Lapse Rates 3 MET
The stability is established by comparing the environmental lapse rate (the actual fall of temperature with height) to two standard rates: the DALRDALR —Dry Adiabatic Lapse Rate of 3 degrees per 1000' and the SALRSALR —Saturated Adiabatic Lapse Rate of 1.8 degrees per 1000'.
Here, calculate the difference in temperature (the temperature at the lower FLFL —Flight Level minus the other, to get the sign right) and divide this by the difference in height. This will give you the lapse rate.
If the lapse rate is greater than 3, the layer is absolutely unstable. If it is between 1.8 and 3, it is conditionally unstable (note there is no such term as conditionally stable.) If it is less than 1.8 (including 0 or negative) the layer is absolutely stable.