Measurement of variations in atmospheric refractive index with an airborne microwave refractometer

A micro refra cto meter for aircraft use is described , and some of the so urces of error in the measuremen t of variations of a t mospheric re fractive index with t his instrumen t a re discussed. Observat ions were made up to 10,000 feet on 2 days near vVashington, D . C. Two refractive-index so undings taken 1Yz h ours apart showed changes that, in the coarser aspects, resembled changes shown by radiosonde data. The flu ctuation intensity changed erratically with ti me and place, but was usually greatest where the vertical gradient was changing. A rough analysis of the data showed that the larger fluc t ua tions occurred over distances of several hundred m eters, whereas over distances less than 5 mete rs t he flu ctuations were negligible (the r esponse of the i nstr ument wou ld have a llowed t he detection of ftuctuations occu rr ing within 0.5 meter). Large increases in i ndex were observed on entering cumulus clouds and intense flu ctuations we I'e noted within the clouds.