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Understanding Moisture in Weather and Climate

Learn about the hydrologic cycle and the role of water vapor in the atmosphere. Explore concepts like saturation, dew point, and relative humidity to understand how moisture levels impact weather conditions and human comfort.

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Understanding Moisture in Weather and Climate

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  1. MOISTURE Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University 1

  2. Ahrens, Ch. 4

  3. Hydrologic Cycle

  4. Water in the atmosphere • All phases are present in the atmosphere • Gas • Water vapor in the air • Liquid • Cloud droplets, fog, drizzle, rain • Solid • Ice crystals (in clouds), snow

  5. Hydrologic Cycle • Cycle that the water goes through as it moves among the earth, atmosphere and oceans and transforms from liquid to water vapor and back again.

  6. Vapor and Humidity

  7. Water Vapor in the Atmosphere • The amount of water vapor in the atmosphere varies dramatically • 0% to 4% by mass • The term humidity is used to describe the amount of water vapor in the air • Humidity is an expression of the atmosphere’s level of saturation

  8. Definition • Saturation – when the amount of water vapor in the air is the maximum possible at the current temperature and pressure • Condensation and evaporation are in equilibrium • Warmer air can hold more water vapor without becoming saturated • Colder air can hold less water vapor

  9. Ways to describe the amount of vapor in the atmosphere • Vapor Pressure • Pressure exerted by the water vapor present in the air (0 to 40 mb) • Dew Point • Temperature that the air would have to be cooled in order to obtain saturation at constant pressure • Relative Humidity • Ratio of actual amount of water vapor to maximum amount possible at T and p

  10. Dew Point Temperature (Td ) • Temperature to which air must be cooled for saturation to occur at constant p • Td is always ≤ T • It depends solely on the amount of water vapor actually present in the air • It can be used to forecast minimum temperatures • When T = Td, then relative humidity = 100%

  11. Relative Humidity (RH) • Ratio of amount of water vapor in air to amount required for saturation

  12. Relative Humidity (RH) • Ratio of amount of water vapor in air to amount required for saturation

  13. Relative Humidity (RH) • Ratio of amount of water vapor in air to amount required for saturation CALCULATED FROM DEW POINT

  14. Relative Humidity (RH) • Ratio of amount of water vapor in air to amount required for saturation CALCULATED FROM THE TEMPERATURE (WARMER AIR CAN HOLD MORE VAPOR WITHOUT BECOMING SATURATED)

  15. Relative Humidity (RH) • Ratio of amount of water vapor in air to amount required for saturation

  16. Relative Humidity (RH) • Ratio of amount of water vapor in air to amount required for saturation VAPOR PRESSURE IS A FUNCTION OF DEW POINT

  17. Relative Humidity (RH) • Ratio of amount of water vapor in air to amount required for saturation SATURATION VAPOR PRESSURE IS A FUNCTION OF TEMPERATURE

  18. Relative Humidity (RH) • Ratio of amount of water vapor (VP) in the air to amount required for saturation (SVP) • Usually expressed in terms of percentage (%) • Indicates only the “relative” level of saturation – not the actual amount of vapor in the air

  19. Relative Humidity (RH) • To increase relative humidity

  20. Relative Humidity (RH) • To increase relative humidity • Increase the dew point (VP)

  21. Relative Humidity (RH) • To increase relative humidity • Increase the dew point (VP) • Decrease the temperature (SVP)

  22. Variations in Dew Point and RH

  23. VARIATIONS IN RH AND Td ACROSS TWO DIFFERENT AIRMASSES Fig. 4-8, p.85

  24. POLAR AIRMASS Cold Can’t hold much vapor before becoming saturated Fig. 4-8, p.85

  25. TROPICAL AIRMASS Hot Can hold alot of vapor without becoming saturated Fig. 4-8, p.85

  26. DEW POINT MUCH LOWER IN POLAR AIRMASS DEW POINT MUCH HIGHER IN TROPICAL AIRMASS Fig. 4-8, p.85

  27. THERE IS MORE WATER VAPOR IN THE DESERT AIR THAN IN THE COLD POLAR AIR Fig. 4-8, p.85

  28. POLAR AIRMASS HAS 100 % RH (SATURATED) TROPICAL AIRMASS HAS VERY LOW RH Fig. 4-8, p.85

  29. RH and Human Comfort

  30. Humans perceive different temperatures depending upon atmospheric conditions Our internal temperature is 98.6 °F There is a constant heat exchange between our skin and the environment The temperature we perceive depends on our ability to maintain our body temperature If we gain more heat than we lose, we feel hot If we lose more heat than we gain, we feel cold Air temperature, wind and humidity can affect this ability RH and Human Comfort

  31. Warm Weather Concerns • An important source of cooling is the evaporation of perspiration (conversion of body heat into latent heat) • If it is humid, surface water evaporates slowly • We feel hot • If it is dry, surface water evaporates easily • We cool down • Heat Index • Combines air temperature and relative humidity to determine how hot it actually feels (apparent temperature)

  32. Fig. 4-9, p.87

  33. Fig. 4-9, p.87

  34. Fig. 4-9, p.87

  35. Heat Cramps: caused by rapid loss of salt & water • Heat Exhaustion: fatigue, headache, nausea, fainting due to excessive water loss & increasing body temperature • Heat Stroke: complete failure of circulatory functions occurs when body temp rises above 106°F; death may occur

  36. Table 4-1, p.87

  37. Review

  38. Hydrologic Cycle • Definition • Saturation • Definition • Vapor pressure, relative humidity, and dew point • Definitions • How they vary • Heat index • Definition

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