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# O Level - Weather & Climate - Factors Affecting Relative Humidity and Rain

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Factors Affecting Relative Humidity and Rain

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### O Level - Weather & Climate - Factors Affecting Relative Humidity and Rain

1. 1. Weather & Climate
2. 2. Humidity
3. 3. By the end of the lesson… We will be able to ► Explain the relationship between temperature and relative humidity ► Explain the formation of convectional and relief rain.
4. 4. Relative Humidity ► Relative humidity (RH) is the ratio between the actual amount of water vapour present in the air and the maximum amount of water vapour the air can hold at a given temperature. ► Expressed as a precentage (%) ► Formula:
5. 5. Relative Humidity Example PART 1: ► The actual amount of vapour in the air in the morning was 10g/m3. Given that the air could contain a maximum of 50g/m3, what was the relative humidity of the air in the morning? Imagine: 50g 10g
6. 6. Relative Humidity Example PART 2: ► As the temperature increased in the afternoon, the maximum amount of vapour that could be contained in the air now increased to 75g/m3. Given that the actual amount of vapour in the air remained the same as before, what was the relative humidity of the air now?
7. 7. Relative Humidity Imagine: 75g 50g 50g RH: 20% 10g RH: 13.33% 10g
8. 8. Relative Humidity Instrument used: Measured using a sling psychrometer:
9. 9. Relative Humidity Terms: ► Saturation Point  The point when the air can no longer absorb any more water.  Relative humidity is at 100% ► Dew Point Temperature  The temperature when saturation point is reached.  Condensation occurs
10. 10. Relative Humidity Imagine: 50g 50g RH: 20% 10g RH: 100% 10g
11. 11. PitStop 3 Q1 and 2
12. 12. 1) Describe and explain the relationship between temperature and relative humidity. When temperature increases, relative humidity decreases. This is because:  as the temperature rises, the surrounding air will expand.  Thus, if the amount of water vapour in the air remains the same, the maximum capacity of the air to hold water vapour will now increase, causing the relative humidity level to drop.
13. 13. Clouds
14. 14. Clouds Evaporation Water converts to vapour as it evaporates and rises up into the atmosphere.
15. 15. Clouds Condensation Water vapour will condense on tiny particles in the air to form water droplets known as condensation nuclei.
16. 16. Clouds Cloud formation: A cloud is essentially a huge mass of water droplets or ice crystals suspended in the atmosphere.
17. 17. Q: Are fogs and clouds the same? There is no basic difference between a fog and a cloud. A fog is a cloud in contact with the ground. They are caused by a cold current of air from above striking down upon the warmer surface of the land or water.
19. 19. No two clouds are exactly alike, and they are always changing their shape. The reason we have different types of clouds is that clouds formation takes place at different heights and temperatures.
20. 20. Nimbostratus Cumulonimbus Cirrus
21. 21. Clouds ► High-level clouds: Cirrus clouds  Appear 6 kilometres in the sky and as white lines or streaks.  Do not produce rain and are usually made up of ice. ► Mid-level clouds: Altocumulus clouds  White puffy clouds with darker areas underneath  Around 4 kilometres high and often indicate an approaching storm.
22. 22. Clouds ► Low-level clouds: Stratus clouds  Often dense and dark or cottony white clumps in blue skies.  Appears at around 2 kilometres high. ► Storm clouds: Cumulus clouds  Produces thunderstorms  Are large white and dark grey, extending in height from 1 to 7 kilometres above the Earth’s surface.
23. 23. PitStop 3 Question 3
24. 24. Rainfall and Precipitation
25. 25. Precipitation ► Refers to waster in any form that falls from the atmosphere. ► Includes hail, snow, sleet etc. ► Classification  High rainfall: > 1,500mm  Low rainfall: < 250mm
26. 26. The Hydrological Cycle 2. CONDENSATION 3. PRECIPITATION 1. TRANSPIRATION 1. EVAPORATION 4. RUNOFF Land Land 5. STORAGE
27. 27. Precipitation Measured using a rain gauge:
28. 28. Convectional Rain
29. 29. Storm Clouds Gathering
30. 30. Convectional Rain 1. Occurs when the Earth’s surface is intensely heated by the sun. 2. The air becomes unstable causing it to expand and rise rapidly. 3. As it expands, it loses heat and cools. When it reaches dew point temperature*, condensation occurs. 4. Cumulonimbus clouds are formed. 5. When the water droplets become too large and heavy, they fall to the ground. * The temperature when saturation point is reached.
31. 31. Relief Rain
32. 32. Relief Rain Key Terms: ► Windward side  On the side facing the wind ► Leeward  On or toward the side sheltered from the wind
33. 33. Relief Rain Windward Side Leeward Side
34. 34. Relief Rain
35. 35. Relief Rain ► Relief rain occurs when moist air is forced to rise over physical barriers such as a mountain range. ► As the air rises over the windward side of the mountain range, altitude increases and temperature falls. ► Condensation occurs as the moist air reaches past saturation point, hence resulting in heavy rainfall along the windward mountain ranges.
36. 36. Relief Rain ► Over at the leeward side, the clouds passing through have shed most of their moisture. ► Hence, the leeward side receives little to no rainfall.
37. 37. PitStop 3 Q4, 5 and 6
38. 38. 4. Is convectional or relief rain more likely to occur in SG? Explain your answer. Ans: Convectional Rain – Temperatures are regularly high which allows for the intensive heating of the earth’s surface and therefore higher rates of evaporation. – Also, there are no landforms such as a mountain range which forces warm, moist air to rise – a condition necessary for the formation of relief rain.
39. 39. 5. a) Describe the climate of Osaka Questions to consider: ► Is the weather constant throughout the year? Are there any variations? ► Are there any months whereby the weather is cooler or warmer? ► Is the rainfall constant throughout? Is there a co-relation between rainfall and temperature? ► Is the relative humidity constant throughout?