Soil scientists restrict the word 'soil' to the surface material which, over a long period of time, has come to have distinct layers or horizons. A soil has certain distinctive physical, chemical and biological qualities, which permit it to support plant growth and which differentiate it from the infertile substratum lying below. While soil is composed of poth mineral and organic particles, the underlying layers may usually be composed of mineral material.
Soil is a dynamic layer-a changing and developing body-as many complex chemical, physical and biological activities continue constantly in it Soils become adjusted to conditions of climate, landform and vegetation and change internally when the controlling conditions change.
The scientific study of the characteristics, development and distribution of soils is called pedology; the process of soil formation is known as pedogenesis.
Showing posts with label Wheather. Show all posts
Showing posts with label Wheather. Show all posts
Thursday, October 29, 2009
HYDROLOGICAL CYCLE AND GLOBAL WATER BALANCE
HYDROLOGICAL CYCLE AND GLOBAL WATER BALANCE
The cyclic movement of water between the atmosphere, the land and the sea is called the water cycle. It involves the movement of water in a great series of continuous interchanges 'of both geographical position and physical change. Water is released into the atmosphere as water vapour through evaporation from the oceans, rivers, and lakes, and through evapo-transpiration from plants and the ground surface. Oceans are the biggest reservoirs of free water, from which the evaporation totals about 455,000 cu km per year. Evaporation from soil, plants and water surfaces of the continents totals 62,000 cu km per year.
Within the atmosphere water vapour condenses to form clouds and is returned'to the land and to its water bodies as precipitation. This quantity of precipitation must be equal to the quantity of evaporation that takes place. Precipitation is unevenly distributed between lan~ and oceans. The amount of precipitation gained by the land is more than the evaporation that takes place from the land surface.
This excess quantity flows over or under the ground surface to reach the sea; collectively called runoff, this forms a continuous cycle. However, great inequalities exist in the global amounts of water stored in gaseous, liquid and solid states.
Water flowing exposed or ponded upon land is surface water; water occupying openings in the soil, overburden or bedrock is subsurface water.
The cyclic movement of water between the atmosphere, the land and the sea is called the water cycle. It involves the movement of water in a great series of continuous interchanges 'of both geographical position and physical change. Water is released into the atmosphere as water vapour through evaporation from the oceans, rivers, and lakes, and through evapo-transpiration from plants and the ground surface. Oceans are the biggest reservoirs of free water, from which the evaporation totals about 455,000 cu km per year. Evaporation from soil, plants and water surfaces of the continents totals 62,000 cu km per year.
Within the atmosphere water vapour condenses to form clouds and is returned'to the land and to its water bodies as precipitation. This quantity of precipitation must be equal to the quantity of evaporation that takes place. Precipitation is unevenly distributed between lan~ and oceans. The amount of precipitation gained by the land is more than the evaporation that takes place from the land surface.
This excess quantity flows over or under the ground surface to reach the sea; collectively called runoff, this forms a continuous cycle. However, great inequalities exist in the global amounts of water stored in gaseous, liquid and solid states.
Water flowing exposed or ponded upon land is surface water; water occupying openings in the soil, overburden or bedrock is subsurface water.
WORLD PATTERNS OF PRECIPITATION
WORLD PATTERNS OF PRECIPITATION The pattern of rainfall in the world is very complex and only a very broad zonal pattern can be detected. Equatodal areas have the most precipitation due to high temperatures and consequent large moisture holding capacities of air, and also the presence of large oceanic water surfaces to supply the moisture. Most of the rainfall is convectional. Conversely, polar areas receive scanty rainfall because of low air temperatures.
