Aim: What is Humidity and Dew Point? Do Now: What instruments do scientist use to measure air pressure? Explain why in the daytime a breeze comes from the ocean. Notepack 36

Where does water go after it evaporates? Water covers 70% of the surface of the Earth. Half the planet is in sunlight at any given time. Evaporation happens because a lot of energy from the Sun is put into the water. This causes the liquid water to change into water vapor (gas). Water vapor has a low density so it rises into the air.

What happens to the water in the air? Warm air can hold more water vapor than cold air. The higher the temperature the air is, the more water it can hold or wetter it is. The cooler the air, the less water it can hold or drier it is.

Humidity Humidity is the amount of water vapor in the air. When the air can hold no more water it is said to be saturated (100% full) Hot air can hold more water than cold air. Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Example: If the air at a certain temperature can hold 4 grams of water but it is only hold 2 grams; than the relative humidity is 50% - 2 grams / 4 grams

What happens if there is too much water in the air? Because evaporation is always happening and the air is always heating up and cooling down, the amount of water the air can hold is always changing. The point at which the most amount of water that the air cannot hold is called the dew point or saturation point.

Dewpoint As air cools, it cannot hold all the water vapor in it, so the air begins to pushed out water vapor into liquid drops. The dewpoint is the temperature when condensation (gas  liquid) occurs in the air.

Sling Psychrometer A double thermometer – One is ‘normal’ = dry bulb – One is covered with a wet piece of cloth = wet bulb When water evaporates, it removes heat. Therefore, the wet bulb will always record a lower temperature than the dry bulb. Dry Bulb Wet Bulb

Calculating Dew Point Subtract the wet bulb temperature from the dry bulb temperature and record this value. – You should now have 3 values recorded: Dry bulb ____ °C Wet bulb ____ °C Difference ____ °C

Calculating Dew Point, cont. Open your ESRT to page 12. Locate the dry bulb temperature on the vertical (left) axis of the Dew Point Temperature Chart. Locate the difference between the wet and dry bulb temperature on the horizontal (top) of the chart. Locate where the two values intersect: this is the dew point.

The “Dry Bulb” Don’t let it fool youis just a thermometer. It measures the air temperature. 20°C

The “Wet Bulb” Has a little wet sponge tied to the bottom. Gets cool when water evaporates. 12°C

A Dry Day… A lot of moisture will evaporate. The wet bulb will be a lot cooler than the dry bulb. 8°C 20°C Difference between wet bulb & dry bulb is 12 °C. 20°C14°C 12°C

A Humid Day… A little bit of moisture will evaporate. The wet bulb will not be much cooler than the dry bulb. 14°C 20°C Difference between wet bulb & dry bulb is 6 °C.

Page 12 of your Handy Dandy Earth Science Reference Tables Warning #1: Be sure to READ the correct chart: DPT or RH Warning #2: Dew Point Temperature IS NOT “Difference between wet bulb and dry bulb”. Warning #3: The wet bulb temp IS NOT the DPT.

Dry-bulb temperature is your air temperature. 20 °C

14 °C

4 °C

Subtract (the difference) between the dry bulb and wet bulb

14°C 20°C 20-14=6

16°C 18°C 18-16=2

8°C 20°C 20-8=12 Put it all together Relative Humidity = 11%

10°C 14°C 14-10=4 Relative Humidity = 60%

10°C 14°C 14-10=4 Dew Point = 6°C The Dew Point Chart works the same way

Try These. Dry Bulb Wet Bulb R Humidity DPT 26°C 20 °C 6 °C 5 °C 12 °C 8 °C 18 °C 17 °C 14 °C 0 °C -3 °C 57% °C 17 °C 86% °C 4 °C 57% °C 4 °C 91% °C 16 °C 100% °C 14 °C 45% °C -9 °C

More Difficult. Dry Bulb Wet Bulb R Humidity DPT 21°C19°C 11°C 6 °C 17 °C 17°C 2°C -3 °C 9°C 6.5 °C 70°F 60 °F 82% °C 18 °C 45.5% °C -.5 °C 100% °C 17 °C 20% °C -17 °C 69% °C 3.5 °C 52% °C (50 °F) 11 °C (50 °F) °C 21 °C °C 15 °C