During World War II, military researchers developed radar, a tool that would allow them to detect warplanes and ships at distance. This tool provided critical early warning of an attack, and could show how many planes or ships were involved, and where they were coming from. Radar utilized a microwave transmitter that emitted radio waves. When the waves hit something solid, they would be reflected back to the transmitter, and by figuring out how long it took the waves to make the trip out and back, the distance to the target could be calculated.
There was a problem with radar, though. Whenever precipitation was present between the radar set and the target, it prevented the transmitted signal from reaching the target efficiently, and the signal returned from the target was weak, absent, or inconclusive. This required further experimentation with different radio frequencies to try to find those which were not affected by precipitation, and led to the development of radar sets that were more effective for their stated military purpose.
After the war, some people realized that the “interference” which got in the way of the radar was actually something worth observing, and that, just as the frequency was adjusted to minimize interference, it could also be adjusted so that the radar would be optimized for detecting precipitation. This led to the development of weather radar, and the technology has seen major improvements. We have gone from looking through a shaded hood and seeing a rotating line that glowed when precipitation was detected to multicolored radar images widely available on television and the Internet.
To interpret what these displays actually mean requires some understanding of how weather radar works. Basically, conventional weather radar is tuned to detect small targets in the atmosphere (meaning raindrops and snowflakes), and when it does so, a portion of the radio energy is reflected back to the receiving antenna. This information is then processed very quickly to yield the familiar patterns of colors on a map background. The strength of the returned signal is an indication of the density of the targets. Thus if the returned signal is stronger, it means that there are more (or larger) raindrops or snowflakes present, and this is usually shown on the display with brighter colors. This typically means that heavier or more intense precipitation is present in these areas.
There are a few pitfalls, though. First, if particularly heavy precipitation is present near the radar, then not enough of the radio signal will get past the precipitation to detect precipitation farther from the radar. This can mean that in some situations, precipitation that occurs farther from the radar may not be detected.
Second, one must keep in mind that the radar beam extends in a straight line from the transmitter. Because of the curvature of the earth, the beam is higher off the ground with increasing distance from the radar. This means that in some cases, the radar will be detecting raindrops/snowflakes that are well above the surface, and that may be evaporating as they fall and not reaching the ground as precipitation. It also means that fog and drizzle, which are very low-level phenomena, are often not detected by radar.
Third, terrain can get in the way of weather radar, and mountains will occasionally show up on the screen. Other ground obstructions near the radar, where the beam is close to the ground, also generate signal returns. This is called ground clutter, and can sometimes mask precipitation near the radar site. One way to sort this out is to look at consecutive radar images, and notice the portion of the display that is not moving. Typically these are ground clutter or terrain features, and the features that are moving are precipitation.
Weather radar data available on the Internet (http://radar.weather.gov is a good starting point) can be very useful to help make decisions about a near-shore outing on the water. Large areas of lower signal returns typically indicate a general light rainfall while more spotty returns indicate more showery precipitation that might not last that long in any given location. When the signal returns are stronger (usually shown by brighter colors) this indicates heavier precipitation. If the stronger returns are concentrated in small areas it can mean that strong thunderstorms or squalls are present, and if a long, thin line of intense returns is displayed, this indicates a squall line, or a strong front. In this case, heavy precipitation (possibly including hail produced by a severe thunderstorm) is being detected by t he radar. Of more concern to boaters is the strong and gusty wind that usually accompanies these features. Lightning is a concern as well. Keep in mind that the range of most shore-based weather radar is a few hundred miles at most, so if your plans carry you well offshore, the information is not as useful.
The navigational radars that are present on many recreational vessels today are not optimized to detect precipitation, but rather are tuned to be able to “see” terrain, other vessels and navigational aids. Most of them will detect precipitation, however, especially heavier precipitation, and some have the capability to be tuned to frequencies where they are more effective at this. If a mariner is in a situation where squalls are in the area, use of the onboard radar to determine where the strongest signal returns are, and how they are moving, can be useful to help avoid areas where brief but intense wind gusts could cause difficulty.
In summary, weather radar is a tool that can be used to help the mariner be meteorologically aware, but it is only one tool among many, and should not be relied on by itself to provide a complete weather picture.
About the writer
Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at the Massachusetts Institute of Technology. After working as a meteorologist for nearly 10 years for a large private consulting firm in Massachusetts, he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine in 1991. A large portion of his business at Locus Weather involves providing custom weather forecast services for oceangoing yachts, both racers and cruisers. Ken serves as an instructor for the Ocean Navigator School of Seamanship, and also as an adjunct instructor at the STAR Centers for Professional Maritime Officers in Dania, Fla., and Toledo, Ohio, and for MITAGS in Baltimore, Md. He has also taught meteorology at Maine Maritime Academy. He resides in Rockport, Maine with his wife and two sons. Ken’s website is: www.locusweather.com
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