Middle latitudes, between these two extremes, have a complicated distribution where high rainfall patterns relate particularly to the westerlies in both the hemispheres and also to their cyclone tracks. Regions of the lowest rains coincide with regions of subsiding air because of high pressure conditions. This occurs mainly in the sub tropics and on the eastern sides of the oceans (or western margins of continents), e.g., the Sahara desert. Rainfall pattern is greatly affected by the mountain ranges; for example, the Rockies and the southern Andes, where the windward sides record high rainfall, while the leeward sides are marked by rain-shadow areas. Altitude also plays an important role on the local scale. There is a general increase in precipitation with height up to about 2 km, while beyond this, precipitation diminishes due to the coolness of air.
All places having the same rainfall are represented on a map by lines called isohyets.
Middle latitudes, between these two extremes, have a complicated distribution where high rainfall patterns relate particularly to the westerlies in both the hemispheres and also to their cyclone tracks. Regions of the lowest rains coincide with regions of subsiding air because of high pressure conditions. This occurs mainly in the sub tropics and on the eastern sides of the oceans (or western margins of continents), e.g., the Sahara desert. Rainfall pattern is greatly affected by the mountain ranges; for example, the Rockies and the southern Andes, where the windward sides record high rainfall, while the leeward sides are marked by rain-shadow areas. Altitude also plays an important role on the local scale. There is a general increase in precipitation with height up to about 2 km, while beyond this, precipitation diminishes due to the coolness of air.
All places having the same rainfall are represented on a map by lines called isohyets.
CONDITIONS FOR PRECIPITATION
CONDITIONS FOR PRECIPITATION There are three possible ways by which precipitation is produced.
(i) Convectional Precipitation is caused by heating of moist air in the lower layers of atmosphere which rises, expands, and is cooled adiabatically to its dew point. Towering cumulonimbus clouds may form. Convection rain is often accompanied by lightning and thunder. In tropical latitudes this type of rain is usually torrential. It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity, It is most common in equatorial regions and regions having a tropical monsoon climate.
(ii) Orographic means 'related to mountains'. The precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises on the windward side, it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results; when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rainshadow, may exist on the leeward side.
Several of the important dry deserts of earth are of this type. Orographic rainfall is most common where no-shore winds rise up over hilly mountain regions lying parallel to the coast, e.g., British Columbia (Canada) and Scotland. In USA, prevailing westerlies bring moist air from the Pacific Ocean over the coast ranges of central and northern California and the great Sierra Nevada Range, Heavy rainfall is experienced on the windward side, while on the leeward side, air descends and becomes dry down the eastern face of sierras. This produces part pf America's great desert zone. Much of orographic rainfall is actually of convectional type, in that it takes the form of heavy convectional showers and thunderstorms. In monsoon areas, this type greatly augments the normal monsoon rainfall e.g., at Cheerapunji, on the windward slopes of the Khasi hills.
(iii) Cyclonic Precipitation (depression or frontal) occurs when large masses of air of different temperatures meet. The warm moist air of one air mass moves over the cold heavier air of another. Or, it is caused by air rising through horizontal convergence in an area of low pressure. Cyclonic rain is common throughout the doldrums where the trade winds meet. It is the precipitation along the frontal surfaces of a depression in mid and high latitudes. In tropical cyclones, the rainfall is often very heavy, but lasts only for a few hours. In temperate cyclones, it is much lighter but lasts for many hours, even days.
Thunderstorm It is an intense local storm accompanied by lightning and thunder that develops in large cumulonimbus clouds, which result from rapid ascent of air under very unstable conditions. Such conditions occur either at the cold front of a depression or when the ground is intensely heated, and there must always be sufficient moisture in the air for cloud formation. The equatorial areas produce the highest frequency of thunderstorms a& the air is usually very humid and the hot sun produces the necessary up currents of air. Very heavy rainfall or hail occurs, accompanied by the discharge of electrical energy, producing lightning and thunder.
Based on the cause of initial lift of air column, resulting in a storm, a classification can be made:
(i) Thermal or air mass thunderstorm set off by thermal convection caused by solar heating of the ground and lower layers of air; time or occurrence is typically in late afternoon when air temperatures near the ground are the highest;
(ii) Orographic thunderstorms occur when the air is forced to rise
over a mountain range; the torrential monsoon rains of the Asiatic and East Indian mountain ranges are largely of this type; (iii) Frontal thunderstorm is caused when a layer of warm air is forced to rise over a layer of cold air.
