full text of the classic FAA guide
Canopy static, similar to the precipitation static sometimes encountered at lower levels, is produced by particles brushing against plastic-covered aircraft surfaces. The discharge of static electricity results in a noisy disturbance that interferes with radio reception. Discharges can occur in such rapid succession that interference seems to be continuous. Since dust and ice crystals in cirrus clouds are the primary producers of canopy static, usually you may eliminate it by changing altitude.