AIM
The aim of agricultural meteorology practical is to expose the student to the meteorological instruments and how they function. It also provides the students with advanced techniques which will enable them to address environmental and agricultural problems facing our global community.
INTRODUCTION
Agricultural meteorology involves the integration of climatologically and meteorological data and techniques into agricultural problems such as crop production, soil moisture, moisture stress and migration of pests. That is to say, the relationship between weather and climate to agricultural processes and how they affect agricultural products.
EFFECT OF METEOROLOGY IN AGRICULTURE
Weather and climate is a resource and considered as basic input or resources in agricultural planning. Every plant process related with growth, development and yield of a crop is affected by weather.
Similarly, every farm operation such as ploughing, harrowing, land preparation, weeding, irrigation, manuring, spraying, dusting, harvesting, threshing, storage and transport of farm produce are affected by weather.
The effect of agricultural meteorology can be illustrated through the following few applications
1. Characterization of Agricultural Climate:- This helps in determining crop growing season, solar radiation, air temperature, precipitation, wind, humidity etc which are important climatic factors on which the growth, development and yield of a crop depends on. Agro-meteorology considers and assesses the suitability of these parameters in a given region of maximum crop production and economical benefits.
(2) Crop planning for stability in production:- To reduce risk of crop failure on climatic part, so as to get stabilized yields even under weather adversity, suitable crop / cropping patterns / contingent cropping planning can be selected by considering water requirements of crop, effective rainfall and available soil moisture.
(3) Crop management:- This involves various farm operation such as sowing, fertilizer application, plant protection, irrigation scheduling, harvesting etc can be carried out on the basis of specially tailored weather support. For this, the use of operational forecasts available from agro- meteorology advisories is made e.g
(a) Weeding, harrowing, mulching etc are undertaken during dry spells forecasted.
(b) Fertilizer application is advisable when rainfall is not heavy, wind speed is less than 30km/hr and soil moisture is between 30-80%.
(c) Spraying/dusting is undertaken when there is no rainfall, soil moisture is 90% and wind speed is 25km/hr.
(4) Crop monitoring:- To check crop health and growth performance of a crop, suitable meteorological tools such as crop growth models, water balance technique or remote sensing etc can be used.
(5) Crop modelling and yield-climate relationship:- suitable crop models devised for the purpose can provide information or predict the result about the growth and yield when the current and past weather data is used.
DISCUSSION
Measurements of weather elements or weather observations are carried out at locations known as the weather stations. In some institutions, metrological stations are established for the purpose of weather observation and measurement of weather elements. Four types of weather stations can be recognized depending on the number of weather elements measured, the frequency of measurement and the status of the observer (whether professional or amateur). The four types of weather stations are as follows:
Synoptic Stations:-These station are manned by full-time professional observers who maintain continuous weather watch and make hourly instrumental observations of the weather elements on which information is required for the compilation of the synoptic charts or weather used in weather forecasting.
Agricultural Stations:- These stations are manned by part-time observers making at least twice daily instrumental observation of the major weather elements. Here, evaporation, grass and soil temperatures, and solar radiation are usually measured in view of their obvious importance in agriculture.
Climatologically Stations:- These stations are manned by part-time observers making only once or twice daily instrumental observations of temperature, humidity, rainfall and wind.
Rainfall Stations:- These stations are manned by part-time observers who take daily reading of rainfalls only.
Stevenson Screen
This screen is found in a meteorological station. It is used in keeping some meteorological instruments such as the wet and dry bulb thermometer, minimum and maximum thermometer, measuring cylinder, magnet (in some cases) etc.
The Stevenson screen is built with a white wooden box which is raised at 1.2m (4 feet) above the ground supported by stilts. The roof of Stevenson screen is double layered because it consist of an outer cover which is zinc and the inner part made up of wood. This double layered roof is made to give room for air spaces and avoid direct sun rays. The sides of the screen are louver-like venetian blind (it has little open spaces). The purpose of the screen in a meteorological station is to exclude the intensity of the sun’s radiant heat on the instruments associated with it. The roof of the screen is 603mm (23 inches). It has a length of 419mm (161/2 inches) while the double louvered sides has a width of 254mm (10 inches).
In the meteorological station, certain meteorological instruments are found. Such includes
– Minimum and maximum thermometer
– Dry and wet bulb thermometer
– Rain gauge
– Wind vane and anemometer
Minimum thermometer:- It is used to measure the minimum temperature of a particular area when temperature of the area is at its minimum level. It is usually taken very early in the morning between 1 a.m – 3 a.m. The thermometer is made up of a glass tube containing alcohol and an indicator. The principle of its function is that alcohol contracts when temperature is low/drops. With this principle, we can determine the minimum temperature of a place. When recording the minimum temperature, it is recorded in the column for the current day. It is calibrated to have a 0.5 difference. Its S.I unit is in oC or oF.
