Use of the Tephigram. Path of air parcel on tephigram. Consider a parcel of air lifted from the surface, e.g. by flow over a hill. 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
Consider a parcel of air lifted from the surface, e.g. by flow over a hill
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
Lifting condensation level, LCL = 780 mb
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.
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
Boundary layer top
Lifting Condensation Level = 930 mb
LCL in the afternoon
Surface warms from 18°C to 23°C
Convective available potential energy = area where Tparcel>Tenvironment
Level of free convection
Convective inhibition = area where Tparcel<Tenvironment
Initial profile. We will use the four coloured altitudes as examples
Lift the bottom point through 50 mb. It does not reach the LCL and remains unsaturated
Lift the next point through 50 mb. It does reach saturation
Lift the next two points through 50 mb.
Join up the dots to get the new temperature profile.
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