• Relative humidity is the ratio of amount of water vapour present in the air and the amount of water vapour required for saturation at a particular temperature and pressure.
Relative humidity (RH) = Water vapour present in the air/ Water vapour required for saturation × 100
• Relative humidity is expressed as Percentage or ratio.
• Relative humidity is widely used in agriculture compared to absolute and specific humidity.
• RH 90% means still 10% water vapour is required for saturation.
• If RH is low, evaporation and transpiration will be high.
• Vapour pressure deficit (VPD) is the difference between saturation vapour pressure and actual vapour pressure.
• Vapour pressure deficit is expressed in bars or pascals.
• When VPD is around or less than 1.5kpa air is said to be humid.
• When vapour pressure deficit is more than 2.5 kpa then air is said to be dry.
• Relative humidity is measured by hygrometers or psychrometers.
• Asmann’s psychrometer is used to measure relative humidity in crop canopy.
• Hair hygrometer is used to measure relative humidity inside rooms.
• Hygrograph is used to measure humidity continuously.
• Crescograph is used for continuous recording of plant growth.
• Evaporation is measured by evaporimeter/atmometer/ atmidometer.
• Evaporation is measured by using United States Weather Bureau (USWB) open pan evaporimeter, sunken screen evaporimeter, can evaporimeter and portable evaporimeter.
• USWB open pan evaporimeter is most widely used for measuring evaporation.
• Evaporation is expressed as mm/day.
• USWB open pan evaporimeter consists of cylindrical vessel of 25 cm height and 122 cm diameter.
• Portable evaporimeter is used to measure evaporation for very short period.
• Piche atmometer is a porous paper wick atmometer for measuring evaporative rate of water.
• Lysimeter is used to measure evapo-transpiration.
• Falling of any type of condensed moisture to ground surface is called precipitation.
• Rainfall is precipitation in the form of liquid drops larger than 0.5mm in diameter.
• Size of ordinary rain drop varies from 0.5 to 4mm diameter.
• Rainfall is measured by rain gauge or pluviograph.
• Automatic rain gauges are used to measure rainfall continuously.
• Drizzle is rain of very light intensity composed of fine droplets less than 0.5 mm in diameter barely reaching the ground.
• In mist water droplets evaporate completely before reaching the ground.
• Glaze or freezing rain is formed when rain falls on any material on ground having subfreezing temperatures and freezes into sheet or coating of ice.
• Rime is freezing fog. Thick, frostly deposit is formed when objects with subfreezing temperatures encounter fog.
• Snow is formed by sublimation of water vapour at subfreezing temperatures.
• Snow is solid precipitation in the form of ice crystals or flakes.
• When temperature of air falls below a certain limit before any water vapour is condensed, water vapour passes directly from vapour state to solid state.
• Sleet is also solid precipitation in the form of small particles of clear ice which are originally formed as rain drops and later frozen as they fall through a layer of cold air.
• Hail consists of hard rounded pellets of ice and compact snow.
• Hail is similar to sleet but larger in size.
• Hails falls from cumulo nimbus clouds along with thunder storms.
• In Hail formation rain water drops moves upward instead of falling due to strong vertical air currents, and due to freezing temperatures at higher altitude water becomes ice and falls to ground.
• Clouds are classified based on height, shape, colour, transmission or reflection of light.
• Basic cloud forms are
1. Cirrus (feathery or fibrous)
2. Stratus (stratified or in layers)
3. Cumulus (in heaps)
• If a basic cloud form occurs above its normal height of 1950m, then the cloud will be thin and the word ‘alto’ should be prefixed to its form.
• If any cloud is associated with rain, then the word ‘nimbus’ is prefixed or suffixed to the basic form.
• According to 1956 International Cloud Atlas of world meteorological Department, clouds are classified into 10 characteristic forms.
• Cloud cover is measured in Okta units.
• Clouds which are present at highest height are cirrus clouds.
• Height of cirrus cloud – 5 to 13 km
• Cirrus clouds appear as bright red or orange before sunrise and after sunset.
• Cirrus clouds comprise of thin crystals or needles of ice, not droplets of water.
• Cirro-cumulus clouds are often arranged in bands or fused into waves or ripples resembling those of sand on sea shore.
• Cirro stratus clouds are responsible for hallows and also milky white appearance of sky.
• Alto-stratus clouds give good amount of rainfall in middle and high latitudes.
• Stratus clouds have no particular structure or form.
• Nimbo-stratus clouds are associated with steady precipitation in the form of rain or snow.
• Cumulous clouds are flat based, with pronounced vertical thickness and extends upwards as dome or cauliflower shaped.
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