lapse rates & stability learning outcomes: - what are condensation nuclei? - what is the...

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

Lapse Rates & Stability

Learning outcomes:

- What are condensation nuclei?

- What is the difference between DALR & SALR?

- What are the different states of air stability?

Page 2: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

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

Page 3: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

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

Page 4: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

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

height6.5 oC / 1000m

Page 5: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

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

Page 6: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

There are 2 ALRs

1. Dry Adiabatic Lapse Rate

- energy (heat) used up by expansion causes fall in temp

- no condensation, air not saturated

- 9.8oC / 1000m- Aver. 1oC / 100m

Page 7: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

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

4-9oC / 1000m• Average 5.4oC / 1000m,

about 0.5oC / 100m

Page 8: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

Freezing Adiabatic Lapse Rate

• Happens when temp below zero

• Same rate as DALR

• Very little moisture at low temps, as no evaporation

Page 9: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

STABILITY

PARCEL OF WARM AIR COOLS AT

DALR (9.8oC/1000M)

Ground level temp 20oC

1000m up parcel of aircools to 10.2oC

Surrounding air

Cools at ELR

6.5oC / 1000m

Surrounding air 14oC at 1000m

Will the parcel of air continue to rise above 1000m? Why?

What will happen to the parcel?

Page 10: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

Stability

• Parcel will sink because…..

• No clouds formed because…..

• Creates stable conditions because….

• Weather conditions associated with….

Page 11: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

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

Page 12: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

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

Page 13: Lapse Rates & Stability Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states

Summary

• Learning outcomes:

• - What are condensation nuclei?

• - What is the difference between DALR & SALR?

• - What are the different states of air stability?