(i) Convectional Precipitation is caused by heating of moist air in the lower layers of atmosphere which rises, expands, and is cooled adiabatically to its dew point. Towering cumulonimbus clouds may form. Convection rain is often accompanied by lightning and thunder. In tropical latitudes this type of rain is usually torrential. It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity, It is most common in equatorial regions and regions having a tropical monsoon climate.
(ii) Orographic means 'related to mountains'. The precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises on the windward side, it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results; when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rainshadow, may exist on the leeward side.
Several of the important dry deserts of earth are of this type. Orographic rainfall is most common where no-shore winds rise up over hilly mountain regions lying parallel to the coast, e.g., British Columbia (Canada) and Scotland. In USA, prevailing westerlies bring moist air from the Pacific Ocean over the coast ranges of central and northern California and the great Sierra Nevada Range, Heavy rainfall is experienced on the windward side, while on the leeward side, air descends and becomes dry down the eastern face of sierras. This produces part pf America's great desert zone. Much of orographic rainfall is actually of convectional type, in that it takes the form of heavy convectional showers and thunderstorms. In monsoon areas, this type greatly augments the normal monsoon rainfall e.g., at Cheerapunji, on the windward slopes of the Khasi hills.
(iii) Cyclonic Precipitation (depression or frontal) occurs when large masses of air of different temperatures meet. The warm moist air of one air mass moves over the cold heavier air of another. Or, it is caused by air rising through horizontal convergence in an area of low pressure. Cyclonic rain is common throughout the doldrums where the trade winds meet. It is the precipitation along the frontal surfaces of a depression in mid and high latitudes. In tropical cyclones, the rainfall is often very heavy, but lasts only for a few hours. In temperate cyclones, it is much lighter but lasts for many hours, even days.
Thunderstorm It is an intense local storm accompanied by lightning and thunder that develops in large cumulonimbus clouds, which result from rapid ascent of air under very unstable conditions. Such conditions occur either at the cold front of a depression or when the ground is intensely heated, and there must always be sufficient moisture in the air for cloud formation. The equatorial areas produce the highest frequency of thunderstorms a& the air is usually very humid and the hot sun produces the necessary up currents of air. Very heavy rainfall or hail occurs, accompanied by the discharge of electrical energy, producing lightning and thunder.
Based on the cause of initial lift of air column, resulting in a storm, a classification can be made:
(i) Thermal or air mass thunderstorm set off by thermal convection caused by solar heating of the ground and lower layers of air; time or occurrence is typically in late afternoon when air temperatures near the ground are the highest;
(ii) Orographic thunderstorms occur when the air is forced to rise
over a mountain range; the torrential monsoon rains of the Asiatic and East Indian mountain ranges are largely of this type; (iii) Frontal thunderstorm is caused when a layer of warm air is forced to rise over a layer of cold air.
PRECIPITATION
PRECIPITATION
Formation of water particles or ice within the cloud that fall towards earth's surface is precipitation. It occurs when condensation takes place rapidly within the cloud. Main types are rain, drizzle, sleet, snow and hail. Any of these may evaporate before reaching the ground surface, appearing as streamers below cloud base-a phenomenon known as virgo.
(i) RAIN Form of precipitation consisting of the drops of water formed by the coalescence of minute condensation droplets within the clouds. In strict terms, these drops have a diameter ranging from about 0.5 mm to 5.0 mm, although smaller drops can be called rain if they are widely scattered.
(ii) DRIZZLE Form of precipitation in which the water droplets are very fine (less than 0.5 mm) and are close together. Normally drizzle is produced by stratus and stratocumulus clouds.
