Measuring the weather: instruments and weather stations

Detail

1. Traditional weather prediction: reading nature's clues

• Traditional weather prediction means guessing the coming weather from signs in nature, not from instruments.
• Signs people watched:
• Ants carry their eggs to higher ground before heavy rain.
• Frogs croak more loudly when the air is moist.
• Birds fly low before a storm.
• Squirrels gather nuts before a hard season.

• Pine cones close in humid air and open in dry air.

• Ancient Indian texts read the sky:

• Krishi-Parashara predicted rain from the positions of the Sun and Moon.
• Varahamihira's Brihat Samhita used the nakshatras. These are 27 lunar mansions, the star groups the Moon passes through in its monthly path. The Brihat Samhita dates to about 500 CE and records ideas on how clouds form and how rain falls [2].
• Kalidasa's Meghadutam follows the path of monsoon clouds across India.

• The Arthashastra records rainfall measurement. Rainfall was measured to fix land revenue and to plan relief in drought.

• Limits:

• This knowledge is local, because it works only for one place.

• It is qualitative, because it describes the weather and does not measure it.

• Meteorology is the systematic, measured study of weather. It uses instruments, fixed observation times and numbers that can be compared from place to place.

2. Weather instruments: what each one measures

Instrument	Element measured	Unit / how it is read
Thermometer (maximum–minimum)	Temperature	°C. Daily range = max − min; daily mean = (max + min) ÷ 2
Rain gauge	Precipitation	millimetres (mm)
Barometer	Air pressure	millibar (mb)
Wind vane, wind sock	Wind direction	Compass direction (N, NE, …)
Anemometer	Wind speed	km/h
Hygrometer	Humidity	Relative humidity, %

• Maximum–minimum thermometer: it records the highest and the lowest temperature of the day.

• Example: max 34 °C and min 24 °C → range 10 °C and mean 29 °C.

• Rain gauge: a funnel collects rain into a measuring jar. The depth of water is read in mm.

• Wind vane: the arrow points to the direction from which the wind blows. Winds are named after that direction, so a "south-west wind" comes from the south-west.

• A wind sock is a cloth cone, often seen at airports. It shows both direction and rough strength.

• Anemometer: cups spin in the wind. The faster they turn, the higher the wind speed.

• Hygrometer: it measures relative humidity. This is how much water vapour the air holds compared with the most it could hold at that temperature, given as %.
• Rain gauge, wind vane, anemometer and hygrometer are studied again with their own elements, under moisture and precipitation and under pressure and winds.

3. The barometer and pressure values

• A barometer is an instrument that measures atmospheric pressure. This is the weight of the column of air pressing down on a surface.

Type	How it works	Key fact
Mercury barometer	Air pressure pushes up a column of mercury in a glass tube. The column's height is read.	Torricelli, 1643. About 760 mm at sea level
Aneroid barometer	A sealed metal box with part of its air removed bends in or out as pressure changes. A needle shows the change.	"Aneroid" means without liquid. It is small and portable.
Barograph	An aneroid barometer linked to a pen on a rotating drum	Records pressure continuously as a graph

• Pressure values to remember:
• Normal sea-level pressure is about 1,013 mb, which equals about 760 mm of mercury.
• Below about 1,000 mb marks a depression. A depression is a low-pressure area that often brings clouds, rain and storms.
• At Khardung La, Ladakh, pressure is about 650 mb, roughly two-thirds of sea-level pressure.
  • Air is thinner higher up, so less air presses down from above.
  • (NCERT: the pass is over 5,600 m high. Modern surveys give a lower height.)

4. Housing the thermometers: the Stevenson screen

• A Stevenson screen is a white wooden box with slatted (louvred) sides that stands on legs. Thermometers are kept inside it.
• Why it is built this way:
• The white colour reflects sunlight, so direct sun does not heat the thermometer.
• Air flows freely through the slats, so the thermometer reads the true air temperature.

• The box is raised above the ground, so the heat of the ground does not affect the reading.

• The result is that readings from different stations can be compared fairly.

5. Weather stations and the IMD network

• A weather station is a place where all the instruments are kept together. They are read at fixed times every day, so data from many stations can be put together on one weather map.
• Growth of observation in India:
• 1785: the East India Company set up an observatory at Calcutta. 1796: an observatory was set up at Madras [2].
• 1875: the Government of India set up the India Meteorological Department (IMD) [2]. It celebrated its 150th Foundation Day in January 2025 [3].
• 1889: Sir John Eliot became the first Director General, at Calcutta. Over time the headquarters moved Calcutta → Shimla → Pune → New Delhi [2].

• Earlier, between 1835 and 1855, Henry Piddington studied tropical storms and coined the word "cyclone" [2].

• Network growth, end-2014 to end-2025 [4]:

Facility	End-2014	End-2025
Automatic Weather Stations	675	1,008
Doppler Weather Radars	15	47
Rain gauge stations	3,500	6,726
Runway Visual Range systems (airports)	20	186

• The 47 Doppler Weather Radars cover about 87% of India's area (2025) [4].
• In 2026, the IMD plans to install 50 AWS each in Delhi, Mumbai, Chennai and Pune for city-level weather data [5].

