NCERT Class 11 Practical Geography Chapter 8: Weather Instruments, Maps and Charts YouTube Lecture Handouts

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NCERT Class 11 Practical Geography Chapter 8: Weather Instruments, Maps and Charts

NCERT Class 11 Practical Geography Chapter 8: Weather Instruments, Maps and Charts

Weather & Forecast

Weather & Forecast
  • Weather: The condition of the atmosphere at a given place and time with respect to atmospheric pressure, temperature, humidity, precipitation, cloudiness and wind. These factors are known as weather elements.
  • Weather Forecast: Prediction with a reasonable amount of certainty about the conditions of weather that would prevail in the coming 12 to 48 hours in a certain area.
  • The Indian Meteorological Department (IMD) was established in 1875, with its headquarters at Calcutta. The IMD headquarters are presently located at New Delhi

Weather Observations – 3 Levels

Weather Observations – 3 Levels

The World Meteorological Organization (WMO) , a specialized agency of the United Nations, coordinates these observations

Spaced Based Observations (E. G. INSAT)

Spaced Based Observations

Weather satellites make comprehensive and large-scale observations of different meteorological elements at the ground level as well in the upper layers of the atmosphere. The geo-stationary satellites provide space-based observations about weather conditions

Surface Observations – IMD Class 1 (Highest Category)

Surface Observations
  • A typical surface observatory has instruments for measuring and recording weather elements like temperature (maximum and minimum) , air pressure, humidity, clouds, wind, and rainfall. Specialized observatories also record elements like radiation, ozone atmospheric trace gases, pollution, and atmospheric electricity. These observations are taken all over the globe at fixed times of the day
  • Observations are taken in these observatories normally at 00, 03, 06, 09, 12, 15,18, 21 hours (Greenwich Mean Time) around the globe.
  • For logistics – some observations are limited on daily numbers

Stevenson Screen: Thermometers

Stevenson Screen: Thermometers
  • The Stevenson screen is designed to protect thermometers from precipitation and direct sunlight while allowing air to circulate freely around them. It is made from wood with louvered sides to allow free and even flow of air. It is painted white to reflect radiation. It stands on four legs and is about 3 feet 6 inches above the level of the ground. The legs must be sufficiently rigid and be buried sufficiently in the ground to prevent shaking.
  • The front panel is hinged at the bottom to form a door, which allows for maintenance and reading of the thermometers. The door of Stevenson screen is always towards the north in the northern hemisphere and towards the south in the southern hemisphere because direct sunrays also affect mercury. The purpose of the Stevenson screen is to create a uniform temperature enclosure that closely represents the same temperature as the air outside

Maximum & Minimum Thermometers

Maximum & Minimum Thermometers
  • Thermometer is used to measure air temperature. Most thermometers are in the form of a narrow closed glass tube with an expanded bulb at one end. The bulb and the lower part of the tube are filled with liquid such as mercury or alcohol. Before the other end is sealed off, the air in the tube is released by heating it.
  • Celsius - melting ice is marked 0 degree C and that of boiling water as 100 degree C, and the interval between the two is divided into 100 equal parts
  • On the Fahrenheit thermometer, the freezing and boiling points of water are graduated as 32 degree F and 212 degree F respectively
  • While the maximum thermometer and minimum thermometer are used to measure the air temperature, the dry bulb and the wet bulb thermometers are used to determine the humidity in the air. A set of these thermometers is kept in the Stevenson Screen.
  • The maximum thermometer is designed to record the highest temperature during a day. As the temperature increases, the mercury moves up into the tube; however, as the mercury cools, it cannot move downwards because of a constriction in the tube. It must be reset again to bring it down
  • Minimum thermometer - In this thermometer, alcohol is used in place of mercury. When the temperature decreases, the metal pin in the tube goes down and strikes at the minimum temperature.

Wet & Dry Bulb Thermometers

Wet & Dry Bulb Thermometers
  • The dry bulb and wet bulb thermometers are two identical thermometers fixed to a wooden frame. The bulb of the dry thermometer is kept uncovered and is exposed to the air while the bulb of the wet bulb thermometer is wrapped up with a piece of wet muslin, which is kept continuously moist by dipping a strand of it into a small vessel of distilled water. The evaporation from the wet bulb lowers its temperature.
  • Dry bulb readings are not affected by the amount of water vapour present in the air, but the wet bulb readings vary with it since the rate of evaporation is dependent upon the amount of water vapour present in the air. The greater the humidity in the air, the slower the rate of evaporation and hence, the difference between the readings of the dry bulb and wet bulb will be small. On the other hand, when the air is dry, the evaporation from the surface of the wet bulb is rapid, which would lower its temperature and the difference between the two readings would be larger.
  • Difference of the readings of the dry bulb and the wet bulb thermometers determines the state of the atmosphere with regard to its humidity.
  • The larger the difference, the more arid is the air.

