Weather and its elements
On a winter morning you wear a sweater, in summer light clothes: your body is sensing the weather.
Weather is the state of the atmosphere at a particular place and time. The atmosphere is the layer of gases around the Earth. The lowest layer, the troposphere, is where almost all weather happens.
Key points
- The troposphere is 6 to 18 km high; thinner at the poles (cold air contracts), thicker in the tropics (warm air expands).
- Five elements of weather: temperature, atmospheric pressure, wind, precipitation (rain, snow, sleet, hail) and humidity.
- We need common units; 'cold' differs from person to person, but 20 °C means the same to everyone.
- People once read ants, frogs and pine cones to guess rain; this is traditional knowledge from experience.
- Easy : What is the study of weather called? → Meteorology.
- Medium : Why is the troposphere thinner at the poles? → Cold air contracts.
- Exam-type : Write the five elements of weather. → Temperature, pressure, wind, precipitation, humidity.
Atmospheric pressure is the weight of the air above us.
Weather and forecast are not the same; weather is the present state, a forecast is a prediction.
Five elements = T-P-W-P-H. Memory clue : Count on fingers: Temperature, Pressure, Wind, Precipitation, Humidity.
Spot the difference
| Point | Weather | Forecast |
|---|---|---|
| Time | Present state | Coming hours/days |
| Basis | Observation | Measured data and study |
| Use | Clothes, travel | Warnings, disaster readiness |
Practice questions
- 1Which is the lowest layer of the atmosphere?
- 2Name one element of weather.
- 3What is precipitation?
- 4What is humidity?
- 5Why do we need common units for measurement?
- 6Why is the troposphere lower at the poles?
- 7Which signs did people once use to guess weather? Give two examples.
- 8Write in two lines how weather and forecast differ.
Show answers (try them yourself first)
- 1Troposphere
- 2Temperature, pressure, wind, precipitation, humidity (any one)
- 3Any form of water falling from the sky: rain, snow, sleet, hail
- 4Amount of water vapour in air
- 5'Cold' differs by person; a measurement is understood by all
- 6Cold air contracts
- 7Ants moving eggs to higher ground, frogs croaking, pine cones opening/closing
- 8Weather = present state; forecast = likely future state from measured data.
Measuring temperature and rainfall
We use a thermometer for fever; news says '30 mm of rain': both are measured with instruments.
Temperature is measured with a thermometer (°C or °F) and rain with a rain gauge. 15 °C = 59 °F.
Key points
- Range of temperature = maximum − minimum (usually in 24 hours).
- Mean daily temperature = (maximum + minimum) ÷ 2.
- In a rain gauge water runs through a funnel into a tube; the scale shows depth: 5 mm depth = 5 mm rainfall.
- The India Meteorological Department (IMD) was set up in 1875; its motto 'ādityāt jāyate vṛiṣhṭi' means 'from the sun arises rain'.
- Easy : Max 34 °C, min 22 °C. Range? → 34 − 22 = 12 °C.
- Medium : Mean temperature (max 36, min 20)? → (36 + 20) ÷ 2 = 28 °C.
- Exam-type : Where to place a rain gauge? → In the open, away from trees and walls, on a flat surface where it will not topple.
Range = maximum − minimum.
When finding the mean, do not forget to divide the sum by 2.
Range = subtract; Mean = in the middle. Memory clue : 'Range = Remove (subtract), Mean = Middle (add and halve)': R for Remove, M for Middle.
Spot the difference
| Point | Range | Mean |
|---|---|---|
| Rule | Max − min | (Max + min) ÷ 2 |
| Example (34, 22) | 12 °C | 28 °C |
| Tells | Day's swing | A day's typical temperature |
Practice questions
- 1Name the instrument that measures rainfall.
- 2In which year was the IMD set up?
- 3What is 15 °C in °F?
- 4Max 38 °C, min 24 °C: what is the range?
- 5Max 32 °C, min 18 °C: what is the mean?
- 6How does 5 mm of rain show in a rain gauge?
- 7Choose a school site for a rain gauge and give reasons: vegetable garden, open raised platform on the terrace, compound wall.
- 8Weekly maximum temperatures are 30, 31, 33, 32, 34, 33, 31 °C. What is the difference between the highest and lowest of these?
Show answers (try them yourself first)
- 1Rain gauge
- 21875
- 359 °F
- 438 − 24 = 14 °C
- 5(32 + 18) ÷ 2 = 25 °C
- 6Water 5 mm deep in the tube
- 7Open platform: no trees/walls block rain, flat and safe
- 834 − 30 = 4 °C
Atmospheric pressure and wind
Climbing a hill makes you breathless; a 'wind sock' at an airport shows wind direction.
Atmospheric pressure is the pressure of the weight of air; a barometer measures it in millibars (mb). Wind is air moving from high pressure to low pressure.
Atmospheric pressure and wind
In the app you can move this figure with your finger.
Open in the appKey points
- Normal pressure at the coast is about 1013 mb; below 1000 mb is a 'depression', which can grow into a storm or cyclone.
- At height the air is thin, pressure and oxygen are low, so people feel breathless or dizzy. At Khardung La, Ladakh (over 5600 m), pressure is about 650 mb.
- A wind vane shows direction; an anemometer measures speed and direction: stronger wind spins the cups faster; speed in km/h.
- Wind helps pilots, sailors and farmers; strong wind dries soil faster.
- Easy : Which instrument measures pressure? → Barometer.
- Medium : Why breathless on a mountain? → Thin air, less oxygen.
- Exam-type : In which direction does wind blow? → From high pressure to low pressure.
