The Earth’s atmosphere at a given location is rarely standard. Thus, a radar beam does not often travel as it would in the Standard Atmosphere. This is significant since the radar beam will be at a higher or lower altitude than we assume. This can lead to precipitation height estimates which are in error and echoes from the ground which may be mistaken for precipitation.

Non-standard propagation, or anomalous propagation (AP), occurs when non-standard vertical distributions of pressure, temperature and moisture exist within the atmosphere. As long as any of these three variables depart from the Standard Atmosphere, anomalous propagation (AP) can occur. Temperature and water vapour can change significantly with very small changes in height and such variations have the most marked effect on propagation of the radar beam.

Let us consider the effect of pressure, temperature and water vapour gradients on the refractive index. Pressure always decreases with height, and this causes a radar beam to be bent downward. The vertical gradient of pressure decreases with height and so refraction of the radar beam decreases with height. That is, the amount the radar beam is bent due to the vertical pressure gradient is less at greater heights. Typically the vertical pressure gradient varies little from day to day.

Typically, temperature decreases with height and this supports an upward bend of the radar beam. The more rapid the decrease of temperature with height (the greater the lapse rate), the greater the upward bending expected. In layers experiencing dry-adiabatic or super-adiabatic lapse rates, this effect may become more important than the pressure effect. If temperature remains constant with height (isothermal layer), it does not act to refract the beam. If temperature increases with height (inversion layer), the radar beam will be bent downward. The greater the rate of increase of temperature with height, the greater the downward bend (refraction), and the more likely that this effect would dominate over the pressure effect.

Changes in atmospheric moisture content impact the speed of propagation of electromagnetic waves possessing relatively low frequencies, such as those employed by weather radar. When the radar beam illuminates water vapour molecules, these molecules become electronically polarized and reorient themselves rapidly enough to follow the electric field changes. Vapour pressure typically decreases with height; as a result, a radar beam will be bent downward. If vapour pressure decreases more rapidly than usually with height, the amount of downward bending is increased; conversely, if vapour pressure decreases less rapidly than usual with height, the amount of downward bending is decreased. If vapour pressure remains constant with height, it does no act to refract the beam. If vapour pressure increases with height, its impact would be to bend the beam upward. It is evident from Figure 2 that the water vapour content has the greatest effect on the refractive index.