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

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

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

7. Adiabatic Lapse Rates 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 4-9 o C / 1000m Average 5.4 o C / 1000m, about 0.5 o C / 100m

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

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

11. Instability 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

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?