A tomographic optical system and method based on evaluations of plasma radiation in a wavelength range of 559-601 nm were used to acquire temperature distributions in an air plasma cutting torch in planes perpendicular to the arc axis with a time resolution of 1 mu s. The derived frequency spectra and distributions of temperature fluctuations represented by standard deviations have shown significant variations in distributions of instabilities, depending on the time scales which are taken into account. The results confirmed the decisive role of arc current ripple modulation in the arc temperature fluctuations.