A synchrophasor-based thermal line monitoring scheme enabling the measurement of line spans and therewith temperature profiles is presented. Further, the measurement of thermal transients with synchrophasors has become feasible by incorporating non-steady state heat equations and local weather data. The proof of concept is executed with an electro-thermal line modelling approach in combination with sensitivity analyses of various power flow scenarios, data processing time steps and weather updating rates. It has been shown, that the relative measurement error of this method is ±0.2% at ideal thermal static and ±2.0% at ideal thermal transient conditions for various load currents with reference to the original temperature. Finally, the accuracy at highly volatile weather conditions as well as the suitability for practical implementation is discussed.