Radar, an acronym for Radio Detection and Ranging, was invented during World War II to detect aircraft, but precipitation frequently got in the way. The military’s noise is meteorology’s signal.
A radar consists of a transmitter and a receiver. The transmitter emits pulses of radio waves outward in a circular pattern. Precipitation scatters these radio waves, sending some energy back to the transmitting point where it is detected by the radar’s receiver.
The intensity of this received signal, called the radar echo, indicates the intensity of the precipitation. Measuring the time it takes for the radio wave to leave the radar and return tells us how far away the storm is. The direction the radar is pointing locates the storm.
Uniquely, Doppler radar can measure the velocity of precipitation particles (and thus, the wind) in precipitating regions. A Doppler radar receiver “hears” waves of a higher frequency if precipitation particles are moving toward the radar and a lower frequency if particles are moving away. This allows Doppler radars to identify the detailed wind structure within severe thunderstorms. For example, if particles switch from moving toward and then away from the Doppler radar over a small distance, then a tornado is possible.
The National Weather Service is currently replacing its older radars with dual-polarization radars to improve observations of the interior of storm systems. A radio wave is an electromagnetic wave and therefore has electric and magnetic fields that are oriented perpendicular to one another. The orientation of these oscillations is referred to as polarization.
A polarizing filter for a camera, or polarizing sunglasses, can be used to observe the effects of polarization of light in a cloud-free sky. Rotate the filter, or glasses, while looking through them at a portion of the sky away from the sun — at a certain orientation, the intensity of the sky-light will be reduced. The filter is removing polarized light that is not aligned with the filter.
The additional information on polarization improves the precipitation rate measurement as well as the determining of the type of precipitation (snow, rain, freezing rain and possibly hail). The polarization radars can also measure information about both the horizontal and vertical dimensions of precipitation sized particles.