The contribution of atmospheric density to the drop-off rate of Cn2

Calculation and comparison of terms in the expression for Cn2, the atmospheric refractive index structure parameter, indicate that density is the prime factor in determining the drop-off rate of Cn2 with altitude in the lower stratosphere. We have then determined the drop-off rate in atmospheric density using rawinsonde data from sites spread out in latitude from approximately 70°N latitude to approximately 10°N latitude. These height drop-off values in density follow, very nicely, a linear relationship with latitude described by a value of 1.30 dB/km at 70°N latitude and 1.52 dB/km at 10°N latitude. No longitudinal variations were found. Density drop-off rates are then compared to drop-off rates of mean Cn2 measurements obtained using remote radars and also to in situ thermosonde mean Cn2 measurements. At high latitudes, radar Cn2 drop-off rates agree with the density drop-off rates but then diverge from the density drop-off rates as latitude decreases until the radar value is about a factor of two greater than the density drop-off rate at 10°N latitude. The thermosonde values of Cn2 drop-off are approximately equal to or no more than about 14% lower and 17% higher than the density drop-off rates over the latitudes investigated.