Weather and Climate — Class 8 Social Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 8 Social Science, Geography — Chapter 2. The air around us and the conditions it creates.
1. About this lesson
This lesson explains the difference between weather and climate, the atmosphere, the elements of weather, and the isolines used on weather maps.
2. Weather and climate; the atmosphere
- Weather is the day-to-day condition of the atmosphere at a place; climate is the average weather over a long period (about 30 years).
- The atmosphere is about 78% nitrogen and 21% oxygen (plus argon, carbon dioxide and water vapour).
- The troposphere is the lowest layer and the "weather-making zone" — clouds, rain, storms and winds occur here.
3. Temperature and the lapse rate
- Temperature is measured with a thermometer (kept in a Stevenson screen) on the Celsius, Fahrenheit or Kelvin scale.
- In the troposphere, temperature decreases with height — the lapse rate is about 6.5 °C per 1000 m.
4. Humidity and other elements
- Humidity is the water vapour in the air. Relative humidity is the ratio of the actual water vapour to the maximum the air can hold at that temperature.
- Other elements of weather: air pressure (measured by a barometer), winds (anemometer), and rainfall (rain gauge).
5. Isolines on weather maps
| Isoline | Joins places of equal… |
|---|---|
| Isotherm | temperature |
| Isobar | air pressure |
| Isohyet | rainfall |
6. Worked examples
Example 1. Which layer is the weather-making zone? The troposphere.
Example 2. What is the lapse rate? About 6.5 °C per 1000 m — temperature falls with height.
Example 3. What does an isohyet join? Places of equal rainfall.
7. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- The lowest, weather-making layer of the atmosphere is the — (a) troposphere / (b) stratosphere. Ans: (a) troposphere.
- The atmosphere contains about ____ % nitrogen — (a) 21 / (b) 78. Ans: (b) 78.
- The lapse rate is about — (a) 6.5 °C per 1000 m / (b) 10 °C per 1000 m. Ans: (a) 6.5 °C per 1000 m.
- An isobar joins places of equal — (a) air pressure / (b) rainfall. Ans: (a) air pressure.
- The average weather of a place over a long period is its — (a) weather / (b) climate. Ans: (b) climate.
II. Fill in the blanks 6. Temperature is measured with a thermometer kept in a Stevenson screen. 7. An isohyet joins places of equal rainfall. 8. The atmosphere has about 21% oxygen.
III. Answer briefly 9. Differentiate weather and climate. 10. Name the three isolines and what each shows.
8. Common mistakes
- Mistake: Using "weather" and "climate" as the same thing. Fix: Weather = day-to-day; climate = long-term average (~30 years).
- Mistake: Mixing up the isolines. Fix: Isotherm = temperature; isobar = pressure; isohyet = rainfall.
- Mistake: Saying temperature rises with height in the troposphere. Fix: It falls with height (lapse rate ~6.5 °C/1000 m).
9. Quick revision
- Geography Ch 2 · weather and climate.
- Weather = day-to-day; climate = long-term average (~30 years).
- Atmosphere: 78% nitrogen, 21% oxygen; troposphere = weather-making zone.
- Lapse rate ~6.5 °C per 1000 m; temperature falls with height.
- Isolines: isotherm (temperature), isobar (pressure), isohyet (rainfall).
