise in air temperature and the fall or rise in the soil temperature at this depth. In temperate climates such as that of the United Kingdom, the soil temperature determines the length of the growing season for crops as seeds will only germinate above a certain temperature. Rye will germinate at 7°C, maize at 10°C and grass is stimulated into growth above 5.5°C.
The position of the land in relation to the sun, known as the aspect of the land, is important in farming. In the Northern Hemisphere, land facing south is lying towards the sun and will receive more heat than a North-facing slope. The opposite applies in the Southern Hemisphere.
Figure 5: Shows the effect of aspect on soil temperature and heat.
The amount of sunlight received is important to both plants and animals. The process of photosynthesis in plants takes place in sunlight and ceases during darkness. Plants are stimulated to produce their flowers and seeds according to the lengthening of daylight so that grasses, cereals and other plants seed, at about the same time each year. Animals are stimulated in the same way and birds begin to lay their eggs when the length of the day increases. Hens can be brought into lay by using electric light to lengthen the lighting time in the henhouse. In animals light affects the pituitary gland under the brain.
One factor which reduces the amount of light received by plants and animals is the formation of clouds, which diffuses the sunlight falling on the earth and reflects heat back to the sun and earth.
Figure 6: Shows how heat is trapped and reflected back to earth as well as the sun.
Figure 7: Shows all the different cloud formations.
The identification of cloud formation can help a farmer to forecast local weather conditions. Basically, there are two types of clouds. Cumulus clouds can be large or small but have a definite outline. Stratus clouds spread completely across the sky. Cumulus clouds produce rain in the form of thunderstorms which can bring heavy rain, but do not go on for too long.
Stratus Clouds build up into sheets, cover the whole sky and produce the prolonged rain which comes when the International Tropical Convergence Zone (I.T.C.Z.) is lying across the country.
- 2. FROST
The likelihood of frost is important because these days many crops are grown in winter, e.g. coffee, tea, wheat, barley, tobacco seedlings for early irrigated tobacco crops, etc. During winter days the air cools down quickly and draws heat out of the soil by a process called radiation. As hot air rises, the heat escapes upwards particularly on clear nights with no cloud. Frost occurs when the temperature falls to 0°C or below and the moisture in the air and on the surface of the ground freezes. An air frost occurs when the temperature falls to 0°C or just below and the moisture freezes on the surface of the soil or on the grass.
A ground frost occurs when the temperature falls to well below 0°C and the soil itself freezes. In parts of Russia the soil is completely frozen hard to a depth of 1 – 2 metres during the winter. During cloudy winter weather, frost is less likely to occur because the heat rising from the earth is reflected back by the cloud cover.
Figure 7: Shows the affect clouds have on keeping heat in thus effecting temperature.
Frost is more likely to occur under cloudless nights as the temperature drops below -0oC in the winter months.
FACTORS AFFECTING THE LIKELIHOOD OF FROST
South Africa gets very severe frosts in the winter. The north of Zimbabwe is less likely to get frost than the South whereas Zambia gets very little. Therefore the further north in latitude one goes the less likely it is to receive frosts.
The higher the ground is above sea level the more likely it is to get frost. Pretoria at 4 500 feet gets more frost than does the Lowveld at a little over 1 000 feet.
This is the topography of the ground, the hills and valleys. Cold air travels downhill and collects in valleys and dips in the ground causing what are called Frost Pockets.
Figure 8: Shows the flow of cold air down slopes at night causing pockets of cool air and then essentially frost pockets.
- Dryness of the air: Dry air will cause more frost than damp air and it is dryer in winter on the West side of the country than in the East. The Eastern Districts often have damp ‘guti’, weather in the winter, which reduce the risk of frost. Much of the Country’s tea and coffee is grown in the Eastern Districts.
Methods of frost protection can be divided into Long and Short Term.
LONG TERM METHODS
Long term methods are generally much cheaper than short term and include the following aspects when considering areas to plant to crops.
- Choosing an area which is frost free in which to grow your crop; this applies to tea, coffee and tobacco seedbeds and many other frost sensitive crops;
- Adjusting the planting dates and consequently the growing season to avoid frost hitting the crop when it is most susceptible which applies to wheat and barley;
- Soil husbandry, is affected by ploughing and mulching. Heat is available in the soil and will travel up to the surface faster or further down in the profile in a damp soil than in a dry soil because water is a better conductor of heat than air. Soils should be ploughed so that the moisture is conserved in the soil;
Figure 9: Shows the heat transfer through wet and dry soil profiles through the different seasons experienced.
Figure 10: Shows the differences in look of wet and dry soil, while Figure 11: shows a properly mulched soil to protect against heat and moisture loss.
Source: tankonyvtar Source: shaunsbackyard
- Laying mulch: e.g. a layer of straw or compost on the surface of the soil will keep the heat in the soil and prevent it escaping into the air;
- Planting quick-growing trees or bushes to act as a frost barrier which is often done at the coffee and tea plantations and;
- Plant breeding may produce new, hardy varieties which are less susceptible to frost. This is a very long-term programme.
SHORT TERM METHODS:
- Covering the plant or bush with sacks, straw or plastic sheeting to trap the heat;
- Lighting a large number of small fires or using smoke pots to raise the local temperature and produce a layer of smoke which acts like a cloud layer and prevents heat from escaping;
- Creating wind by means of fans to disperse the cold air; and
Figures 12 and 13: Shows plastic bags around the plants to protect them against cold and frost (left) and shows tomato plants that have been affected by the cold (right).
Source: greertoday Source: greertoday
Figure 14: Shows plants surviving in areas of extreme cold under plastic sheeting.
- Covering the crop with a layer of water by means of overhead irrigation sprinklers. Provided the water is applied before the start of the frost and the crop is covered completely, this method is very efficient. The plants are completely covered by a layer of water which freezes at 0°C, turning to ice and protecting the plant from further heat loss. This method is used for coffee trees. The secret is in the timing of the application of water and estimating when frost is likely to occur.
Figure 15: Shows the use of fans to disperse cold air on cold or frost sensitive crops.