Maximum Thermometer:- It is used to measure the maximum temperature of the day in a locality/area. It is made up of a glass tube which has mercury and an indicator. When the temperature of the day rises, the mercury expands. The principle of its expansion is used in determining the maximum temperature for the day and can be reset with a magnet. It is determined during the day between 2p.m-5p.m. When recording the maximum temperature, it is recorded at the column for previous day. It is calibrated to have a 0.5 difference. Its S.I. unit is in oC or oF.
Dry bulb thermometer:- This works hand-in-hand with the maximum thermometer as it gives the current temperature of the day. It cannot read higher than maximum thermometer. It also has mercury indicator inside. It is calibrated to take a reading on how dry the temperature is. It is raised at 4 feet above sea level when measuring. Its S. I unit is in oC or oF.
Wet bulb thermometer:- This is used to measure how moist the air is (presence of water vapour). It is the same as the dry bulb thermometer just that it has a wick connected to a container filled with distilled water. The water flows from the container through the wick to the thermometer to cool it. As water evaporates from the wet bulb, it absorbs heat from the thermometer, driving down its temperature reading. Its S.I unit is in oC or oF.
Rain Gauge:- This is used to measure rainfall and other forms of precipitation. It consists of a copper cylinder with a metal funnel either 13cm or 20cm (5 or 8 inches) in diameter which leads into a small copper container or a glass bottle. The hole in the funnel that leads down to the container is very small so that evaporation of the collected rain is minimized. The rain gauge should be at least a third of a meter (one foot) above the ground and firmly fastened to avoid splashing. Rain water is measured using a graduated cylinder with a 3.8cm (11/2 inch) in diameter. The reading is done at eye-level and to an accuracy of 0.25mm (0.01 inch). If a greater accuracy is needed, a special kind of taper measure is required. The accuracy is up to 0.125 (0.005) inch). The rain gauge must be examined everyday.
Wind Vane:- This is used to measure the direction of wind in an area. It consist of a pole raised to a height of about 2.5m. It has the four cardinal points (North, South, East and West) with an indictor that shows where the wind is coming from. The indicator has a knob on a side and a blade on the other side. In determining wind direction, it is read at the region North, South, East, and West.
Anemometer:- This is used in measuring the speed of wind. Like the cup anemometer, it rotates on a pole depending on the strength of the wind. It is fixed at the top of a high pole and it rotates freely. Its S. I unit is in m/s.
OBSERVATION
It is good to note that a metrological station should be sited well away from tall buildings, high trees and other objects which would shelter it so as to avoid obstructions.
During the practical lectures, some readings were taken so as to let the students understand their aim more. The reading includes
Minimum temperature = 16.8 oC
Maximum temperature = 33oC
Dry bulb thermometer = 32.8oC
Just as the S.I units of temperature can be in oC or oF, so, it can also be converted to either ways thus:
oF = (180 x oC) + 32 or 9/5oC + 32
100
oC = (oF – 32) x 100 or 5/9 (oF – 32)
180
The absolute humidity of is said to be the actual amount of water vapour present in the air. While the relative humidity is the ratio of the actual amount of water vapour present in the air to the total amount the air can hold at a given temperature. Therefore, the difference between the wet bulb and dry bulb thermometers is used in the calculation of relative humidity.
It was also pointed out that the environment will be totally saturated when the dry bulb and wet bulb thermometers reads the same value of temperature.
It wad said that 25mm (1 inch) of rainfall means that the amount of water that would cover the ground to a depth of 25mm is the same provided that none evaporated, drained off, or percolated away. It was also noted that a rain day is reckoned at a period of 24hours with a least 0.25mm or more. If the amount exceeds 1mm (0.04 inch), it is considered a wet day and it is only torrential downpour that can amount to more than 25mm of rainfall in a day.
During the practical lecture, the wind vane gave a result of North-west and an assignment was also given by the instructor, that the students should check the reading of the wind vane by 12 noon the next day. The assignment was carried out as it was exactly 12 noon the next day, the wine vane gave a result of south-east.
CONCLUSION
Just as an adage says “experience is the best teacher”, that is exactly what the practical lecture did because it impacted much information to the students who have not been opportuned to witness a meteorological station. Also, with this knowledge, the students will be able to know how to handle their agricultural practises in relation to weather elements.