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Atmospheric Moisture

Atmospheric Moisture. Atmospheric Moisture. State Changes of Water Humidity Adiabatic Cooling What Makes Air Rise? Atmospheric Stability. Terms. Adiabatic Without the transfer of heat Exothermic Heat-releasing Endothermic Heat-absorbing. Terms. Humidity

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Atmospheric Moisture

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  1. Atmospheric Moisture

  2. Atmospheric Moisture • State Changes of Water • Humidity • Adiabatic Cooling • What Makes Air Rise? • Atmospheric Stability

  3. Terms • Adiabatic • Without the transfer of heat • Exothermic • Heat-releasing • Endothermic • Heat-absorbing

  4. Terms • Humidity • How much water vapor the air is holding • Parcel • A volume of air assumed to have the same properties throughout (temp, humidity, etc.) • Assumed to behave independently of surrounding (ambient) air • Environmental lapse rate (ELR)

  5. Terms • Environmental lapse rate (ELR) • Rate at which temperature drops with increasing altitude in the troposphere • Variable, but average = 6.5°C/km

  6. State Changes of Water PSCI 131: Atmospheric Moisture

  7. Water’s State Changes PSCI 131: Atmospheric Moisture • Transfer of heat between water molecules & their surroundings • Either endothermic or exothermic • Heat source: reradiation of solar energy from ground

  8. PSCI 131: Atmospheric Moisture: Water’s State Changes Blue arrows: exothermic Red arrows: endothermic

  9. Water’s State Changes PSCI 131: Atmospheric Moisture • Why does sweating cool you off? • Sweat evaporates (endothermic), absorbing heat from your skin

  10. Humidity PSCI 131: Atmospheric Moisture

  11. Humidity PSCI 131: Atmospheric Moisture • Evaporation from Earth’s surface (especially oceans) adds water vapor to the air • A given mass of air at a given temperature can only hold so much water in vapor form • If this limit is exceeded, excess vapor condenses to liquid

  12. Ways of Expressing Air’s Humidity PSCI 131: Atmospheric Moisture: Humidity • Relative humidity • Dewpoint temperature

  13. Relative Humidity PSCI 131: Atmospheric Moisture: Humidity • Saturation content • Maximum amount of vapor an air parcel can hold • Controlled by air’s temperature

  14. Relative Humidity PSCI 131: Atmospheric Moisture: Humidity • Vapor content • How much vapor the parcel is actually holding • RH = vapor content / saturation content

  15. Relative Humidity PSCI 131: Atmospheric Moisture: Humidity • Relative humidity will change if either of the following happens: • Temperature changes (more common cause) • Vapor content changes

  16. Relative Humidity PSCI 131: Atmospheric Moisture: Humidity • Example • Parcel’s vapor content is 10g (given) • Parcel temp: 25 degrees C • Therefore, saturation content is 20g (from table) • RH = 10g/ 20g = 50%

  17. Relative Humidity PSCI 131: Atmospheric Moisture: Humidity • Example (cont.) • Parcel’s vapor content changes to 14g • Parcel temp remains 25 degrees C • Therefore, saturation content is still 20g • RH = 14g/ 20g = 70% • RH has risen because vapor content has risen

  18. Relative Humidity PSCI 131: Atmospheric Moisture: Humidity • Example (cont.) • Vapor content is still 14g • Parcel cools to 20 degrees C • New saturation content is 14g • RH = 14g/ 14g = 100% • RH has risen because temp has fallen • Air is saturated

  19. Relative Humidity: Summary PSCI 131: Atmospheric Moisture: Humidity • Higher vapor content = higher RH • Lower temp = lower sat. content = higher RH

  20. Relative Humidity: Summary PSCI 131: Atmospheric Moisture: Humidity • RH highest when temp is lowest • & vice versa

  21. Dewpoint Temperature PSCI 131: Atmospheric Moisture: Humidity • Temp at which air parcel is saturated (100% RH) • More vapor in parcel (more humid) = higher dewpoint

  22. Adiabatic Cooling PSCI 131: Atmospheric Moisture

  23. PSCI 131: Atmospheric Moisture: Adiabatic Cooling Adiabatic Cooling Air parcel rises, pressure drops, air cools (no heat energy transferred).

