Go through these RBSE Class 11 Geography Notes Chapter 15 Condensation and Rainfall contain important concepts so that students can score better marks in the exam.
Rajasthan Board RBSE Class 11 Geography Notes Chapter 15 Condensation and Rainfall
→ The water vapour present in the atmosphere is called humidity. Average atmospheric humidity is found to be 2%.
→ Like every substance, water is also found in three states – solid, liquid and gas. In solid form it is called ice, water in liquid state, and water vapour in gaseous state.
→ Water vapour in the atmosphere comes by evaporation. When favourable conditions occur, the water vapour condenses and falls on the earth in the form of rain, sleet, snow, etc.
→ The real volume of water vapour present in the atmosphere is called absolute humidity or the actual volume of the total water vapour present in a fixed volume of air is called absolute humidity.
→ The volume of absolute humidity depends on the probability of rainfall, and it is expressed in gram/cubic meter.
→ At a certain temperature, the ratio of the humidity of the saturated air with fixed volume and the ratio of the actual humidity contained in it is called relative humidity. It is represented as a percentage value.
→ When there is so much humidity in air at a certain temperature, that addition of any more water vapour is not possible, it is called saturated air.
→ The temperature at which the air is saturated is called dew point.
→ The humidity of the atmosphere is measured by an instrument called hygrometer.
- The process through which the liquid or water phase changes into gas or water vapor is called evaporation.
- The volume and rate of evaporation depends on the speed, temperature, and dryness of the air.
- Most evaporation on the continents ranges from 10°north to 10° south latitudes and highest evaporation on the oceans is found between 10° to 20° latitudes in both the hemispheres.
- The volume of evaporation decreases gradually towards higher latitudes.
- The volume of evaporation depends on temperature, air dryness, expanse of water area, cloud cover and wind velocity.
- The process of transformation of water from gaseous state into liquid or solid state is called condensation.
- The temperature at which the air is saturated is called dew point.
- The process of condensation is caused by decrease in temperature or by increase in humidity.
Forms of Condensation
- Major forms of condensation include dew, frost, cloud and fog.
- The condensation of water vapour in the air and its deposition on the leaves of the plants in the form of small droplets and on other surfaces is called dew.
- When the water vapour present in the air is condensing and the temperature of the air is 0°C or less, then the water vapour begins to take the solid form instead of the dew, and is called frost.
- When the visibility of the surface or atmospheric area is less than 1 kilometre, this form of condensed suspended water vapour is called fog.
- Fog visibility is measured by an instrument called transmissometer.
- When visibility becomes extremely low, this condition is called mist.
- The mass volume of water particles or ice particles formed due to the condensation of water vapour in the open air at very high altitudes is called cloud.
Type of Clouds
- On the basis of height, density and transparency, the clouds are classified as Cirrus Cloud, Cumulus Cloud, Stratus Cloud, Nimbus Cloud.
- The cirrus clouds are found at an altitude of 8000 to 12000 meters, and they spread like a white sheet in the entire sky.
- Cumulus clouds are highly elaborate and deep dark-coloured and heavy clouds, and they are found at altitudes ranging from 4000 to 7000 meters.
- The stratus clouds are found in the form of layers similar to fog.
- Nimbus clouds are dense and dark. They are found at the lowest height, and they cause rainfall.
- When the water vapour present in the atmosphere gets converted into liquid or solid state and falls on the surface, it is called precipitation.
- When precipitation occurs in the form of water, it is called rain and precipitation in the form of snow particles is called snowfall.
- In precipitation, hail, snow and water can all be included, under rainfall.
- For rainfall, there must be sufficient water vapour in the air and such a factor is necessary to cool down the vaporised air and cause it to condense.
Type of Precipitation
- Rainfall on the earth is divided into three types: convectional rain, orographic rain and cyclonic rain, on the basis of origin.
- Convectional rainfall is due to the process of convection in the equatorial linear regions.
- The rising up of air due to being hot due to receipt of excessive temperature and the sudden rising of water in the form of vapour is called convection.
- The orographic rain occurs due to the obstruction of the mountains, and due to the rising and the cooling of the air.
- There is very little rain on the leeward slope of the mountains, which is why this part is called Rain Shadow area.
- Cyclonic rainfall is due to fronts formed by the presence of different types of air masses.
- The lines which are drawn to join the places of similar rainfall on the world map are called isohyets.
- Rainfall is measured by an instrument called rain-gauge. Rainfall is measured in inches or millimetres.
- Factors affecting the rainfall include latitude, elevation, prevailing winds, water currents, distance from sea, water and land conditions, and land topography (mountains, etc.).
Distribution of Rainfall
- Distribution of rain on the surface is uneven, for this inequality in the temperature, distribution of land and water, wind direction, orientation of the mountains is mainly responsible.
- On Earth, there are six belts of rain, which include the Equatorial linear belt, rain belt of trade winds, Sub-tropical minimum rain belt, Mediterranean rain belt, mid-latitudinal heavy rain belt and Polar low belt.
- Equatorial linear belt extends up to 10° latitude on both side of the equator, there is Convectional type of rain in this belt.
