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Understanding Moisture Dynamics: Relative Humidity, Dew Point, and Precipitation

This guide explores key concepts in meteorology regarding moisture, including relative humidity, dew point, and cloud formation. It details how air temperature affects the amount of water vapor it can hold, the processes of state changes (freezing, evaporation, condensation), and the various types of precipitation, such as rain, snow, and hail. Additionally, it introduces ICAO lapse rates and their implications for cloud base and thermal dynamics. Perfect for glider pilots and meteorology students, this resource offers insights into humidity's role in weather phenomena.

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Understanding Moisture Dynamics: Relative Humidity, Dew Point, and Precipitation

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  1. GPS NGS: Meteorology 405.03 Moisture References:-A-CR-CCP-242/PT-005 Chapter 2 -Transport Canada Study and Reference Guide Glider Pilot - -FTGU pages 135-138, 146-147 Instructor 40 Minutes

  2. 405.02 Moisture • MTPs: • Relative Humidity and Dew Point • Changes of State • Cloud Formation • Precipitation • ICAO Lapse Rates

  3. MTP: Relative Humidity and Dew Point • Relative Humidity • Ratio of water present in air compared to the amount the same volume could hold if it were saturated • Dew Point • Temperature to which air must be cooled at constant pressure to become saturated

  4. MTP: Relative Humidity and Dew Point • The warmer the air, the more water vapour it can hold...Why? 1 kg air at 0 degrees = 5g water 1 kg air at30 degrees = 30g water

  5. MTP: Relative Humidity and Dew Point • Define Relative Humidity. • The ratio of water present in the air compared to the amount the same volume could hold if it were saturated • Define Dew Point • Temperature to which air must be cooled at constant pressure to become saturated

  6. MTP: Changes of State • Freezing • Evaporation • Melting • Sublimation • Condensation

  7. Freezing SOLID LIQUID Melting ICE WATER Evaporation Sublimation Sublimation Condensation GAS WATERVAPOUR

  8. MTP: Changes of State • What do you call a change of state from liquid to solid? • Freezing • From solid to gas? • Sublimation • From vapour to liquid? • Condensation

  9. Cloud Formation

  10. MTP: Cloud Formation • Invisible water vapour becomes visible as water droplets or ice • Condensation of water vapour

  11. MTP: Cloud Formation • What is required: • High relative humidity • Condensation nuclei • Cooling of the air • Cold surface • Adiabatic cooling

  12. MTP: Cloud Formation • What are the three things required for cloud formation? • High relative humidity • condensation nuclei • cooling of the air

  13. Precipitation

  14. MTP: Precipitation • Water droplets grow in size and weight and fall due to gravity • Above freezing – coalescence due to vertical currents • Can also occur below freezing (water vapour and ice crystals) The average rain drop is a million times larger than a cloud water droplet

  15. MTP: Precipitation • Below freezing – aggregation of ice crystals • If below cloud is above freezing = coalesce and rain • If temp below is cold enough to allow crystals to fall to ground = snow

  16. MTP: Precipitation • Regions of a cloud Aggregation Aggregation and coalescence Coalescence Large drops and heavy rain = strong vertical motion

  17. MTP: Precipitation • Drizzle • very small drops of water which appears to float • Rain • Large water droplets

  18. MTP: Precipitation • Hail • Hard transparent layer of ice covering soft white core • Snow Grains • Tiny snow crystals that have acquired a coating of rime

  19. MTP: Precipitation • Snow Pellets • Soft white ice (hail without hard transparent layer • Snow • Agglomeration of ice crystals hexagonal/star shaped

  20. MTP: Precipitation • Ice Prisms • Tiny ice crystals in the form of needles • Ice Pellets • Formed by freezing of raindrops

  21. MTP: Precipitation • What are the 8 different types or precipitation? • Drizzle • Rain • Hail • Snow • snow grains • snow pellets • ice pellets • ice prisms

  22. MTP: ICAO Lapse Rates • Lapse rate • Decrease in temperature with altitude • Inversion • Increase in temperature with altitude • Isothermal Layer • Layer in which temperature remains the same

  23. MTP: ICAO Lapse Rates • Lapse rates: • Recall ICAO Standard Atmosphere: • Air is perfectly dry gas • Mean sea level pressure of 29.92 • Mean sea level temp of 15°C

  24. MTP: ICAO Lapse Rates • Can determine base of clouds: • Temperature on ground 10 degrees • Dew point 7 degrees • Lapse Rate Saturated adiabatic (1.5/1000 ft) • Cloud base = 2000 feet

  25. 405.03 Confirmation • What is the dry adiabatic lapse rate? • 3 degrees per 1000 feet • What is hail? • Hard transparent layer of ice covering soft white core • In temperatures above freezing, rain is formed by the _________ of water droplets • Coalescence

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