(iii) SLEET Generally it implies. a form of precipitation consisting of either partly-melted snowflakes or rain and snow falling together. In USA, it implies a form of precipitation consisting of frozen raindrops that have subsequently eartially melted.
(iv) SNOW A form of precipitation consisting of crystals of ice. It is produced when condensation takes place at a temperature below freezing point, so that the minute
crystals (spicules) of ice form directly from the water vapour. These may fall as they are but more commonly they combine together to form snowflakes, which display an infinite variety of patterns.
(v) HAIL Precipitation in the form of ice-pellets (hail stones) that develop in and fall from cumulonimbus clouds, either at a cold front or where intense heating of surface causes rapidly-ascending convection currents. The hail stone develops in the updraught of air as water vapour freezes onto the surface of a nucleus embryo of ice in the cloud. When it has grown sufficiently, its weight overcomes the force of the updraught and it falls. Hail consists of rounded lumps of ice, having an internal structure of concentric layers much like an onion.
Formation of water particles or ice within the cloud that fall towards earth's surface is precipitation. It occurs when condensation takes place rapidly within the cloud. Main types are rain, drizzle, sleet, snow and hail. Any of these may evaporate before reaching the ground surface, appearing as streamers below cloud base-a phenomenon known as virgo.
(i) RAIN Form of precipitation consisting of the drops of water formed by the coalescence of minute condensation droplets within the clouds. In strict terms, these drops have a diameter ranging from about 0.5 mm to 5.0 mm, although smaller drops can be called rain if they are widely scattered.
(ii) DRIZZLE Form of precipitation in which the water droplets are very fine (less than 0.5 mm) and are close together. Normally drizzle is produced by stratus and stratocumulus clouds.
(iii) SLEET Generally it implies. a form of precipitation consisting of either partly-melted snowflakes or rain and snow falling together. In USA, it implies a form of precipitation consisting of frozen raindrops that have subsequently eartially melted.
(iv) SNOW A form of precipitation consisting of crystals of ice. It is produced when condensation takes place at a temperature below freezing point, so that the minute
crystals (spicules) of ice form directly from the water vapour. These may fall as they are but more commonly they combine together to form snowflakes, which display an infinite variety of patterns.
(v) HAIL Precipitation in the form of ice-pellets (hail stones) that develop in and fall from cumulonimbus clouds, either at a cold front or where intense heating of surface causes rapidly-ascending convection currents. The hail stone develops in the updraught of air as water vapour freezes onto the surface of a nucleus embryo of ice in the cloud. When it has grown sufficiently, its weight overcomes the force of the updraught and it falls. Hail consists of rounded lumps of ice, having an internal structure of concentric layers much like an onion.
CUMULUS CLOUDS
CUMULUS CLOUDS These are clouds of great vertical extent from 1500 to 9000 metres. They have a low base level (500-2000 m), the base being flat and cloud masses isolated, either rounded or towering, and with a clear outline, resembling the head of a cauliflower. When these clouds are sunlit, they are brilliantly white and are called 'wool clouds'. They occur mainly in summer and are caused by convection. Small cumulus clouds are associated with fair weather.
Cumulonimbus Under different weather conditions, a cumulus cloud may develop into cumulonimbus, the thunderstorm cloud mass of enormous size which brings heavy rainfall, thunder and lightning and gusty winds. It may extend from a height of 300-600 m at the base up to 9000 to 12000 m. When seen from a distance, the top of cumulonimbus may appear white but to observers below, the sky may be darkened to almost night-time blackness.
Cumulonimbus Under different weather conditions, a cumulus cloud may develop into cumulonimbus, the thunderstorm cloud mass of enormous size which brings heavy rainfall, thunder and lightning and gusty winds. It may extend from a height of 300-600 m at the base up to 9000 to 12000 m. When seen from a distance, the top of cumulonimbus may appear white but to observers below, the sky may be darkened to almost night-time blackness.
