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Understanding Relative Humidity and Dew Point

Learn how relative humidity and dew point affect weather conditions. Discover how to measure moisture using tools like a sling psychrometer. Understand the relationship between temperature, humidity, and dew point.

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Understanding 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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