A National Doppler Radar installation is conveniently located on Bunker Hill south of Ferndale. To see its value in forecasting rain or snow you should visit www.weather.gov/eureka. The following is an introduction to this amazing technology.

Weather radar operates at two wavelengths, 5 cm & 10 cm, with ranges of 124 and 248 nautical miles. The microwave beams sweep a full 360 degrees and tilt at various angles above the horizon. Pulses of microwaves are emitted every millisecond and are scattered back by rain or snow (or flocks of birds), but not clouds or fog. The amount of energy returned decreases with distance squared (assuming the target fills the beam's width) and increases with the 6th power of raindrop diameter. Snow is not as reflective as rain, but melting snowflakes are very reflective because of the size effect. The distance to the particles is determined by the time delay of the returned echo: Distance = (time delay/2)x(speed of light). Click on Reflectivity and Loop to see a time-lapsed 1 second movie of 1.5 hours of storm motion.

When particles are approaching or receding, the wavelength (and frequency) of the reflected microwaves is changed, and although this Doppler effect is too small to measure directly, it does give rise to a displacement of the waves — termed a "phase shift" — between one echo and the next. The change in phase shift between one echo and the next (divided by the time interval between pulses) yields the radial speed at which wind-driven rain is approaching or receding. Click on Velocity to see radial wind speed in knots toward (green) or away from (red) the antenna.

When rain threatens, take comfort from knowing that you can watch it coming, and that our NOAA experts in Eureka are always watching.