TYPES OF CLOUDS
Clouds are classified on the basis of appearance, form and altitude. On the basis of form, there are two major groups: (i) stratiform or layered types, and (ii) cumuliform or massive, globular !)'pes.
1. STRATIFORM CLOUDS are blanket-like, often covering vast areas but are fairly thin when compared to their horizontal dimensions. They are sub-divided on the basis of elevation.
(a) High Clouds: 6000 to 12000 metres above sea-level, e.g., (i) Cirrus Cloud, a wispy, fibrous looking cloud, uften indicating fair weather; (ii) Cirrocumulus, a thin cloud often globular and rippled (this is the mackerel sky of popular
description);
(iii) Cirrostratus looks lilse a thin white sheet producing a halo around the sun or moon.
(b) Medium Clouds: 2100 to 6000 metres above sea level.
(i) Altocumulus Clouds: A layer of individual cloud masses
fitted closely together in geometric pattern; the masses appear white or somewhat grey on shaded sides, and the blue sky can be seen between individual patches of rows
of these bumpy-looking clouds with a flattened base. They indicate fine weather. (ii) Altostratus: A banked layer, often smoothly distributed over the entire sky; greyish in appearance, they usually have a smooth under-side and often
show the sun as a bright spot in the clouds. These are associated with development of bad weather.
(c) Low Clouds: Below 2100 metres (6900 feet) (i) Stratus: A dense, fog-like, low-lying dark-grey layer; brings 'dull weather, usually accompanied with a drizzle. (ii) Nimbostratus: If rain or snow is falling from a stratus cloud, it is Okta is a unit used in meteorology to measure cloud cover. One okta is equal to a cloud cover of one-eighth of the whole sky, Le., one okta represents one-eighth of cloud cover. Therefore, eight oktas are equivalent to a total cloud cover. called nimbostratus.
The prefix 'nimbo' means that precipitation is coming from the cloud. (Hi) Stratocumulus: A lowlying cloud layer consisting of distinct greyish masses of cloud between which blue sky is' visible; oriented at right angles to the direction of wind and cloud motion, they are generally indicative of fair or clearing weather.
1. STRATIFORM CLOUDS are blanket-like, often covering vast areas but are fairly thin when compared to their horizontal dimensions. They are sub-divided on the basis of elevation.
(a) High Clouds: 6000 to 12000 metres above sea-level, e.g., (i) Cirrus Cloud, a wispy, fibrous looking cloud, uften indicating fair weather; (ii) Cirrocumulus, a thin cloud often globular and rippled (this is the mackerel sky of popular
description);
(iii) Cirrostratus looks lilse a thin white sheet producing a halo around the sun or moon.
(b) Medium Clouds: 2100 to 6000 metres above sea level.
(i) Altocumulus Clouds: A layer of individual cloud masses
fitted closely together in geometric pattern; the masses appear white or somewhat grey on shaded sides, and the blue sky can be seen between individual patches of rows
of these bumpy-looking clouds with a flattened base. They indicate fine weather. (ii) Altostratus: A banked layer, often smoothly distributed over the entire sky; greyish in appearance, they usually have a smooth under-side and often
show the sun as a bright spot in the clouds. These are associated with development of bad weather.
(c) Low Clouds: Below 2100 metres (6900 feet) (i) Stratus: A dense, fog-like, low-lying dark-grey layer; brings 'dull weather, usually accompanied with a drizzle. (ii) Nimbostratus: If rain or snow is falling from a stratus cloud, it is Okta is a unit used in meteorology to measure cloud cover. One okta is equal to a cloud cover of one-eighth of the whole sky, Le., one okta represents one-eighth of cloud cover. Therefore, eight oktas are equivalent to a total cloud cover. called nimbostratus.
The prefix 'nimbo' means that precipitation is coming from the cloud. (Hi) Stratocumulus: A lowlying cloud layer consisting of distinct greyish masses of cloud between which blue sky is' visible; oriented at right angles to the direction of wind and cloud motion, they are generally indicative of fair or clearing weather.
