WEATHER ELEMENTS- AN OVERVIEW

WEATHER ELEMENTS- AN OVERVIEW

Introduction

Weather is state or condition of lower layer of atmosphere and it pertains to smaller area and for smaller duration of time.

Success and failure of crops is intimately related to the weather during the crop periods. Weather has significant influence on every phase of agricultural activities; from preparatory tillage to harvesting and storage. lets see elements of weather.

Weather Elements

1. Solar Radiation
2. Air temperature
3. Atmospheric pressure
4. Wind movement
5. Relative humidity
6. Precipitation
7. Clouds

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Solar Radiation

• It is the source of energy for all the physical processes taking place in the atmosphere. It also drives the process of photosynthesis, evaporation, heating the soil and air.
• Solar constant, i.e., 1.94cal/cm.square/min is emitted by sun continuously. Solar radiation changes only when enters the atmosphere as by absorption, reflection by clouds and scattering by dust particles.
• Solar radiation is received in the form of electromagnetic waves. It consists of flow of particles called quanta or photons.
• Global radiation is sum of direct solar radiation and diffused solar radiation. Diffused solar radiation is the radiation scattered by the suspended particles and depends on latitude, cloudiness and seasons. Before sunrise and sunset all the energy received is scattered radiation. Diffused solar radiation penetrates into plant canopy more effectively than direct solar radiation.
• Spectral distribution– In direct solar radiation 42% is photosynthetically active radiation (PAR) while 65% in diffused radiation.
• Albedo- It is solar radiation that is reflected without any change in its quality. Such radiation is very important in remote sensing studies. Rocks, soil, sand and vegetation reflects 10-30%, whereas cloud and snow are very effective reflector. Water surface and sea are very poor reflectors.

Temperature

• It is degree of hotness or coldness of a substance and measured by thermometer.
• Conduction, Convection and Radiation- Heat flow between and within substances takes place by these three processes.
  Transfer of heat by molecular activity is called conduction. As the first molecule is heated it speeded up and transferred to adjacent molecules. Heat transfer within liquid and gases resulting from the motion of fluids is called convection. It is much faster than conduction.
  Radiation is the transfer of energy by electromagnetic waves moving at the speed of light. Radiation does not require medium for heat transfer.
• Air temperature- During average cloudiness, the absorption of solar radiation by earth’s surface is about 43% and by atmosphere it is about 22%, whereas 35% is reflected back. solar radiation which is absorbed either by the earth surface or atmosphere is converted into thermal energy. The heat energy from the earth is transferred to the air above by the process of conduction, convection and radiation.
  The temperature variation between day and night is called Diurnal variation of temperature.
  Isotherms are lines connecting points of equal temperature. Temperature decreases gradually from the equator to pole wards.
  Temperature decreases as altitudes increases. The decrease in temperature in higher altitudes in the air is known as Vertical temperature gradient. It is due to air heated by the earth, reduction in water vapor content that reduce the capacity to hold heat and, and expansion of air at higher elevations.
  Lapse rate, Adiabatic lapse rate and Inversion- Vertical temperature decrease is expressed as lapse rate and the normal lapse rate is 6.5⁰C/km. the rate at which the temperature changes as air rises or falls is called Adiabatic lapse rate. It is constant for dry air and dry adiabatic lapse rate is 10⁰C/km. The condition of abrupt rise instead of fall in temperature is called Inversion.

• Soil temperature- heat transfer into the soil is mainly by conduction and to a lesser extent by convection. The heat flow into or out of the soil is called soil heat flux. The movement of temperature into the soil called thermal conductivity depends on texture, moisture and organic matter of the soil. Thermal conductivity is high for sandy soil as compared to clay soils. As soil moisture content increases thermal conductivity increases while soil organic matter lowers the thermal conductivity. Thermal conductivity is decreases as increase in porosity. Soil porosity is reduced due to presence of water and thus increases thermal conductivity. When soil is ploughed its porosity increases and thus thermal conductivity reduces. Irrigation and ploughing significantly influence the heat flow into the soil. Thermograph is a mechanical device to record air temperature continuously.

Atmospheric Pressure

• It is the pressure exerted by the atmosphere on the surface of the earth. Generally, it is low in areas of high temperature and is high in areas of low temperature. In the lower layer of atmosphere, the atmospheric pressure is more due to weight of all other upper layers rest on it. With increase in elevation, the atmospheric pressure decreases.
• Atmospheric pressure does not have direct influence on crop growth. Atmospheric pressure is measured by Aneroid barometer and Barograph and expressed in millibars.

Wind & Humidity

• Wind is air in horizontal motion which travels from a high-pressure area to a low-pressure area. Windwards refers to the direction from which the wind comes and leeward refers to the direction towards which it blows. The speed with which wind is flowing is called wind velocity. Wind velocity is measured by Anemometer whereas, wind direction is measured by wind vane.
• Humidity refers to the water vapor content of the atmosphere. The amount of water vapor in the atmosphere depends on wind and temperature. Higher the temperature more is the amount of water vapor that can be held by the atmosphere. The air is said to be saturated when it holds maximum amount of water vapor at a particular temperature. If the temperature rises the atmosphere becomes unsaturated as it can accommodate some more water vapor.
• The quantity of water vapor held in the atmosphere at any particular time can be expressed as-
  Absolute humidity: is the actual quantity of water vapor by weight present in a given volume of air. It is the ratio of weight of water vapor to the volume of air.
  Specific humidity: is the weight of water vapor per unit weight of air including water vapor.
  Relative humidity (RH): is the ratio between the amount of water vapor present in the air and the amount of water vapor required for saturation at a particular temperature and pressure. RH is expressed as percentage or ratio.

When RH is 80%, it means that there is deficit of 20% water vapor for saturation. If RH is low or deficit is more there is possibility of more evaporation and transpiration. That’s why RH is widely used in agriculture compared to absolute and specific humidity.
RH is measure by Psychrometer or Hygrometer.

Precipitation and Clouds

• Precipitation refers to falling of any type of condensed moisture to the ground surface. Rainfall is precipitation in the form of liquid drops larger than 0.5mm in diameter falling on the earth. Ordinary rain drop size varies from 0.5 to 4.0mm in diameter. Rainfall is measured by Rain gauge. Other form of precipitation are drizzle, mist, glaze, rime, snow, sleet and hail.
• Cloud is an aggregate of minute drops water suspended in the air at higher altitudes. Evaporation supplies water vapor to the air. As the mass of air goes up or rise upwards due to increasing temperature, it extends due to low pressure and cools. If the cooling proceeds up to saturation, water vapor condenses and cloud formation takes place. There are three basic clouds form– cirrus (feathery or fibrous), stratus (in layers or stratified) and cumulus (in heaps).

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