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Lab 6: Saturation & Atmospheric Stability

Lab 6: Saturation & Atmospheric Stability. Review Lab 5 – Atm. Saturation. Relative humidity? Mixing ratio / saturation mixing ratio? Function of temp.. Clausius-Clapeyron curve Sling psychrometer – what does this give us? Dew point?

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Lab 6: Saturation & Atmospheric Stability

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  1. Lab 6: Saturation & Atmospheric Stability

  2. Review Lab 5 – Atm. Saturation • Relative humidity? • Mixing ratio / saturation mixing ratio? • Function of temp.. Clausius-Clapeyron curve • Sling psychrometer – what does this give us? • Dew point? • Looking at RH equation above, when temp is reduced, all else being equal, what happens to the RH of a sample of air? Does RH go up or down? • Air is saturated when RH=100%

  3. Lab 6 • Lab 6: Saturation and Atmospheric Stability • processes that influence atmospheric saturation – i.e., cause cooling and/or increase in water vapor content • atmospheric processes that change either the temp and/or water vapor content of an air sample • In this lab, we’ll focus on atmospheric mixing and adiabatic cooling and some processes that drive these conditions

  4. Saturation & Atmospheric Stability • Two main ways for air to reach saturation: • Cooling to its dew point temperature (most common) • Increasing water vapor content Remember Condensation produces: Fog Dew Clouds *ALL require saturated air to form! 1. 2. 3.

  5. Atmospheric Mixing • When two air masses of different temps and water vapor content mix • When they mix, the new air mass will change in temp and water vapor • resulting in new mixing and saturation mixing ratios • Changes relative humidity

  6. Assuming the two mixing air masses are the same size and you know the temps and RH find: • The new temp of mixed air mass • The new mixing ratio of the mixed air mass • From the above, you can find the new RH (due to change in temp and water vapor) Mixing Ratio = SMR * RH “Saturated air” 100% RH “Unsaturated air” 0 – 99.9% RH

  7. Adiabatic Cooling • Adiabatic temperature changes: • Temperature changes in which heat was neither added nor subtracted (closed system) • Average internal energy decreases with expansion – changes in average kinetic energy • Compressed air = warm air • Expanded air =cooler air • NOTE: If a parcel moves ↑, it passes through regions of successively lower pressure: • Ascending air: EXPANDS • Descending air: COMPRESSES

  8. Saturation & Atmospheric Stability DRY adiabatic rate: unsaturated • cools at a constant rate of 10°C/1km of ascent • warms at constant rate of 10°C/km of descent WET adiabatic rate: saturated (has RH 100%) • Slower rate of cooling caused by the release of latent heat • Rates vary between 5°C & 9°C/1km • Amount of LH released depends on quantity of moisture in the air Dew Point rate: • 2°C/1km tothe LCL • At the WALR after the LCL LCL = altitude at which a parcel reaches saturation & cloud formation begins

  9. Saturation & Atmospheric Stability DALR = 10°C/1km WALR = 5 – 9°C/1km WALR Air decreases by 2.5°C 10.5° LCL 13° 1.5 Air decreases by 5°C 18° DALR 23°

  10. Lifting Condensation Level (LCL): • Reached when ascending air coolsto its dew point (saturation = 100% RH) – clouds form • If it continues to rise: • Cools at the wet adiabatic lapse rate (between 5°& 9°C) • Calculated based on: • Surface temperature & dew point temperature

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