1. Aim: What is relative humidity? Do Now: Answer the following in your notebooks; 1)What do you think of when you hear it is humid out? 2)Explain what type of pressure is outside and how you know. 3)When it is windy here, what does that tell you about the pressure around RVC?

2. 1)What do you think of when you hear it is humid out? 2)Explain what type of pressure is outside and how you know. 3)When it is windy here, what does that tell you about the pressure around RVC? it’s wet, its hot, it’s uncomfortable There is high pressure outside because it is happy weather (dry, calm, clear) The surrounding areas must have different (higher) pressures; wind blows from High --> Low pressure

3. I. Vocabulary a) Humidity – is a measure of how much water vapor is in the air. b) Parcel – a section or part of something. A parcel of air is a section of the atmosphere we are discussing.

4. - Warmer air can hold more water vapor than cooler air because. Cold air Warm air WHY? Which parcel air can hold the most water vapor? - Warm air expands and has more space between molecules to hold water vapor

5. II. Relative Humidity (RH) : describes “how full” the air is with water vapor (expressed as a percent (%)). - compares the size of the air to the amount of water in the air. c The cup represents the air’s size and the water represents the vapor filling the air. What is the R.H. of the cup??

6. What caused this parcel of air to change it’s size (expand)? - heat causes air to expand and allows it hold more water vapor. What happens to the RH as the air temperature changes?

7. - Saturation the air is 100% filled with water vapor. Cold air Warm air They have the same RH = 100% When saturated, which parcel of air will hold more water vapor? WHY? Which parcel of air has a higher relative humidity? Warm air is more spread out and can hold more water vapor

8. - When the Relative humidity = 100%, the air is Saturated and Condensation can occur (makes clouds and rain)

9. Take the worksheet from the back top bin and work on the side the says What is Humidity? only

10. III. Changing the Relative Humidity a. Water vapor enters or leaves the air. (evaporation or precipitation) b. The temperature changes. (changes the size of the air) How can you change the Relative Humidity of the air? (Think, what is the definition of RH?)

11. IV. Dew Point Temperature (DP) - indicates when the air will be saturated with water vapor. (RH = 100%) - always lower than the air temperature RH = 50% Air Temp. = 87 0 F Example: Below, the parcel of air has a dew point temperature of 75 0 F. RH = 100% Air Temp. = 75 0 F As the air temp. reaches the dew point temperature, what happens to the air?

12. - As air temperature gets closer to the dew point temperature… –the RH increases and there is better chance for precipitation. 1) At what time is the RH the highest? 2) At what time is RH the lowest? 3) When is the greatest chance of rain? 6am 3pm 6am

13. V. Measuring DP and RH: - Sling Psychrometer measure the RH and DP. -uses a dry bulb temperature and wet bulb temperature. -The wet bulb temperature is always lower. Why would the wet bulb thermometer always be at a lower temperature? What is the water doing?

14. Example: A student measure the temperature of the air using a sling psychrometer and found that the Dry Bulb Temp. = 24 0 C and the Wet Bulb Temp. = 20 0 C. What is the wet bulb depression temperature? Relative humidity? Dew Point? -find the difference between dry and wet bulb temps. This is called the wet bulb depression. -Use ESRT p.12 VI. Calculating DP and RH:

15. Example: Dry bulb = 24 0 C Wet Bulb = 20 0 C Depression = ? RH = ? DP = ? -Subtract the wet bulb from the dry bulb value. - Use the ESRT page 12 charts to look up the Dew Point Temperature and the Relative Humidity. When trying to determine Relative Humidity and Dew Point, create a list of the information you need, and the information you have to make solving the problem easier. 40C40C 18 C 69% How can we find some of the missing information?

16. Work on the other side of the worksheet you already have