The heat transferred to the atmosphere by fumarole clouds is linearly proportional to the cloud area that is visible in vertical air-photos. Using measured outputs of a few accessible fumaroles, the total heat output of all fumaroles in a fumarole field can be assessed from the total area of the steam clouds. Normalised heat outputs, Q(j), and normalised cloud areas, A(j), were used as variables. Two surveys of the large Karapiti fumarole field (Wairakei, New Zealand) show that the total heat discharged by fumaroles with cloud areas >10 m(2) was 99 MW and 88 MW in 1999 and 2000, respectively. The output of vents with smaller cloud areas has to be obtained from ground surveys. The method also gives a representative (order of magnitude) estimate of the total heat discharged by fumarole clouds if measurements from a single, large fumarole can be used in conjunction with a recent air-photo.