The AFGL thermosonde instrument, developed by Brown et al3, measures the difference in temperature of the ambient atmosphere between two probes spaced 0.5 meters apart. The two

Turbulence parameters are determined from measurements along a 183 m line-of-sight at Palm Bay, Florida and a 300 m path at Sudbury, Massachusetts. A horizontal scintillometer is used to obtain o 2, the variance of log amplitude of the irradiance, and the path- averaged C 2, the atmospheric refractive index structure constant. Thermosondes are also used to obtain C 2 at several locations along the optical path. The mean C 2 is determined from three thermosondes and compared to that from the scintillometer. Statistics obtained from processing the 2 sec sampled data over 10 minute periods are presented. Excellent agreement is obtained between the two systems when averaged over 10 minutes. Statistics ob­ tained over shorter periods show a considerable variation in the C 2 estimates within the 10 minute period. The range of variation within 10 minute intervals can exceed 50%. Volume averaging, wind speed and direction fluctuations are discussed as possible causes of short time variations in the C 2 estimates. Atmospheric seeing conditions are an important limiting effect on modern optical systems. The refractive index structure parameter, C 2, is the fundamental quantity used to characterize atmospheric optical turbulence and its effects on optical systems. One of the most widely known turbulence effects is that of scintillation, or fluctuations in the received intensity. The parameter used to describe scintillation is o 2 , the variance of log amplitude of the irradiance. This paper presents results from a cc&iparison of measure­ ments of o 2 and Cn2. The data derive from both optical and atmospheric measurements. The optical instruments include a NOAA scintillometer and a modified AFGL transmissometer. Both these instruments respond to changes or fluctuations in intensity at the receiver aperture. However, the measurement of these fluctuations is performed differently in the two instru­ ments. Results of the comparison of these two instruments will be presented for a 300 m path at Sudbury, Massachusetts. Further comparisons will also be made between the scintil­ lometer and several AFGL thermosondes located along the scintillometer optical path. The latter comparison is from data obtained over a 183 m path at Palm Bay, Florida. A descrip­ tion of the experimental systems used follows. The horizontal scintillometer was developed at the NOAA Wave Propagation Laboratory by Ochs et al . The transmitter is a collimated, incoherent LED light source oscillating at 7 kHz with a wavelength of 940 nm. At the receiver, the log of the intensity is band-passed from 1.2 to 1000 Hz and processed through an RMS module with a 4 sec averaging time to pro­ vide an estimate of o 2. The operation of the instrument is based on the application of first order scattering^theory the relationship of C 2 to o 2. This instrument is design­ ed to avoid saturation effects and to also maintain the' path-weighting function and calibration throughout its range. The path averaged C 2 is given by the relationship 2