Aim: What is relative humidity? Do Now: Answer the following in your notebooks; What do you think of when you hear it is humid out? What type of pressure does this humidity make? Is it usually humid in the winter?

What do you think of when you hear it is humid out? What type of pressure does this humidity make? 3) Is it usually humid in the winter? it’s wet, it feels warmer, it’s uncomfortable It makes low pressure because it is wet No

a) Humidity – is a measure of how much water vapor is in the air. 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.

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

(expressed as a percent (%)). 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??

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

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

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

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

How can you change the Relative Humidity of the air? 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?)

Example: Below, the parcel of air has a dew point temperature of 750F. IV. Dew Point Temperature (DP) - indicates when the air will be saturated with water vapor. (RH = 100%) - always lower than the air temperature Example: Below, the parcel of air has a dew point temperature of 750F. As the air temp. reaches the dew point temperature, what happens to the air? RH = 50% Air Temp. = 870F RH = 100% Air Temp. = 750F

- 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

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?

Example: VI. Calculating DP and RH: find the difference between dry and wet bulb temps. This is called the wet bulb depression. Use ESRT p.12 Example: A student measure the temperature of the air using a sling psychrometer and found that the Dry Bulb Temp. = 240C and the Wet Bulb Temp. = 200C. What is the wet bulb depression temperature? Relative humidity? Dew Point?

How can we find some of the missing information? Example: Dry bulb = 240C Wet Bulb = 200C Depression = ? RH = ? DP = ? 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. 40C 69% 18 C How can we find some of the missing information? 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.

Work on the other side of the worksheet you already have