CONDENSATION
CONDENSATION
The physical process of transformation from the vapour to the liquid state is condensation. It occurs only when large masses of air experience a steady drop in temperature below the dew point, or when there is enough water vapour within the air mass for it to reach saturation point.
The rising of air mass to high elevations is necessary for this to happen. Condensation occurs when temperature reaches the dew point and the water vapour in the form of tiny droplets becomes visible in the form of clouds at a height of about one kilometre or more above the sea level.
DEW POINT is the temperature at whlch the air is fully saturated and below which condensation normally occurs; water vapour starts to condense to form water droplets.
DEW is the deposition of water droplets on the ground and objects, such as plants, near the ground. It occurs when the temperature of the ground surface falls and the air in contact with it is cooled below its dew point. Water vapour from the air or diffused from the soil then condenses and is deposited as droplets. The favourable conditions are moist air, light winds and clear night skies to ensure
maximum cooling by radiation.
FROST is a weather condition that occurs when the air temperature is at or below ODe. Moisture on the ground surface and objects freezes to form an icy deposit. Con
ditions favourable for its formation are similar to those in the case of dew formation.
FOG is made of the droplets of water suspended in the lower layers of the atmospher~, resulting from the condensation of water vapour around nuclei of floating dust or smoke particles. A visibility of less ~han 1 kilometre is the internationally-recognised definition of fog.
SMOG, also called smoke fog, is a form of fog that occurs in areas where the air contains a large amount of smoke. Smoke particles provide a high concentration of nuclei around which condensation occurs. Condensation can occur around these nuclei even when the air is not saturated and therefore it forms. earlier, becomes denser and lasts longer than fog that develops in unpolluted air. The smoke, because of its chemical contents, gives an acid taste to fog.
MIST is the term for a reduction of visibility between 1-2 km caused by condensation producing water droplets
within the lower layers of atmosphere. It is intermediate between fog and haze.
HAZE is normally formed by water particles that have condensed around nuclei in the atmosphere, but may also be a result of particles of smoke, dust or salt in the air. In meteorology, it is an obscurity of the lower atmosphere that limits visibility to under 2 km but over 1 km. This term is also used for other phenomena that limit visibility.
CLOUDS are masses of minute water droplets and! or ice crystals formed by the condensation of water vapour and held in suspension in the atmosphere. Condensation, which results from cooling, usually takes place around nuclei such as dust, smoke particles and salt. The cooling may be caused by convection, uplift over mountains or
ascent in depressions. Clouds may be present at heights ranging from ground level up to over 13,000 metres.
The physical process of transformation from the vapour to the liquid state is condensation. It occurs only when large masses of air experience a steady drop in temperature below the dew point, or when there is enough water vapour within the air mass for it to reach saturation point.
The rising of air mass to high elevations is necessary for this to happen. Condensation occurs when temperature reaches the dew point and the water vapour in the form of tiny droplets becomes visible in the form of clouds at a height of about one kilometre or more above the sea level.
DEW POINT is the temperature at whlch the air is fully saturated and below which condensation normally occurs; water vapour starts to condense to form water droplets.
DEW is the deposition of water droplets on the ground and objects, such as plants, near the ground. It occurs when the temperature of the ground surface falls and the air in contact with it is cooled below its dew point. Water vapour from the air or diffused from the soil then condenses and is deposited as droplets. The favourable conditions are moist air, light winds and clear night skies to ensure
maximum cooling by radiation.
FROST is a weather condition that occurs when the air temperature is at or below ODe. Moisture on the ground surface and objects freezes to form an icy deposit. Con
ditions favourable for its formation are similar to those in the case of dew formation.
FOG is made of the droplets of water suspended in the lower layers of the atmospher~, resulting from the condensation of water vapour around nuclei of floating dust or smoke particles. A visibility of less ~han 1 kilometre is the internationally-recognised definition of fog.
