1. Use of the Tephigram

2. Path of air parcel on tephigram
1000
900
800
700
Lifting condensation level, LCL
Parcel initially at: 30°C, dew point 14°, pressure 1000 mb. We can see that this corresponds to w = 10 g kg-1
As parcel ascends:
T follows dry adiabatic
Mixing ratio is constant
until saturation (T = TD)
Then parcel follows saturated adiabatic

3. “Normand’s Theorem”
Wet bulb potential temperature, θW is used to label saturated adiabats (value of T where adiabat crosses 1000 mb)
Project the saturated adiabatic to 1000 mb.
It is colder than the (unsaturated) temperature because water is evaporated into the air to keep it saturated, and the energy comes from the air.
This is the definition of the wet bulb temperature.
700
800
LCL
900
1000 TD TW T
Dry adiabatic through the temperature, the mixing ratio line through the dew point, and the saturated adiabatic through the wet bulb temperature, all meet at the LCL

4. Early morning tephigram over land
Tropopause
Boundary layer top
Radiation inversion

5. Mixing out of radiation inversion – solar heating at the ground
Cloud top
Cumulus layer
LCL

6. Energy = area on tephigram
Energy = area on tephigram. What happens if something pushes air parcel upwards?
Convective available potential energy = area where Tparcel>Tenvironment
Level of free convection
Convective inhibition = area where Tparcel<Tenvironment

7. Deep convection Develops when CIN is small and CAPE is large
Need some CIN otherwise there is no ‘build-up’ of energy in the boundary layer
Mechanical forcing often needed to overcome CIN – e.g. flow over mountain, sea breeze, cold front
At other times large-scale forcing e.g. trough provides lift.

8. Orographic uplift – lift each point on tephigram by 50 mb
Coloured dots and lines – initital and final positions of air parcels
Grey line – final temperature profile
Dark Grey – saturated segment (hill fog)
Cloud
At each point on the profile:
No cloud Cloud forms

9. Potential Instability
Grey line: temperature profile lifted 50 mb
Orange line: SALR from uplifted surface air
Slope of grey line > SALR so convectively unstable where saturated: deep thunderstorms would occur on this day over a small hill
Saturation