Introduction

The application of meteorology to agriculture is essential since every facet of agricultural activity in an area depends on its weather. Agro-meteorological information is effectively utilized to increase the quality and quantity of agricultural production and reduce the production cost. This study examines the significance of the meteorological parameters in the implementation of agricultural practices involved in the production of specific crops.

Meteorological measurements for weather forecasting and climatology have been carried out on a regular basis for centuries. However, the data acquired can only be evaluated and interpreted after having statistically recorded medium-term and long-term atmospheric conditions.

Nowadays, transport and communications such as ground, air and sea traffic could not be maintained without these data, which are mainly being collected from measurements and observations in the atmosphere close to the ground (the Boundary Layer). The main meteorological parameters in this field are:

• Wind speed and direction
• Air temperature
• Air pressure
• Air humidity
• Precipitation
• Haze and contents of the air
• Solar and terrestrial radiation

Meteorological parameters in agriculture

Forecasting the size and quality of harvests, particularly of cereal crops in relation to what have been sown, has always been a challenge in front of the farmers. However there are many factors which directly or indirectly affect the final yield. Although the growth of any crop is not only dependent on weather parameters alone, yet they have a great impact on the final yield. The effect of climatic parameters like maximum and minimum temperatures, relative humidity, sunshine hours and rainfall on wheat yield has been studied in detail has shown that even though a number of meteorological parameters are vital for the growth and development of wheat crop in rain fed areas like Potohar, in Pakistan, all of them are not significant throughout the growth period.

It has been observed that at a particular time some meteorological parameters act as the main actor for the physical growth of wheat crop and the final yield in the end. At one stage or the other, one of the various meteorological parameters has a definite impact on the physical growth of the plant. It has shown that an increasing temperature may have a positive impact on agriculture in the mountain areas.Higher rainfall variance as the main factor behind dry-land yield fluctuations. The influence of atmospheric relative humidity on plant growth and how it interacts with salinity to influence salt tolerance and water relations of barley, wheat and sweet corn.

Temperature - is a physical property that underlies the common notions of hot and cold. Celsius scale is used for most temperature measuring purposes.

Relative humidity - The ratio of the vapor pressure to the saturation vapor pressure with respect to water. This quantity is alternatively defined by the World Meteorological Organization as the ratio of the mixing ratio to the saturation mixing ratio.

Water vapor mixing ratio - Mixing or humidity ratio is expressed as a ratio of water vapour mass  per kilogram of dry air, at a given pressure.

Precipitations - is any product of the condensation of atmospheric water vapor that is pulled down by gravity and deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel. It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water condenses, falling out of solution (i.e., precipitates). Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air.

Wind speed - the movement of air or other gases in an atmosphere. It is a scalar quantity, the magnitude of the vector of motion.

Wind direction - is the direction from which a wind originates. It is usually reported in cardinal directions or in azimuth degrees.

Data

Meteorological Data?

Daily meteorological data including temperature, relative humidity, evapo-transpiration, wind speed and rainfall for a period of 10 years from 2002-2011 is collected. The data recorded for the same period for two more stations of Riyadh (24° 43’ N, 46° 44’E) in the central region and Jeddah (21° 31’ N, 39° 13’E) in the western region are also studied , since these two regions along with Tabuk (28° 36’ N, 36° 63’E) come under a triangular geographical position. Daily Meteorological data is analysed to obtain the parameters of interest.

From the daily data, the monthly averages showing the seasonal variations for surface meteorological parameters like temperature (C°), relative humidity (%), wind speed (m/s) and precipitation (mm) and the annual mean indicating the yearly variations are calculated. For temperature and relative humidity, the temporal trends show significant spatial dependence.

These parameters are also significant for such issues as air pollution, avalanche warning, sun simulation, renewable energy industry, agriculture, forestry, water supply and distribution, town and regional planning. For example, the evaluation and interpretation of gas emission measurements is only possible in comparison with meteorological data acquired concurrently.

The structure of the atmosphere close to the ground is extremely important for the local climate. Knowing the solar radiation as well as the air humidity and air temperature is necessary to evaluate chemical reactions of pollutants in the air.

All meteorological parameters are subject to short-term variations, normally caused by turbulences within the atmosphere. All meteorological parameters are influenced by solar radiation, directly or indirectly, and this results in typical daily or yearly trends.

In order to be able to evaluate these typical trends it is necessary to compute the mean values from the actual ones measured over a specific period. For some meteorological parameters it is quite easy to understand their daily cycle. For example, the temperature cycle is normally a simple curve with a minimum value shortly after sunrise and a maximum in the early afternoon. The yearly cycle of a meteorological parameter can be determined by making daily measurements. The average yearly cycle within a climatic region is normally determined by making measurements over a minimum of 30 years.

Of course, meteorological measurements have to be made outside. This means that the sensors and associated electronics must be designed to withstand the local climatic conditions, which may be extreme, from deserts to Antarctica.

Within the atmosphere close to the ground the temporal and spatial characteristics of radiation values are influenced by the characteristics of the ground surface. The most influential factors affecting the received radiation at any particular location are:

• ·Location on the earth
• ·Date and time
• ·Precipitation (cloud, fog, rain, snow)
• ·Constriction of the horizon (field of view)
• ·Air pollution (aerosols and gasses)
• ·Albedo

Due to the physical effects mentioned above it is sometimes not sufficient for many application fields to just measure the ‘Global Radiation’ coming from all around the measurement location. It may also be necessary to measure the ‘Direct Radiation’ coming only from the sun and/or the ‘Diffuse Radiation’ (not coming directly from the sun). The ‘Radiation Balance’ of incoming to outgoing radiation in the short-wave and long-wave may also be required.

Before reaching the ground the solar radiation is influenced by our atmosphere and its physical characteristics and an essential parameter is the absorption in different wavelength ranges. Albedo is affected by surfaces with different reflection characteristics, such as water, ice, snow, stone, grass, crops or woodland.

The different wavelength ranges as well as the properties of the atmosphere and the ground surface must be taken into consideration and this makes it necessary to develop special sensors suitable for individual, very complex, measuring tasks.

In order to be able to develop the proper sensor, it is necessary beforehand to determine which meteorological value shall be measured and how it is defined.

In order to monitor and minimise air pollution, measurements complying with the requirements of national standards for the evaluation of emissions and air quality are also made. These are termed ‘environmental measurements’.

Usually these environmental measurements comprise the measurement of the global radiation, the direct radiation and the radiation balance. The data acquired from global radiation and direct radiation, together with specific spectral data help to determine the amount of gasses and aerosol particles in the air and also provide information on the photo-chemical formation of secondary impurities. Measuring the radiation balance provides information on the vertical exchange and spread of the pollution.

A further field of application for solar sensors is environmental simulation where the effect on materials is tested under ‘artificial suns’. In order to cut down the testing time, very high intensity artificial radiation sources are used that far exceed the maximum natural value of 1,367 W/m² (the Solar Constant). Sensors that are used for these purposes should have measuring ranges that allow for radiation levels in the range 2,000 to 4,000 W/m² and they are often required to work at high temperatures (> 100ºC).

Conclusions

The study is an initial attempt to understand the significance of meteorological parameters in implementing the different agricultural engineering practices and its influence on the productivity of different crops. A statistical analysis of the meteorological parameters like temperature and rainfall an increasing trend with little variation over time. However, the relative humidity was found to have a decreasing trend over the years. It was found that some of the meteorological parameters negatively affect the productivity of crops especially parameters like relative humidity and temperature. Air temperature is often taken into account while planning the crop but soil temperature is often overlooked as being compensated by an adjustment in water supply and is neglected after initial crop establishment.