Which of the following is NOT a cause of spectrum width?

Spectrum width is a measure of the variability in the Doppler velocities observed within a radar sampling volume. It reflects the dispersion of velocities caused by various factors.

Wind shear, which is the variation of wind speed or direction with altitude, can lead to changes in the velocities of targets within the radar's beam, contributing to a broader spectrum width. Similarly, differences in hydrometeor sizes affect the scattering properties and Doppler shifts of the returning signals, also resulting in variations in observed velocities.

The rotation rate of the radar antenna can introduce time-based variations in the measured velocity as well, potentially affecting the calculated spectrum width due to the movement of targets during the scanning process.

In contrast, temperature fluctuations themselves do not directly cause spectrum width. While temperature can influence other parameters, it does not result in variations in the Doppler velocities of precipitation particles; thus, it is not a primary factor contributing to a change in spectrum width. This understanding clarifies why fluctuations in temperature do not directly play a role in this particular measurement.