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Lapse Rates & Stability

Lapse Rates & Stability. Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states of air stability?. Condensation nuclei. Large numbers of these in atmosphere Hygroscopic nuclei (attract water) Volcanic dust

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Lapse Rates & Stability

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  1. Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states of air stability?

  2. Condensation nuclei • Large numbers of these in atmosphere • Hygroscopic nuclei (attract water) • Volcanic dust • Dust from windblown soil • Smoke – urb & ind • Sulphuric acid – urb & ind • Salt from sea spray Over cities – 1 mill nuclei per cubic cm Over oceans – 10 nuclei per cubic cm * If large quantities of nuclei, condensation occurs at low RH e.g. 75% RH = smog in Los Angeles

  3. If air is pure: • No particles or condensation nuclei • Air can be cooled below its dew point • Air can have more than 100% RH • Lab tests have cooled air to -40 before condensation / sublimation • Pure air is very rare

  4. ELR varies with local air conditions: Height Time of year Type of surface that the air is over (land or sea) Environmental Lapse Rate (ELR) Decrease in temperature with height 6.5 oC / 1000m

  5. Decrease in temp Increase in volume Decrease in pressure Parcel of air rises Sinking air Increasing pressure Decreasing volume Air warms up Adiabatic Lapse Rate (ALR)

  6. 1. Dry Adiabatic Lapse Rate energy (heat) used up by expansion causes fall in temp no condensation, air not saturated 9.8oC / 1000m Aver. 1oC / 100m There are 2 ALRs

  7. Saturated Adiabatic Lapse Rate (SALR) Happens when enough uplift to reach Dew Point Temp Cooling causes saturation Condensation occurs Latent heat is releases This partly compensates for loss of temp with height SALR varies with air temp 4-9oC / 1000m Average 5.4oC / 1000m, about 0.5oC / 100m Adiabatic Lapse Rates

  8. Freezing Adiabatic Lapse Rate • Happens when temp below zero • Same rate as DALR • Very little moisture at low temps, as no evaporation

  9. STABILITY Will the parcel of air continue to rise above 1000m? Why? What will happen to the parcel? 1000m up parcel of air cools to 10.2oC Surrounding air 14oC at 1000m PARCEL OF WARM AIR COOLS AT DALR (9.8oC/1000M) Surrounding air Cools at ELR 6.5oC / 1000m Ground level temp 20oC

  10. Stability • Parcel will sink because….. • No clouds formed because….. • Creates stable conditions because…. • Weather conditions associated with….

  11. Common on hot days Surrounding air heated by conduction ELR increases Parcel of rising air remains warmer and lighter Rises above DPT Latent heat released so it continues to rise Cumulonimbus form, thunderstorms likely Instability

  12. Conditional Instability- most common in Britain- instability is conditional upon the air being forced to rise in the first place, until condensation triggers the SALR and further unforced uplift- gives cloudy and showery weather Over mountain or cold air mass

  13. Summary • Learning outcomes: • - What are condensation nuclei? • - What is the difference between DALR & SALR? • - What are the different states of air stability?

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