Above: Taken in April 1973, this photograph shows the first Doppler weather radar system built by the US National Severe Storms Laboratory in Norman, Oklahoma (NOAA Photo Library)
Precise weather forecasting is important for day-to-day planning, such as choosing appropriate clothing, scheduling flights, or planting and harvesting crops. Accurate predictions can also save lives, as highlighted recently by the evacuation orders issued to residents of coastal regions along the path of Hurricane Sandy. One of the most important tools in weather forecasting is radar—an acronym coined in the 1940s by the US Navy from the term Radio Detection and Ranging.
Did you know? Radar was first used during World War II, to detect enemy attacks from air, land, or sea. However, radar operators noticed that rain or snow created radar patterns that masked potential enemy threats.Radar systems consist of a tower-mounted transmitter that emits electromagnetic radiation (radio waves or microwaves) and a receiver that captures the waves that are reflected back. The radar transmitter projects a radio-frequency beam that sweeps 360 degrees around the radar tower, altering the beam elevation with each rotation.
The colourful radar maps often included in weather forecasts correspond to a scale of reflectivity. Dense precipitation reflects more radar waves back toward the receiver and therefore has a higher reflectivity index. As science teacher Jonathan Bergmann explains in a video on How Radar Works, “Radar returns are usually described by color or level. The colors in a radar image normally range from blue or green for weak returns, to red or magenta for very strong returns. … Strong returns (red or magenta) may indicate not only heavy rain but also thunderstorms, hail, strong winds, or tornadoes.”
Did you know? Environment Canada operates a network of 31 radar sites, each which has a range of 250 km. This network provides radar coverage to over 95 percent of the Canadian population.Weather radar shows the location, altitude, and type of precipitation. Pulse-Doppler radar, the type used by Environment Canada, also shows the movement of precipitation. Pulse-Doppler radar emits short pulses of electromagnetic radiation. If the weather system is moving toward the radar tower, the reflected pulses will return with increased frequency (the pulses will return with shorter time between pulses).
Taken alone, information from weather radar alone is not enough to accurately predict the weather. It is combined with observations on the ground (such as temperature and air pressure) and from weather satellites (such as clouds and severe weather systems). All this data is entered into complex computer simulations, which can provide weather models for the next six days or so.
Canadian Weather Radar (Environment Canada)
Doppler radar is a key forecasting tool (USA Today)