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Unit 8 Lesson 5: Relative Humidity and Dew Point

Unit 8 Lesson 5: Relative Humidity and Dew Point. Stop and Think. What does it mean to you when someone says it’s humid out?. Saturation. When the air is holding as much moisture as it can, it is SATURATED The WARMER the temperature, the MORE moisture it can hold

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Unit 8 Lesson 5: Relative Humidity and Dew Point

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  1. Unit 8 Lesson 5: Relative Humidity and Dew Point

  2. Stop and Think • What does it mean to you when someone says it’s humid out?

  3. Saturation • When the air is holding as much moisture as it can, it is SATURATED • The WARMER the temperature, the MOREmoisture it can hold • The COLDER the temperature, the LESS moisture it can hold • If the temperature is lowered enough, then the air will become saturated

  4. Relative Humidity • Expresses how FULL of moisture the air is. • Compares how much moisture is in the air to how much the air COULDhold. • Air is SATURATED at 100% relative humidity

  5. With the same amount of water present: • The HIGHER the temperature goes, the LOWER the relative humidity becomes • The LOWER the temperature goes, the HIGHERthe relative humidity become

  6. Measuring Moisture • sling psychrometer • Two thermometers, one is dry called the “dry bulb” temperature • One is covered in a wick that has been moistened with water called the “wet bulb” temperature • Swing it around until the water from the wick evaporates so you can measure the “wet bulb” temperature. This temperature is always lower because of the evaporating water. • and charts on page 12 of your ESRT

  7. How to find relative humidity: • Locate dry-bulb reading on the left-hand side of chart • Subtract wet-bulb reading from dry-bulb reading • Locate this difference on the top of chart • Follow top column down until you meet dry-bulb temp. on left

  8. Dry-bulb is 14oC and wet-bulb is 9oC 14 - 9 = 5

  9. Dry-bulb is 28oC and wet-bulb is 25oC 28 - 25 = 3

  10. Dry-bulb is 10oC and wet-bulb is 10oC 10 - 10 = 0

  11. Humidity is 13% and dry bulb is 10. What is the wet bulb?

  12. Dew Point • The temperature to which the air must be lowered to become saturated • If temp. falls below dew point condensation occurs!!!! • Can also sublimate directly to a solid (think of frost on grass!)

  13. The more water vapor in the air, the higher the dew point • The less water vapor in the air, the lower the dew point

  14. How to find dew point: • Same as Relative Humidity…just make sure you use the DEW POINT CHART!!!!

  15. Dry-Bulb is 20oC and wet-bulb is 15oC 20 – 15 = 5

  16. Dry-Bulb is 0oC and wet-bulb is -2oC 2 - 0 = 2

  17. Dry-Bulb is 12oC and wet-bulb is 5oC 12 – 5 = 7

  18. To say that the relative humidity on a given day is 70% means that the air • Is composed of 70% water vapor • Holds 70% of its water vapor capacity • Contains 70 parts of water to 100 parts of dry air • Contains the same amount of water that it would at 70oF

  19. If the amount of water vapor in the air increases, then the dew point of the air will • Decrease • Increase • Remain the same

  20. As the air temperature rises, the relative humidity • Increases • Decreases • Remains the same

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