Normal coastal pressure is about 1013 mb.
A wind vane and an anemometer are not the same; the vane shows direction, the anemometer speed.
Wind goes 'High → Low'. Memory clue : 'Vane = Where (direction), Anemo = Amount (speed)'.
Spot the difference
| Point | Wind vane | Anemometer |
|---|---|---|
| Shows | Direction only | Speed (and direction) |
| Build | Rotating arrow and tail | 3-4 cups on a vertical shaft |
| Unit | None | km/h |
Practice questions
- 1Which unit measures atmospheric pressure?
- 2What is the normal pressure at the coast?
- 3From where to where does wind blow?
- 4What is a depression (low-pressure system)?
- 5Why is a wind sock placed at an airport?
- 6Why are people going to high places advised to pause on the way?
- 7How does an anemometer work?
- 8How do pilots, sailors and farmers use wind information?
Show answers (try them yourself first)
- 1Millibar (mb)
- 2About 1013 mb
- 3From high to low pressure
- 4A state where pressure is below 1000 mb
- 5It shows wind direction
- 6The body gets time to adjust to less oxygen
- 7Cups spin in wind; the count of spins gives speed
- 8Take-off and landing, sailing, guessing rain direction
Humidity
In the rainy season clothes dry slowly and we sweat more, because the air holds more vapour.
Humidity is the amount of water vapour in air; it is given as relative humidity (per cent) and measured by a hygrometer.
Humidity
In the app you can move this figure with your finger.
Open in the appKey points
- 0 % = no vapour at all (impossible in nature), 100 % = air fully saturated with vapour.
- Dry weather is about 20 to 40 %, humid weather about 60 to 80 %.
- Evaporation cools; if air already has much vapour, evaporation is slow.
- Humidity is monitored in food processing and in museums (to keep exhibits dry).
- Easy : If humidity is 52 %, how will clothes dry? → Fairly quickly.
- Medium : Delhi 52 %, Kochi 84 %: where more sweat? → Kochi.
- Exam-type : Why does low humidity dry clothes faster? → Less vapour in air allows faster evaporation.
Relative humidity lies between 0 and 100 %.
Do not assume that places near the sea always have higher humidity; measure to be sure.
High humidity = more sweat, slow drying. Memory clue : 'Humid = Heavy with water'.
Spot the difference
| Point | Dry weather | Humid weather |
|---|---|---|
| Relative humidity | About 20-40 % | About 60-80 % |
| Drying clothes | Fast | Slow |
| Sweat | Less felt | More felt |
Practice questions
- 1Name the instrument that measures humidity.
- 2In what unit is relative humidity given?
- 3What is the usual relative humidity in humid weather?
- 4What effect does evaporation have?
- 5Why do clothes dry slowly on a rainy day?
- 6Why is measuring humidity important in museums?
- 7Delhi 52 % and Kochi 84 %, same temperature. Compare drying and sweating in the two places.
- 8Which of Kochi and Jaipur may have higher humidity, and how can you be sure?
Show answers (try them yourself first)
- 1Hygrometer
- 2Per cent (%)
- 3About 60 to 80 %
- 4Cooling
- 5Air already holds much vapour, so evaporation is slow
- 6To keep exhibits dry
- 7Clothes dry faster in Delhi, more sweat in Kochi
- 8Kochi more likely (near the sea), but confirm with a hygrometer
Weather stations and forecasts
Fishermen are told not to go to sea before a storm; this is possible because of weather forecasts.
A weather station keeps all instruments together and records readings at regular intervals. An Automated Weather Station (AWS) records temperature, humidity, wind, precipitation and pressure with sensors, without human help.
Key points
- Meteorologists study long records to forecast hours, days or weeks ahead.
- Climate change makes droughts, floods and cyclones more frequent, so accurate forecasts matter.
- The IMD uses colour codes on maps: Watch (be updated), Alert (be prepared), Warning (take action).
- In 2023 an AWS was set up at a glacial lake in Sikkim above 4800 m, giving early information.
- Easy : Full form of AWS? → Automated Weather Station.
- Medium : How does a storm forecast help fishermen? → They avoid going to sea.
- Exam-type : What does the administration do when a cyclone is likely? → Evacuates coasts and readies resources.
The IMD motto is 'ādityāt jāyate vṛiṣhṭi'.
A forecast is not a certain future; it is a likelihood based on measurements.
Watch → Alert → Warning: Look, Prepare, Act. Memory clue : 'Watch = Wait and see, Alert = Be ready, Warning = Work now'.
Spot the difference
| Point | Watch | Alert | Warning |
|---|---|---|---|
| Meaning | Stay updated | Be prepared | Take action |
| Danger | Low | Rising | Urgent |
| What you do | Follow news | Gather essentials | Move to a safe place |
Practice questions
- 1What does AWS stand for?
- 2In which year was the IMD set up?
- 3What does a weather station contain?
- 4Name the three levels of IMD advisories.
- 5Why have accurate forecasts become more important with climate change?
- 6What is the use of the AWS in Sikkim?
- 7Give three uses of weather forecasts.
- 8In two lines, compare traditional signs with modern instruments.
Show answers (try them yourself first)
- 1Automated Weather Station
- 21875
- 3Instruments for temperature, rain, pressure, wind, humidity
- 4Watch, Alert, Warning
- 5Extreme weather is becoming more frequent
- 6Gives early weather information near a glacial lake
- 7Fishermen's warnings, farm planning, disaster readiness
- 8Traditional signs rest on experience; instruments give precise numbers