Atmospheric Moisture

1. Atmospheric Moisture Chapter 4

2. Learning Targets • LT #1: I can describe the movement of water through the hydrologic cycle. • LT #2: I can summarize the processes by which water changes from one state to another in terms of kinetic energy and molecular motion. • LT #3: I can define latent heat and describe what happens to latent heat during each phase change. • LT #4: I can distinguish among the various types of water-vapor content in the air. • LT #5: I can identify the two ways in which relative humidity can be changed and list three major ways in which air temperatures change in nature. • LT #6: I can understand the importance of dew point temperature.

3. The Hydrologic CycleLT #1 • Hydrologic Cycle: the circulation of Earth’s water supply • The cycle illustrates the continuous movement of water from: • The oceans to the atmosphere • The atmosphere to the land • The land back to the sea

4. The Water CycleLT #1

5. The Importance of WaterLT #2 • Water Vapor: gas that can change from one state of matter (solid, liquid or gas) to another at the temperatures and pressures experienced on Earth • The processes that change that state of matter of water include: • Evaporation (liquid to gas) • Condensation (gas to liquid) • Melting (solid to liquid) • Freezing (liquid to solid) • Sublimation (solid to gas) • Deposition (gas to solid)

6. States of MatterLT #3

7. Atmospheric MoistureLT #4 Hypothetical jar containing pure water with a flat surface and an overlying volume that initially contains no water vapor.

8. Vapor PressureLT #4 • Water vapor contributes to the total pressure exerted by the atmosphere. • Vapor pressure is the part of the total atmospheric pressure due to water vapor. • Depends mostly on changes in density (abundance of water molecules), but is also slightly influenced by temperature • The higher the temperature of the atmosphere, the more vapor pressure it can withstand. • Warmer air can hold more water vapor before becoming saturated.

9. Water SaturationLT #4 • Saturation-the maximum amount of water that can exist in the atmosphere as vapor. • Saturation produces a balance between the number of water molecules leaving the surface of the water and the number of returning. • Saturation vapor pressure- the vapor pressure of the atmosphere when it is saturated. (vapor pressure is part of the total atmospheric pressure due to water vapor) • Because the saturation vapor pressure is temperature dependent, at high temperatures more water vapor is required for saturation to occur.

10. Saturation vs. Temperature.

11. HumidityLT #4 • The general term used to describe the amount of water vapor in the air • Methods to express humidity • Absolute humidity: density of water vapor • Specific humidity: the mass of water vapor existing in a given mass of air (# of g of water vapor per kg of air) • Mixing ratio: the mass of water vapor in a unit mass of dry air • Vapor pressure: part of the total atmospheric pressure due to water vapor. • Relative humidity: ratio of the air’s actual water vapor content compared with the amount of water vapor required for saturation at a specific temperature • Dew Point: temperature needed in order for air to reach saturation.

12. Dew PointLT #6 • The temperature to which the air must be cooled to become saturated • An expression of water vapor content • The closer the air temperature is to dew point indicates lots of water in the air…humid day

13. Relative HumidityLT #4 • RH: relates the amount of water vapor in the air to the maximum possible at the current temperature • More water vapor exists in warm air than in cold air • RH = (specific humidity/saturation specific humidity) x 100% • RH depends on both moisture content and the air temperature • Adding moisture to the air while keeping the temperature constant increases the relative humidity • Removing moisture lowers the relative humidity

14. Relative HumidityLT #5 • In (a), the temperature is 14oC and has a saturation specific humidity of 10g of water vapor per kilogram of air. If the actual specific humidity is 6g the RH is 60% • In (b) the specific humidity is still 6g, but the higher temperature of 25C results in a greater saturation specific humidity. • The RH is less in (a) even though the amount of water vapor is the same

15. Relative HumidityLT #5

16. Relative HumidityLT #5

17. Relative Humidity PracticeLT #5 • If a parcel of air at 25oC contains 10 grams of water vapor per kilogram of air, what is its relative humidity? • Step One: determine how much water 25oC can hold. (check chart on page 105) • 20 grams • Step Two: take the amount of water vapor in the air and divide it by the amount it can hold • 10g/20g = 50% relative humidity

18. Relative Humidity PracticeLT #5 • If a parcel of air at 35oC contains 5 grams of water vapor per kilogram of air, what is its relative humidity? • Step One: determine how much water 35oC can hold. • 35 grams • Step Two: take the amount of water vapor in the air and divide it by the amount it can hold • 5g/35g = 14% relative humidity

19. Relative Humidity PracticeLT#5 • If a parcel of air at 15oC contains 5 grams of water vapor per kilogram of air, what is its relative humidity? • Step One: determine how much water 15oC can hold. • Step Two: take the amount of water vapor in the air and divide it by the amount it can hold

20. Relative Humidity PracticeLT #5 • If a parcel of air at 10oC contains 5 grams of water vapor per kilogram of air, what is its relative humidity? • Step One: determine how much water 15oC can hold. • Step Two: take the amount of water vapor in the air and divide it by the amount it can hold

21. Where in the US is it most humid? Where is it most dry?

22. Water Vapor Content ChangesLT #5 • When water vapor content of air remains at a constant level, a decrease in air temperature results in an increase in relative humidity and an increase in temperature causes a decrease in relative humidity • In nature, there are three major ways that air temperature changes to cause changes in RH • Daily changes in temperature • Temperature changes that result as air moves horizontally from on location to another • Changes caused as air moves vertically in the atmosphere.

23. Relative Humidity changes throughout the dayLT #5

24. Measuring HumidityLT #4 • Amount of coolness that takes place is directly proportional to the dryness in the air • Dryer the air, the greater the cooling • Larger the difference between the wet and dry bulb the lower the humidity; the smaller the difference the higher the humidity • If the air is saturated, no evaporation will occur and the 2 thermometers will have identical readings

25. Measuring Humidity • Sling Psychrometer: instrument for measuring humidity • Wet Bulb Depression: depends on the moisture of the air • Dry Bulb Depression: ambient air temperature

26. The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the dew point temperature.

27. The value corresponding to the row for the dry bulb temperature and the column for the wet bulb depression yields the relative humidity.