1. AOS 101-304 Moisture and Stability March 22

2. Humidity • The amount of water vapor in the atmosphere. • Mixing ratio, w: ratio of mass of WV to mass of dry air (g/kg) grams of water vapor w = kilograms of dry air

3. Vapor Pressure • Vapor pressure, e: partial pressure of water vapor (Pa) • The total air pressure of the atm. equals the partial pressures of each constituent gas Pair = PN2 + PO2 + PAr + e + PCO2 + ... • Typical air pressures are around 1000 hPa, vapor pressure is typically 1-25 hPa.

4. Saturation Vapor Pressure, es • Definition: For a given temperature, the amount of WV required so that the rates of condensation and evaporation are equal. • Warmer air has a higher saturation vapor pressure than cooler air. • Exponential relationship between es and temperature, i.e. es = f(T)

6. Relative Humidity: ratio of observed vapor pressure and the saturation vapor pressure (for a given temperature) • Dewpoint (Td): Temperature of air to which a parcel must be cooled to reach saturation. • Completely relies on vapor pressure. e = f(Td) e RH= f(T)

7. Given T = 70oF and RH = 20%, what are es, e and Td?

8. Given T = 70oF and RH = 20%, what are es, e and Td? T = 70oF → es = 25 hPa RH = .2 = e/25 → e = 5 hPa e = 5 hPa → Td = 27oF

9. T = 70oF → es = 25 hPa RH = .2 = e/25 → e = 5 hPa e = 5 hPa → Td = 27oF • If you know 2, you know all 5. T ↔ es Td↔ e RH ↔ e / es

10. Stability • Refers to an equilibrium position • Stable equilibrium: perturbing an object will result in the object returning to original position • Unstable equilibrium: perturbing an object will result in the object continuing to move away.

11. In the atmosphere… • Perturbed objects are parcels (bubbles of air that can’t mix with surroundings). • Atmospheric stability has to do with buoyancy • A parcel warmer (less dense) than its surroundings will rise. • A parcel cooler (more dense) than its surroundings will sink. • Stability assessed by moving parcels vertically and comparing parcel temperature with the temperature of its surroundings.

12. However… • As a parcel rises it does not keep the same temperature, instead it cools ADIABATICALLY. • Adiabatic: reversible process which results in no loss/gain of heat energy by the parcel • Instead, during ascent the parcel expands resulting in cooling (or warming by compressing during sinking)

13. Dry Adiabatic Lapse Rate (DALR): The rate at which an unsaturated parcel cools as it rises (or warms as it sinks) is constant: Γd = 9.8oC / km • If a parcel is saturated, latent heat is released as water vapor condenses slightly counteracting the cooling during ascent. This is the Moist Adiabatic Lapse Rate (MALR): Γm ~ 6.0oC / km (varies 4-7oC/km) Note: if condensed water leaves the parcel the process is no longer reversible.

14. Assessing stability • On a given day, the atmosphere will have a certain environmental lapse rate (Γe). • Negative sign needed so that if temperature decreases with height the lapse rate is positive. T(z2) – T(z1) ― Γe= z2 – z1

15. For a certain atmospheric layer… • If Γe > Γd, atmospheric layer is absolutely unstable. • If Γe < Γm, atmospheric layer is absolutely stable. • If Γd > Γe > Γm, atm. layer is conditionally unstable, atm. only unstable if saturated.

16. Absolutely Unstable Γe > Γd (9.8oC/km) DALR MALR T(z=1 km) = 10oC 1 km Γe T(z=0 km) = 25oC 10 15 20 25

17. Absolutely Stable Γe < Γm(6.0oC/km) DALR MALR T(z=1 km) = 25oC 1 km Γe T(z=0 km) = 25oC 10 15 20 25

18. Conditionally Unstable (9.8) Γd > Γe> Γm(6.0) DALR MALR T(z=1 km) = 17oC 1 km Γe T(z=0 km) = 25oC 10 15 20 25

19. On average… • The environmental lapse rate is 6.5oC/km, meaning the atmosphere is usually conditionally unstable or barely stable. • The closeness to saturation determines if the atmosphere is unstable or not

20. Atmospheric Soundings • Radiosondes measure the temperature and moisture profiles of the atmosphere • These profiles can be used to assess the stability of the atmosphere

21. Next Week… • Severe thunderstorms usually result from instability in the atmosphere. • The summer sun warms parcels near the ground, allowing them to become unstable and rise to the tropopause