Method of detection for identifying and monitoring one or more thick clouds in an area of the sky observed from the ground, wherein the following steps are carried out: a) collecting at least one portion of the thermal-infrared fluxes emitted in said observed area of the sky and sending same to at least one thermal-infrared detector, said at least one thermal-infrared detector comprising at least one sensor sensitive to said fluxes in a predetermined wavelength band, b) taking at least one measurement of the actual temperature and relative humidity at ground level and deducing the vertical distribution of the temperature and water vapour therefrom, c) simulating or obtaining all of the data relative to the thermal-infrared signal emitted by a reference sky for said vertical distribution of temperature and water vapour thus deduced, d) subtracting, from the entire dataset measured by said at least one sensor, the set of data thus simulated or obtained such as to determine the presence or absence of one or more clouds in said observation area of the sky, e) processing the dataset thus obtained in order to determine the optical thickness and the altitude of each cloud in said observation area of the sky, said method being characterised in that the step e) of processing the dataset comprises a step of determining the altitude of a cloud by postulating a relationship between the luminance perceived by the measuring instrument, and the modelled luminances of a cloud and of the atmospheric layer thereof located underneath and by implementing correspondence tables of the "look-up" type, with the relationship between the luminance perceived by the measuring instrument, and the modelled luminances of a thick cloud and of the atmospheric layer thereof located underneath, being defined as: L instrument = Lthick cloud + Lclear sky under the cloud = Lthick cloud + x*Ltotal clear sky with the construction of a look-up table causing to correspond for a given clear sky luminance value: the altitude, the luminance perceived by the measuring instrument, the value x, the luminance of the atmosphere under the cloud and the luminance of the cloud, and in that once the measurement of the luminance perceived by the measuring instrument is taken, the known value for the clear sky luminance is used to choose the correct look-up table and to deduce the altitude of the cloud therefrom. a) collecting at least one portion of the thermal-infrared fluxes emitted in said observed area of the sky and sending same to at least one thermal-infrared detector, said at least one thermal-infrared detector comprising at least one sensor sensitive to said fluxes in a predetermined wavelength band, b) taking at least one measurement of the actual temperature and relative humidity at ground level and deducing the vertical distribution of the temperature and water vapour therefrom, c) simulating or obtaining all of the data relative to the thermal-infrared signal emitted by a reference sky for said vertical distribution of temperature and water vapour thus deduced, d) subtracting, from the entire dataset measured by said at least one sensor, the set of data thus simulated or obtained such as to determine the presence or absence of one or more clouds in said observation area of the sky, e) processing the dataset thus obtained in order to determine the optical thickness and the altitude of each cloud in said observation area of the sky, said method being characterised in that the step e) of processing the dataset comprises a step of determining the altitude of a cloud by postulating a relationship between the luminance perceived by the measuring instrument, and the modelled luminances of a cloud and of the atmospheric layer thereof located underneath and by implementing correspondence tables of the "look-up" type, with the relationship between the luminance perceived by the measuring instrument, and the modelled luminances of a thick cloud and of the atmospheric layer thereof located underneath, being defined as: L instrument = Lthick cloud + Lclear sky under the cloud = Lthick cloud + x*Ltotal clear sky with the construction of a look-up table causing to correspond for a given clear sky luminance value: the altitude, the luminance perceived by the measuring instrument, the value x, the luminance of the atmosphere under the cloud and the luminance of the cloud, and in that once the measurement of the luminance perceived by the measuring instrument is taken, the known value for the clear sky luminance is used to choose the correct look-up table and to deduce the altitude of the cloud therefrom.