Humidity & Our Weather

1. Humidity & Our Weather 13.2 Energy and Water (p431-433)

2. The Water Cycle • Evaporation and condensation occur all the time • About 23% of all incoming solar radiation causes liquid water to evaporate into water vapour • This water vapour collects in the air above our earth • The movement of air into cooler regions (or the lowering of air temperature) causes the water vapour to condense into water droplets, forming clouds.

3. How does this Affect our Weather? • The movement of air into cooler regions (or the lowering of air temperature) causes the water vapour to condense into water droplets, forming clouds. • If the air is cold enough, ice crystals form instead of liquid droplets. If these crystals become large enough, they fall to the ground as either snow, sleet, or hail.

4. Small Group Activity Look through the handouts of information provided, and answer the following questions into your binder: • What is the dry bulb temperature? Give an example of when it would be important to consider the dry bulb temperature? • What is the wet bulb temperature? Give an example of when you might experience this in everyday life. • a) What is the Dew Point temperature? b) What does a high relative humidity determine about the dew point? c) What is one profession for which determining the dew point is important, and why? 4) When would the wet bulb and the dry bulb temperatures be the same?

5. Definitions • Humidity – The amount of water vapour in the air. • Absolute Humidity – The actual amount of water vapour in the air, expressed in units such as grams of water vapour per kilogram of air. • Relative Humidity – The percentage of water vapour in the air compared with the amount of water vapour that the air would contain if it were saturated (“full”).

6. Definitions • Dry Bulb Temperature: Usually referred to as air temperature; the temperature of air measured a thermometer freely exposed to the air but shielded from radiation and moisture. It does not indicate the amount of moisture in the air. • Wet Bulb Temperature: The lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one’s skin is wet and is exposed to moving air. It is an indication of the amount of moisture in the air. • Dew Point : The temperature at which water vapour starts to condense out of the air (the temperature at which the air becomes completely saturated). • If the de point temperature is close to dry air temperature – Relative Humidity high • If the dew point is well below the dry air temperature – Relative Humidity low  The wet bulb temperature will always be lower than the dry bulb temperature, but they will be identical with 100% relative humidity (when the air is saturated).

7. Relative Humidity “The percentage of water vapour in the air compared with the amount of water vapour that the air would contain if it were saturated” Relative Humidity = Actual Humidity x 100% Saturated Humidity at that temperature

8. Relative Humidity Practice Problem(p433) The Outdoor temperature is -12°C and relative humidity is 75%. How much water would the air contain? • How many grams of water would saturated air at this temperature contain? (Table 13.4; p431) 1.53 grams of water (Absolute Humidity) • Therefore, the air at this temperature and humidity would contain 1.15 grams of water per 1Kg of air  1.53g x 0.75 (because 75% of 1.53g) = 1.15g The furnace heats the air to 20°C. What is the relative humidity now? - At 20°C, air can hold 15.0g of water per 1.0Kg of air. 1.15g x 100% = 7.7% Relative Humidity 15.0g - Therefore, the relative humidity is 7.7%.

9. Relative Humidity Worksheet TipsReading Guide 13.2 (Part II) Q#7 • Air Temperature (°C) - Dry Bulb Temperature • Saturated Moisture (g/kg) – Absolute Humidity (p431 in Table) • Dew Point(°C) – Wet Bulb Temperature • Dew Point Moisture (g/kg) – Actual Humidity (p431 in Table) • Relative Humidity (%) – Actual Humidity x 100 Relative Humidity