1. Use of the Tephigram

2. Converting dew point to mixing ratio
What is the mixing ratio of a parcel of air which has a dew point of 10°C at 800 mb?
Plot Td on the tephigram
Read off the value of mixing ratio from the scale at the bottom: 9.8 g kg-1 . TD

3. Converting dew point to RH
What is the relative humidity of a parcel of air which has a temperature of 15°C and dew point of 10°C at 800 mb?
We need RH = 100*w/ws
w = 9.8 g kg-1 as before
ws = 13.6 g kg-1
So RH = 72% T TD

4. Parcel approximation
LCL is the Lifting Condensation Level, the height that a parcel of air must be lifted to reach saturation.
In a convective boundary layer it usually corresponds to cloud base
Air parcel lifted without mixing and without adding or removing heat
SALR z
LCL
DALR T

5. Path of air parcel on tephigram
Consider a parcel of air lifted from the surface, e.g. by flow over a hill
Cloud
Lifting condensation level, LCL = 780 mb
Parcel initially at: 25°C, dew point 8.5°, pressure 1000 mb. We can see that this corresponds to w = 7 g kg-1
As parcel ascends:
T follows dry adiabatic
Mixing ratio is constant
until saturation (T = TD)
Then parcel follows saturated adiabatic

6. “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.
Cloud
LCL TW TD 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

7. Early morning tephigram over land
No cloud
Tropopause
Boundary layer top
Lifting Condensation Level = 930 mb
Radiation inversion

8. Mixing out of radiation inversion – solar heating at the ground
Cloud top
Cumulus layer
LCL in the afternoon
Surface warms from 18°C to 23°C

9. 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

10. 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.

11. Orographic uplift – lift each point on tephigram by 50 mb
Initial profile. We will use the four coloured altitudes as examples

12. Orographic uplift – lift each point on tephigram by 50 mb
Lift the bottom point through 50 mb. It does not reach the LCL and remains unsaturated
50 mb

13. Orographic uplift – lift each point on tephigram by 50 mb
Lift the next point through 50 mb. It does reach saturation
50 mb

14. Orographic uplift – lift each point on tephigram by 50 mb
Lift the next two points through 50 mb.
50 mb

15. Orographic uplift – lift each point on tephigram by 50 mb
Join up the dots to get the new temperature profile.
Hill Cloud

16. 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