6. Automatic Weather Station (AWS)

• An Automatic Weather Station (AWS) is a set of electronic sensors that record temperature, humidity, wind, rainfall and pressure. It sends the data automatically, with no human operator.
• Uses:
• Farming: advice on when to sow, irrigate or spray.
• Aviation and navigation: safe take-off, landing and shipping.
• Early warning: floods, cyclones and cloudbursts.

• Remote places: high mountains, deserts and oceans where people cannot stay.

• Case: AWS at Sikkim's glacial lakes, September 2023

• An NDMA-led team set up AWS at South Lhonak and Shako Cho glacial lakes in North Sikkim. The lakes lie above 5,000 m. (NCERT, Class 7: above 4,800 m.)
• The aim was early warning of a GLOF (Glacial Lake Outburst Flood: a sudden flood that happens when a lake dammed by ice or loose rock bursts its banks).
• What happened next:
  • On the night of 3–4 October 2023, a mass of frozen moraine collapsed into South Lhonak Lake. Moraine is the loose rock and soil left behind by a glacier [6].
  • The lake released about 50 million m³ of water into the Teesta valley [6].
  • The flood destroyed the 1,200 MW Teesta III dam at Chungthang [6].
• Response:
  • Two AWS were installed in Sikkim [6][7].
  • More early-warning systems are planned with C-DAC, ISRO and the Space Applications Centre, Ahmedabad [6][7].
  • The National GLOF Risk Mitigation Project (NGRMP) was approved for four states, including Sikkim, at a cost of ₹150 crore [7].

7. Upper-air observation: radiosondes, radars and satellites

• A radiosonde is a small instrument box carried up by a weather balloon. As it rises, it radios back the temperature, humidity and pressure of the upper air. When it is tracked for wind, the observation is called RS/RW (Radiosonde/Radiowind).
• History in India:
• 1905: the first pilot balloon was released from Shimla to track upper winds [8].
• 1930: RS/RW observations began [8].

• Today the IMD launches radiosondes from 56 locations and pilot balloons from 62 locations [8].

• Doppler Weather Radar (DWR): it sends out radio waves and reads the echoes from rain drops. This shows where rain and storms are and how they are moving. It is used to track cyclones and cloudbursts.

• Satellites: India was the first developing country to have its own geostationary satellite (INSAT) for weather watch [2]. A geostationary satellite stays above the same spot on Earth.
• Mission Mausam:
• It was launched in September 2024 with an outlay of about ₹2,000 crore [9].
• Its goal is to make India "Weather-ready and Climate Smart" [4].
• Planned additions: 50 DWRs, 60 RS/RW stations, 100 disdrometers (rain-drop-size sensors), 10 wind profilers, 25 radiometers and 10 marine AWS with upper-air observation [9][10].

Prelims Hooks

• Instrument–element pairs: barometer → pressure; hygrometer → humidity; anemometer → wind speed; wind vane → wind direction; rain gauge → precipitation (mm).
• Trap: a wind vane points to where the wind comes from, not where it goes.
• Mercury barometer → Torricelli, 1643 → about 760 mm at sea level, which equals about 1,013 mb. The aneroid barometer uses no liquid.
• Barograph records pressure continuously. A thermograph does the same for temperature.
• Below about 1,000 mb → depression. Khardung La (Ladakh) → about 650 mb.
• Stevenson screen: a white, louvred box that holds thermometers.
• Chronology: Calcutta observatory (1785) → Madras (1796) → IMD (1875) → first pilot balloon from Shimla (1905) → RS/RW (1930) → Mission Mausam (September 2024) [2][8][9].
• South Lhonak Lake → North Sikkim → Teesta basin → GLOF of 3–4 October 2023 → Teesta III dam at Chungthang destroyed [6].
• Text–author pairs: Brihat Samhita → Varahamihira (nakshatras); Meghadutam → Kalidasa (monsoon clouds); Krishi-Parashara → Sun–Moon positions; Arthashastra → rainfall measurement.
• IMD network, end-2025: 47 DWRs (about 87% of India's area covered) and 1,008 AWS [4].

Mains Points

• Why the observation network matters (GS-III, disasters): a dense network of AWS, DWRs and radiosondes allows early warning, and early warning saves lives.
• DWRs grew from 15 to 47 between 2014 and 2025 [4].

• This helped India move from rescue after a disaster to preparing before one, for cyclones, cloudbursts and GLOFs.

• Gaps in remote and high areas: the Himalaya has very few manned stations because of height, cold and difficult terrain.

• The South Lhonak GLOF (October 2023) showed this gap [6].

• The AWS + satellite + community-alert model under the NGRMP (₹150 crore, four states) is the policy answer [7].

• Traditional and scientific knowledge: signs from nature and texts such as Krishi-Parashara are local and qualitative. Measured meteorology can be compared across places and used for forecasting.

• Folk observation can still help with the "last mile", meaning getting a warning to the farmer or villager who needs it.

• Uses across sectors (GS-III): AWS data support farm advisories, aviation (186 runway visual range systems by end-2025), shipping and city flood management. Examples are the planned 50 AWS each in four metros in 2026 and Mission Mausam [4][5][9].