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Meteorology Lecture 8: Dew Point and Wet Bulb Approach to Saturation

These slides supplement the "Meteorology" course by Robert G. Fovell, providing figures and videos to enhance understanding of the concepts discussed. This lecture focuses on the dew point and wet bulb approach to saturation, explaining the process of air cooling and the formation of fog.

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Meteorology Lecture 8: Dew Point and Wet Bulb Approach to Saturation

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  1. Meteorology – Lecture 8 Robert Fovell rfovell@albany.edu

  2. Important notes • These slides show some figures and videos prepared by Robert G. Fovell (RGF) for his “Meteorology” course, published by The Great Courses (TGC). Unless otherwise identified, they were created by RGF. • In some cases, the figures employed in the course video are different from what I present here, but these were the figures I provided to TGC at the time the course was taped. • These figures are intended to supplement the videos, in order to facilitate understanding of the concepts discussed in the course. These slide shows cannot, and are not intended to, replace the course itself and are not expected to be understandable in isolation. • Accordingly, these presentations do not represent a summary of each lecture, and neither do they contain each lecture’s full content.

  3. Dew point approach to saturation

  4. Example • Here is an example of air being cooled to its dew point • Start on a pleasant afternoon • The curves depict how T and Td might vary over Stickman’s height • Note T > Td everywhere, so the air is subsaturated

  5. Example • Say after sundown the VS of air does not change, so Td remains the same • But the ground surface under Stickman’s feet has a low thermal inertia. It cools quite well. • That causes the air near the ground to cool. The cooling is largest right near the ground • If cooled to the dew point, dew may form

  6. Example • If air were able to cool to the dew point over a deeper layer near the surface -- then we would have fog. • A fog is just a cloud with base at or very near the ground

  7. Wet bulb approach to saturation

  8. Evaporative vs. isobaric cooling

  9. Boiling

  10. Picture a pot of water on the stove.Liquid water overlain by air

  11. What is the fate of a bubble of water vaporthat forms in the pot?

  12. Atmospheric pressure is pushing down on the water surface, due to air’s mass.The pressure exerted on the bubble is actually slightly larger, as the mass of the overlying water is also contributing

  13. The vapor in the bubble is also exerting its pressure --vapor pressure.If the bubble cannot hold its own against the externalpressure, it collapses.

  14. Success comes with higher temperature.The bubble is composed of vapor at saturation, sothe relevant pressure is the saturation vapor pressure.We know this increases exponentially with temperaure.The boiling point is the T at which the saturationvapor pressure equals the external pressure.

  15. Phase changes occur at constant temperature.In this example, water is brought to a boil.

  16. Phase changes occur at constant temperature. In this example, water vapor is condensing to liquid.

  17. [end]

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