Use of the Tephigram
Path of air parcel on tephigram
Parcel initially at: 30°C, dew
point 14°, pressure 1000
700
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
800
900
1000
Lifting condensation
level, LCL
“Normand’s Theorem”
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.
Wet bulb potential
temperature, θW is used to
label saturated adiabats
(value of T where adiabat
crosses 1000 mb)
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
Early morning tephigram over land
Tropopause
Boundary
layer top
Radiation
inversion
Mixing out of radiation inversion –
solar heating at the ground
Cloud top
Cumulus
layer
LCL
Energy = area on tephigram. What happens
if something pushes air parcel upwards?
Convective available
potential energy = area
where Tparcel>Tenvironment
Convective
inhibition = area
where
Tparcel<Tenvironment
Level of
free
convection
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.
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)
At each point on the profile:
No cloud
Cloud forms
Cloud
Potential Instability
Grey line: temperature
profile lifted 50 mb
Orange line:
SALR from
uplifted surface
air
Saturation
Slope of grey
line > SALR so
convectively
unstable where
saturated: deep
thunderstorms
would occur on
this day over a
small hill