- Rain belt of trade winds are found between 10° to 20° latitudes on both side of the equatorial line, here there are Trade winds in the eastern parts of the continents.
- The Sub-tropical minimum rain belt is found between 20° to 30° latitudes in both hemispheres.
- The expanse of the Mediterranean rain belt is found between 30° to 40° latitude in both hemispheres.
- Mid-Latituduial heavy rain belt is found between 40° to 60° latitude on both sides of the equatorial line. There is cyclonic rainfall in this belt.
- The polar low rainfall belt extends from 60° latitude to the pole in both hemispheres. Most precipitation occurs in the form of snow in this belt.
Condensation and Rainfall Notes Important Terms
→ Condensation: The process through which a substance changes from gaseous (vapour) to liquid state, the condensation of water vapour in the atmosphere forms the cloud and the water vapour turns into water-droplets.
→ Hail: Ice particles with a diameter of about 5 mm or more, which fall on the surface due to the Cumulus rain clouds, the crops are destroyed by the hailstorm, and damage is caused to the public life and property.
→ Precipitation: When water vapour present in the atmosphere gets converted into liquid or solid state and falls on the ground, it is called precipitation.
→ Absolute humidity: The amount of water vapour present in a unit volume of air at a certain temperature; it is often expressed in grams per cubic meter.
→ Relative humidity: The proportion of the amount of absolute humidity of a certain amount of saturated air at a given temperature and the actual amount of humidity existing in it.
→ Saturated air: The air which contains water vapour equal to its maximum capacity to retain humidity at a particular temperature. Along with increase in the temperature of air, its capacity to retain humidity also increases.
→ Dew point: The temperature on which the atmosphere becomes saturated by the water vapour and condensation of water vapour in the form of drops of dew occurs.
→ Hygrometer: An instrument used to measure relative humidity of air. Wet bulb and dry bulb thermometers, hygrograph, hair hygrograph, cycrometer, etc. are the major humidity measurement devices.
→ Dew: When the temperature of the atmosphere cools down below the dew point due to the heat radiation from the Earth during the night, the water vapour present in the air condenses into droplets which get deposited on the earth’s surface and are called dew.
→ Freezing point: The temperature at which any fluid is converted into solid form; freezing point of pure water is 0° Celsius or 32°Fahrenheit.
→ Mist: The amount of very fine, water droplets generated from the condensation of water vapour in air in the lower layers of the atmosphere, which leads to a slight decrease in visibility.
→ Cirrus cloud: High clouds that look like silk scattered at high altitude in the sky are formed by small ice particles, through which the colour of the sunrays turns white during the day time.
→ Cumulus Cloud: Vertically large, thicker and, detailed clouds whose base is horizontal but the upper part is similar to the shape of a dome or a cauliflower.
→ Stratus Cloud: The lower clouds, a layer which appears to look like fog but are found at some elevation from the ground and are not adjacent to the surface. Their colour is usually brown and their layer is thin.
→ Halo: A light ring formed around the sun or the moon, which is formed in the presence of a thin layer of cirrus type of clouds in the high sky, due to the presence of snow particles or water particles in high and light clouds, Halo is formed due to the refelaction of light.
→ Convection Rain: When reaching sufficient heights in the sky, the air cools down to dew and condensation takes place and cumulus clouds are formed and it begins to rain very quickly, it is called convection rain.
→ Orographic Rain: Rainfall caused due to the obstruction of a mountain in the path of wet winds, the rise of winds up the mountain slope and the result of condensation.
→ Cyclonic Rainfall: Rain accompanied by a cyclone or depression. When the relatively hot and humid air mass flows above any cold air mass, there is cyclonic rain due to condensation of humid air.
→ Rainshadow area: Area located on the leeward side of a mountain range where average rainfall is relatively less.
→ Windward slope: Mountain slope in front of the prevailing wind or the front part or face of a mountain. All the winds of this slope flow to the upper part of the mountain.
→ Lee/Leeward/Leeside: The face of mountain located in the rear of the direction of wind flow or part of the mountain which is protected from wind velocity, it is located opposite to the windward slope.
→ Cyclone: A low air pressure area where air travels from the outside to the center (inwards) in circular form.
→ Isohyet: The line drawn on the map which joins those places which receive the same amount of rainfall in a certain period.
→ Raingauge or Pluviometer: A simple device used to measure the precipitation of a place. Rain gauge is kept in an open space, so that there is no obstruction to collecting rainwater.
→ Trade winds: In both the northern and southern hemispheres, the planetary winds blowing from high-pressure belt of the subtropical zone towards the equatorial low-pressure belt are called trade winds.
→ Sub-tropical zone: The belt located between the tropics and the middle of temperate zone, whose extension is from the tropic of cancer to the 45° North Latitude in the Northern Hemisphere and from the tropic of Capricorn to the 45° South Latitude in the Southern Hemisphere.
→ Westerlies: Permanent winds blowing towards sub-polar low-pressure belt from sub-tropical high-pressure belt in both the northern and southern hemispheres.
→ Icefall: Due to the rapid slope change in the path of a glacier, the falling of the snowdrift along the sharp slope in a glaciated area, is called icefall or snowslide or avalanche.