Weather Radar Systems Information

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Weather radar systems locate precipitation, determine precipitation type (hail, snow, rain, etc.), and calculate its motion. They may also be referred to as weather surveillance radar or Doppler weather radar.

Weather radar is founded on basic Doppler radar principles, in which a transmitter emits short pulses of energy at a specific wavelength. These energy waves scatter upon contact with airborne particles, directing some energy back to the radar system. Computers associated with the system analyze key data about the signals—their strength, travel time, and frequency shift, for example—to determine the type and intensity of precipitation. Weather radar systems typically transmit radiation via a magnetron or klystron tube connected to a parabolic antenna.

The pilot training video below provides an overview of the operation and use of a weather radar system.

Video credit: flightcrewguide.com

Specifications and Measurements

Weather radar systems transmit on a particular wavelength. Shorter wavelengths are generally preferable for detection of smaller particles. Common wavelengths include:

S-band systems transmit around the 10 cm microwave wavelength. They are expensive but ideal for weather radar due to their optimum balance between particle detection and attenuation characteristics.
C-band systems transmit at around 5 cm. They are considerably more sensitive than S-band devices but have been historically limited by excessive attenuation. Recent technologies such as dual polarization attenuation correction have largely corrected this disadvantage, however.
X-band systems operate on a wavelength of around 2.5-4 cm. They are most useful for very-short-range cloud studies, as their low wavelengths and high sensitivity is ideal for detecting water particles and light precipitation. Most large aircraft are equipped with X-band radar to measure turbulence and weather characteristics.
K-band systems are similar to X-band, but operate on an even lower wavelength of 1.7-2.5 cm.

Return echoes are measured in dBZ or decibels relative to Z, a logarithmic unit which defines Z as the signal's reflectivity. Radar returns are typically classified by color or level. The US National Doppler Radar abides by a color scale, including the following classifications:

Magenta: 65 dBZ. Typically indicates extremely heavy precipitation and possible hail, thunderstorms, or tornadoes.
Red: 52 dBZ. Heavy precipitation.
Yellow: 36 dBZ. Moderate precipitation.
Green: 20 dBZ. Trace precipitation or light rain.

Aviation weather radar adopts a similar level scale based on the colors above:

Level 1 (green) indicates light precipitation, minimal turbulence, and a minor possibility of reduced visibility.
Level 2 (yellow) describes moderate precipitation, the possibility of very low visibility, moderate turbulence, and a generally uncomfortable ride.
Level 3 (red) indicates heavy precipitation, possible thunderstorms, severe turbulence, and the possibility for structural damage to the aircraft.