  24. PSCI 131: Atmospheric Moisture : Adiabatic Cooling Adiabatic Cooling Rates • Dry adiabatic lapse rate (DAR): 10° C /km • If RH less than 100% • Wet adiabatic lapse rate (WAR): 5-9° C /km • If RH equals100% • Depends on vapor content • Always less than DAR

  25. PSCI 131: Atmospheric Moisture: Adiabatic Cooling Condensation Level • Altitude at which rising, cooling air’s temp reaches dew point and condensation begins

  26. PSCI 131: Atmospheric Moisture: Adiabatic Cooling Condensation Level

  27. PSCI 131: Atmospheric Moisture: Adiabatic Cooling Condensation Level • Several things are true when air has risen to its condensation level: • Air has cooled to its dewpoint • RH = 100% • Condensation can occur (usually as clouds)

  28. PSCI 131: Atmospheric Moisture: Adiabatic Cooling Adiabatic Lapse Rates vs Env Lapse Rate • Adiabatic lapse rates (WAR and DAR): rates at which air parcel cools as it rises • Env lapse rate (ELR): rate at which temperature around parcel decreases with altitude

  29. What Makes Air Rise? PSCI 131: Atmospheric Moisture

  30. PSCI 131: Atmospheric Moisture: What Makes Air Rise? Four Processes • Orographic Lifting

  31. PSCI 131: Atmospheric Moisture: What Makes Air Rise? Four Processes Frontal wedging

  32. PSCI 131: Atmospheric Moisture: What Makes Air Rise? Four Processes Convergence

  33. PSCI 131: Atmospheric Moisture: What Makes Air Rise? Four Processes Convective Lifting

  34. Atmospheric Stability PSCI 131: Atmospheric Moisture

  35. PSCI 131: Atmospheric Moisture: Atmospheric Stability What Is It? • Balance between DAR, WAR, and ELR • How likely a given air parcel is to rise, and how far up it will go • The “weather maker”

  36. PSCI 131: Atmospheric Moisture: Atmospheric Stability Three Combinations • Absolute stability • Absolute instability • Conditional instability

  37. PSCI 131: Atmospheric Moisture: Atmospheric Stability Adiabatic Lapse Rates vs Env Lapse Rate • Adiabatic lapse rates (WAR and DAR): rates at which air parcel cools as it rises • Env lapse rate (ELR): rate at which temperature around parcel decreases with altitude

  38. PSCI 131: Atmospheric Moisture: Atmospheric Stability Absolute Instability • ELR > DAR > WAR • Surrounding air cools faster than parcel, so parcel is always warmer • Parcel “wants” to keep rising • Think of a hot-air balloon

  39. PSCI 131: Atmospheric Moisture: Atmospheric Stability Absolute Instability

  40. PSCI 131: Atmospheric Moisture: Atmospheric Stability Absolute Instability • Weather effects • Lots of condensation • Heavy clouds • Lots of precip • Storms

  41. PSCI 131: Atmospheric Moisture: Atmospheric Stability Absolute Stability • DAR > WAR > ELR • Rising parcel cools faster than surrounding air, so parcel is always colder • “Wants” to descend

  42. PSCI 131: Atmospheric Moisture: Atmospheric Stability Absolute Stability • Will only rise if forced to • Orographic lifting, frontal wedging, etc. • A “cold-air balloon”

  43. PSCI 131: Atmospheric Moisture: Atmospheric Stability Absolute Stability

  44. PSCI 131: Atmospheric Moisture: Atmospheric Stability Absolute Stability • Weather effects • Fair weather • Light to moderate clouds and precip • If forced above condensation level • No storms

  45. PSCI 131: Atmospheric Moisture: Atmospheric Stability Conditional Instability • DAR > ELR > WAR • Rising parcel cools faster than surrounding air while RH is less than 100% and air is cooling at DAR • Parcel always cooler while below condensation level • Must be forced to rise

  46. PSCI 131: Atmospheric Moisture: Atmospheric Stability Conditional Instability • Surrounding air cools faster than parcel once RH = 100% • Parcel always warmer above condensation level • Think of a cold-air balloon that is forced to rise to condensation level, at which it changes into a hot-air balloon

  47. PSCI 131: Atmospheric Moisture: Atmospheric Stability Conditional Instability

  48. PSCI 131: Atmospheric Moisture: Atmospheric Stability Conditional Instability • Weather effects • Fair weather • Heavy clouds and precip/storms if forced above condensation level

  49. End of Chapter

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