SMOG, also called smoke fog, is a form of fog that occurs in areas where the air contains a large amount of smoke. Smoke particles provide a high concentration of nuclei around which condensation occurs. Condensation can occur around these nuclei even when the air is not saturated and therefore it forms. earlier, becomes denser and lasts longer than fog that develops in unpolluted air. The smoke, because of its chemical contents, gives an acid taste to fog.
MIST is the term for a reduction of visibility between 1-2 km caused by condensation producing water droplets
within the lower layers of atmosphere. It is intermediate between fog and haze.
HAZE is normally formed by water particles that have condensed around nuclei in the atmosphere, but may also be a result of particles of smoke, dust or salt in the air. In meteorology, it is an obscurity of the lower atmosphere that limits visibility to under 2 km but over 1 km. This term is also used for other phenomena that limit visibility.
CLOUDS are masses of minute water droplets and! or ice crystals formed by the condensation of water vapour and held in suspension in the atmosphere. Condensation, which results from cooling, usually takes place around nuclei such as dust, smoke particles and salt. The cooling may be caused by convection, uplift over mountains or
ascent in depressions. Clouds may be present at heights ranging from ground level up to over 13,000 metres.
HUMIDITY
Water vapour is amongst the most important atmo
spheric gases. The atmosphere gathers moisture by the process of evaporation, while it loses moisture through
condensation and precipitation. Humidity, which refers to the condition of the air with regard to water vapour, plays a decisive role in evaporation, condensation and precipitation.
HUMIDITY
It is the degree of water vapour present in the air. For any specified temperature, there is a definite limit to the maximum quantity of moisture that can be held by the air. This limit is known as the saturation point. Humidity can be measured by- a hygrometer or sling psychrometer.
RELATIVE HUMIDITY is the ratio between the amount
of water vapour actually present in an air mass and the maximum amount that the air mass can hold at that temperature. It is expressed as a percentage. At the
saturation point, relative humidity is 100 per cent. It varies inversely with temperature, given a fixed amount of water vapour. Secondly, if an exposed water surface is present the relative humidity can be increased by evaporation.
ABSOLUTE HUMIDITY is the actual amount of moisture present in air. The amount of water vapour per unit volume of air is usually expressed in grams per cubic metre. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation. As the absolute humidity cannot remain a constant figure for the same body of air, modem meteorology makes use of another measure of moisture content-specific humidity.
SPECIFIC HUMIDITY is the ratio of the weight of water vapour to the weight of moist air. Expressed in units of grams of water vapour per kilogram of moist air, specific
humidity is often used to describe the moisture characteristic of a large mass of air.
spheric gases. The atmosphere gathers moisture by the process of evaporation, while it loses moisture through
condensation and precipitation. Humidity, which refers to the condition of the air with regard to water vapour, plays a decisive role in evaporation, condensation and precipitation.
HUMIDITY
It is the degree of water vapour present in the air. For any specified temperature, there is a definite limit to the maximum quantity of moisture that can be held by the air. This limit is known as the saturation point. Humidity can be measured by- a hygrometer or sling psychrometer.
RELATIVE HUMIDITY is the ratio between the amount
of water vapour actually present in an air mass and the maximum amount that the air mass can hold at that temperature. It is expressed as a percentage. At the
saturation point, relative humidity is 100 per cent. It varies inversely with temperature, given a fixed amount of water vapour. Secondly, if an exposed water surface is present the relative humidity can be increased by evaporation.
ABSOLUTE HUMIDITY is the actual amount of moisture present in air. The amount of water vapour per unit volume of air is usually expressed in grams per cubic metre. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation. As the absolute humidity cannot remain a constant figure for the same body of air, modem meteorology makes use of another measure of moisture content-specific humidity.
SPECIFIC HUMIDITY is the ratio of the weight of water vapour to the weight of moist air. Expressed in units of grams of water vapour per kilogram of moist air, specific
humidity is often used to describe the moisture characteristic of a large mass of air.
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