L.O: SWBAT to determine dewpoint and relative humidity. Do Now: take your reference table, a PENCIL, and a scrap piece of paper (you can share (you only need half of the paper, must have 1 straight line) or grab one from the back by the printer.

When weather people know the relative humidity and the air’s dewpoint they can predict rain.

Dew Point Temperature Dew Point – The temperature at which air becomes saturated with water vapor.

When air is cooled to the dew point, condensation occurs. The dew point temperature depends on the amount of water vapor in the air. The closer the dew point gets to the temperature, the better the chance of precipitation. (*Important*)

Relative Humidity Relative Humidity – Compares the actual amount of water vapor in the air with the maximum amount of water vapor that the air can hold. (Tells you how much water vapor is in the air) Relative Humidity is expressed as a percent (%)

Relative Humidity is constantly changing Relative Humidity changes based on: 1) Temperature 2) The amount of water vapor

Relative Humidity & the Amount of H2O The Box can hold 20 molecules Relative Humidity & the Amount of H2O 20/20 15/20 10/20 5/20 100% 75% 50% 25%

Cold Air WARM AIR vs. Warm Air is Less Dense Cold Air is More Dense

Cold Air WARM AIR vs. Cold Air holds LESS Water Vapor Warm Air holds Warm Air is Less Dense Cold Air is More Dense Cold Air holds LESS Water Vapor Warm Air holds MORE Water Vapor

Relative Humidity & Temperature The amount of water vapor that air can hold depends on the temperature. Warm Air can hold more water vapor

When the relative humidity is near 100% , it rains When the relative humidity is near 100% , it rains! Because the air is saturated & it can’t hold anymore moisture!

Turn to ESRT pg 12. used to find dewpoint

This chart is used to calculate the relative humidity

this is a psychrometer: psychrometer is used to determine dew point and reletive humidity.

a Psychrometer is just two thermometers a Psychrometer is just two thermometers. One thermometer is called the “dry bulb”(can also be called air temperature), the other is called the “wet bulb”

How can we determine dewpoint temperatures? Step 1. WRITE OUT the wet bulb and the dry bulb temperatures. Step 2. Calculate the difference between wet bulb and dry bulb. (also known as the depression) Step 3. Take out your ESRT’s and turn to page 12

Step 4. Locate the dry bulb temperature reading on the left side of your chart. Step 5. Locate the difference between wet bulb and dry bulb on the top of your chart. Step 6. DRAW A LINE IN PENCIL TO DETERMINE THE DEW POINT!

1. If the dry bulb is 11oC and the wet bulb is 8oC. 2. What is the wet bulb temperature if dry bulb is 16oC and the dewpoint temperature is 9oC? 5 degrees C 12 degrees C

Relative Humidity You can determine the % Relative Humidity the SAME way you found the dewpoint temperature on the chart in the ESRT’s. What happens to the relative humidity as the air temperature approaches the dewpoint temperature? It increases!

Relative Humidity 1. What is the percent relative humidity if the Air Temperature is 20oC and the Wet Bulb is 11oC? 2. What is the percent relative humidity if the Dry-bulb temperature is 14oC and the Wet- bulb temperature is 14oC? 30% 100%

EXIT TICKET Fill in the chart EXIT TICKET Fill in the chart. Depression= difference between Wet&Dry bulb 4oC 24oC 72% 11oC 5oC 54% 17oC 15oC 20oC 74% 3oC 16oC 74.5%