Exercise 14 - Pg 203 Atmosphere and Moisture

3 Forms of Water Three forms or phases - solid liquid, and gas. It is still water; just in different phases. Solid - Ice, Liquid - Water, Gas - Vapor When a solid becomes a liquid heat is absorbed, and when a liquid becomes a gas even more heat has to be absorbed. The opposite is true when a gas becomes a liquid heat is released, and when a liquid becomes solid.

Solid Ice Liquid Water Gas Vapor Melts Heat Absorbed 80 Cal. Boils Heat Absorbed 600 Cal. Condenses Heat Released 600 Cal. Freezes Heat Released 80 Cal. Deposition (G→S) 680 Cal. Released Sublimation (S→G) 680 Cal. Absorbed

Water Vapor Composes 2-3% of the gas in the atmosphere. Responsible for all precipitation that occurs. Also helps to heat the atmosphere. All three occur in balance with each other. This only applies to water. This is how clouds form.

Water Vapor Capacity See chart on page 206 This is the amount of water the atmosphere can hold at a specific temperature. As the temperature increases the amount of moisture that can be held increases. This is because a gas contains a large amount of space between molecules.

Examples Use chart page 206. This is not a linear relationship. 5 O C = 5 g of moisture/1kg of air 10 O C = 7g of moisture/1kg of air 40 O C = 47g of moisture/1kg of air

Relative Humidity Is a ratio of the amount of water vapor / the capacity of the at a specific temp X 100 This alone does not give an accurate indication of the amount of vapor in the air. Any Percent = part over whole X 100

Determine the Relative Humidity The Atmosphere is at a temp of 15 O C, and contains 2g of water. From the chart pg 215; we know the maximum it can hold 10g. What temperature we are calculating

What is the RH of air at 25 O C that contains 5g of water? What is the RH of air at 5 O C that contains 5g of water? The RH is lower in the summer day. The amount of H 2 0 is higher because the overall temperature of the atmosphere is higher.

What’s going on? As temperature decreases relative humidity increases. Why because colder air has less volume thus it takes less moisture to saturate the air. What happen when we get 100% saturation? Condensation This is why grass is damp during early summer mornings…

How to measure Relative Humidity Use pg 209 We use Sling psychrometer We have 2 bulbs one wet and one dry. This works on the process of evaporation. The wet bulb pulls heat from the thermometer. The greater the difference in temperature the lower the relative humidity. The lower the RH the less saturated the atmosphere is.

Example # 1 Difference between wet and dry temp = 8 O C O C= 5/9( O F-32) Wet temp = 55 O F Dry temp = 70 O F

How to read the chart pg 209 Find dry bulb temp on left side. (21°C) Find the difference between wet and dry across top. (8°C) Connect both and where they meet is the Relative Humidity. Note: If your exact temp is not on the chart round up. Our example Relative Humidity is 40%

Dew Point Temperature This is the temperature that the air has to be lowered to reach 100% saturation. This is when Relative Humidity = 100% or when Content = Capacity. Once this temperature is reached condensation occurs. This is why a cold glass sweats.

Determining Dew Point Temperature We know the wet and dry temperatures from the last example dry temp = 21 ° C, and wet temp =13 ° C. Difference between temps = 8 ° C Use the chart on page 210 to determine the dew point. We follow the line and we determine the dew point is between O C

What does this mean? If we lower the temperature to the Dew Point Temperature then we would be at the saturation point. How can this be done and what happens? We will find out…

Adiabatic Process A change in temperature occur due to changes in pressure of a gas As a gas compresses it’s temperature increases. This is because as we compress the gas, the gas molecules bounce off each other more frequently causing temperature to increase. As a gas expands temperature decreases because the gas molecules have more room to move around and therefore collide less frequently. This is how refrigerators and air conditioners work.

How clouds form Hot Air Surface of the Earth Cold Air Hot air is lighter than cold air. So hot air travels up replacing cold air. The hot air cools by expansion. When the dew point is reached condensation occurs forming clouds.

Chapter Summary pg 219 Questions 1 – 4 & 6