South Big Island (PHWA): Located between Naalehu and Pahala at an elevation of 1,460 feet.Because of the elevation of the radar, it is also difficult for it to see the typically shallow trade wind showers beyond 50 miles. However, it provides poor coverage from the Hamakua Coast through Hilo. This location provides good coverage from south clockwise through northeast, including eastern Haleakala. North Big Island (PHKM): Located on the northern side of the Kohala Mountains at an elevation of 3,900 feet.However, it provides poor coverage of the east facing slopes of Haleakala. This location provides good overall coverage of Oahu, Molokai, Lanai, and most of Maui. Molokai (PHMO): Located in the western hills of Molokai at an elevation of 1,400 feet.However, it provides very poor coverage for northern Kauai. It also provides some coverage for western Oahu. This location provides good coverage south of Kauai, and can be used for tropical cyclone surveillance. South Kauai (PHKI): Located near Port Allen at an elevation of 290 feet.The energy transmitted by a radar travels in a straight line, and the terrain of the islands can block the radar beam. The location of the radars is important, and not just to make sure that they cover the entire state. In this mode the radar cycles through 14 elevations every 4.2 minutes. “Storm mode” is used during severe weather.This is the most common VCP that is used in Hawaii. In this mode the radar cycles through nine elevations every six minutes. “Precipitation mode” is used when rain is occurring.In this mode the radar cycles through five elevations every 10 minutes. “Clear air mode” is used primarily when conditions are dry.These are known as volume coverage patterns (VCP). There are predefined scan sets that control how the radar cycles through the different elevation angles. The farther away from the radar the beam travels, the higher above the ground it is located. These elevations range from 0.5° above the horizon to as high as 19.5° above the horizon. Because it’s important to see the elevated structure in features like thunderstorms, the radars regularly cycle through different elevations. Weather radars don’t just scan along one level. In VCP12, the radar will scan 14 elevation angles in 4.2 minutes. However, because the phase only changes when an object moves along the radar beam, the radar can only measure movement toward or away from the radar.Įxample storm mode VCP. The radar measures the change in phase of the beam between two pulses, and uses the difference to estimate the object’s speed. The radar also determines the velocity of the objects it detects, which can be used to infer wind speed in the atmosphere. ![]() For each hour of operation, the radar is only actually transmitting for seven seconds. However, each radar pulse lasts less than 1/600,000 of a second, which is followed by a listening period that lasts almost 1/1,000 of a second. The radar pulses have an average power of 450,000 Watts. Larger or more numerous objects return more energy, and result in a higher reflectivity. ![]() The energy is converted into reflectivity, and the time is converted into distance from the radar. The radar measures how much energy is reflected back, and how long it took to return. (The objects could be anything, not just precipitation one common problem around Hawaii is with sea spray being detected during windy days.) A small fraction of the energy is returned to the radar. The radar transmits an electromagnetic pulse, which reflects off objects in the atmosphere. ![]() This article includes information about weather radar in general, and about the specific radar sites we have in Hawaii. Weather radars are one of the most effective tools for detecting rainfall. This storm brought large hail to Kailua and Kaneohe, including a record-setting hailstone that measured 4.25 inches long, and generated a tornado that damaged homes in Lanikai. Radar image of a supercell thunderstorm from March 9th, 2012.
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