Wind Vane

Wind Vane Device
  • Wind vane is a device used to measure the direction of the wind. The wind vane is a lightweight revolving plate with an arrowhead on one end and two metal plates attached to the other end at the same angle. This revolving plate is mounted on a rod in such a manner that it is free to rotate on a horizontal plane
  • The arrow always points towards the direction from which the wind blows

Rain Gauge

Rain Gauge
  • The amount of rainfall is measured with the help of a rain gauge. The rain gauge consists of a metal cylinder on which a circular funnel is fitted. The diameter of the funnel՚s rim is normally 20 cm
  • Normally, rainfall is measured in the units of millimetres or centimetres. Snow is also measured in a similar manner by turning it into liquid form
    • Temperature- Thermometer
    • Atmospheric Pressure -Barometer -Millibars
    • Wind (Direction) -Wind Vane -Cardinal points
    • Wind (Velocity) - Anemometer -Km/hr
    • Rainfall -Rain Gauge- mm/cm


Barometer Weight
  • The air around us has weight, and it exerts great pressure on the earth՚s surface. At the sea level, under normal conditions, the pressure of air is 1.03 kg per square centimetre. Due to constant movement of air, change in temperature and variation in its vapour content, the weight of the air changes continuously with time and place.
  • Mercury barometer, aneroid barometer and barographs. The unit of measurement is in the millibar.
  • Mercury barometer is an accurate instrument and is used as a standard. In it the atmospheric pressure of any place is balanced against the weight of a column of mercury in an inverted glass tube
  • The mercury will flow out of the tube into the cup and stand at a definite height above the level of the liquid in the cup. This is because the weight of the column of the mercury in the tube, above the surface of the mercury in the cup, is balanced by the weight of the air column of an indefinite height exerted as pressure upon an equal cross-section of the liquid surface. The height of the column of mercury in the tube, therefore, becomes the measure of the pressure of air.
  • Aneroid barometer gets its name from the Greek work, aneros (a- ‘not’ , neros – ‘moisture’ , meaning without liquid) It is a compact and portable instrument. It consists of a corrugated metal box made up of a thin alloy, sealed completely, and made airtight after partial exhaustion of air. It has a thin flexible lid, which is sensitive to changes of pressure.
  • As the pressure increases, the lid is pressed inward, and this, in turn, moves a system of levers connected to a pointer, which moves clockwise over the graduated dial and gives higher reading. When the pressure decreases, the lid is pushed outward and the pointer moves counter clockwise, indicating lower pressure
  • Barograph works on the principle of aneroid barometer. There are a number of vacuum boxes placed one above the other so that the displacement is large. A system of levers magnifies this movement, which is recorded by a pen on a paper attached to a rotating drum. The readings of a barograph are not always accurate, and therefore, they are standardized by comparing them with a mercury barometer reading.

Weather Maps, Charts & Symbols

  • Weather Maps: A weather map is the representation of weather phenomena of the earth or a part of it on a flat surface. It depicts conditions associated with different weather elements such as temperature, rainfall, sunshine and cloudiness, direction and velocity of winds, etc. on a particular day. Such observations being taken at fixed hours are transmitted by code to the forecasting stations. The central office keeps a record of the observations, which forms the basis for making a weather map. The upper air observations, which are procured from hill stations, aeroplanes, pilot balloons, etc. , are plotted separately
  • Meteorological observatories transmit the data to the Central Observatory at Pune twice a day. Data is also collected on ships plying on the Indian seas. A good progress has been made in the field of weather forecasting and observation with the establishment of weather observatories in Antarctica, the International Indian Ocean Expedition, and the launching of rockets and weather satellites.
  • Weather Charts: The data received from various weather observatories are in plenty and detailed. As such, they cannot be incorporated in one single chart unless the coding designed to give the economy of expression is used. These are called synoptic weather charts and the codes used are called meteorological symbols. Weather charts provide the primary tools for weather forecasting. They help in locating and identifying different air masses, pressure systems, fronts, and areas of precipitation.
  • WEATHER SYMBOLS: The messages received from all the observatories are plotted on the map using weather symbols standardized by the World Meteorological Organization and the National Weather Bureaus.

Isobars, Isotherms, Isohyets, Isohels, Isonephs

Isobars, Isotherms, Isohyets, Isohels, Isonephs

Isobars: Lines connecting places of equal air pressure.

Isotherms: Lines connecting places of equal temperature.

Isohyets: Lines connecting places of equal amount of rainfall over a given period.

Isohels: Lines connecting places of same mean daily duration of sunshine.

Isonephs: Lines connecting places of same mean value of cloud cover

Isobars, Isotherms, Isohyets, Isohels, Isonephs

Weather Map Interpretation

Weather Map Interpretation

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