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– 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
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
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
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
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
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
Freezing Adiabatic Lapse Rate
• Happens when temp below zero
• Same rate as DALR
• Very little moisture at low temps, as no evaporation
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?
Stability
• Parcel will sink because…..
• No clouds formed because…..
• Creates stable conditions because….
• Weather conditions associated with….
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
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
Summary
• Learning outcomes:
• - What are condensation nuclei?
• - What is the difference between DALR & SALR?
• - What are the different states of air stability?