From the barren 4,200-meter peak of Mauna Kea on the Big Island to the lush valleys nourished by meters of annual rainfall on Kauai, Hawaii has no shortage of climatic extremes. But it wasn’t until earlier this year that climate divisions were finally defined for the 50th state. This new data set—which establishes official maps of climatically similar regions across Hawaii—will help ensure that the state is included in national climate analyses previously available to only the continental United States and Alaska.
Waiting in the 50th State
From historic county boundaries to the Zone Improvement Plan (ZIP) codes developed by the U.S. Postal Service in the 1960s, individual U.S. states have been continuously subdivided and categorized.
Initial efforts to divide the 48 states in the contiguous United States into climatologically based regions began in the early 20th century. But some of those divisions appeared to have been based more on geography, agricultural land use, or even the ease of communicating via mail. It wasn’t until the 1950s that state climatologists began incorporating climate data to create so-called climate divisions for each of the Lower 48 states.
These regions, which today range in number from 1 to 10 per state, encompass areas that are climatically similar in key indicators such as precipitation and surface temperature. Alaska received its own climate divisions—a record-setting 13—in 2015. But one state was, until earlier this year, conspicuously untabulated.
“We are excluded from a huge percentage of the so-called national products related to weather and climate.”
“Hawaii did not have official climate divisions,” said Xiao Luo, an atmospheric scientist at the University of Hawai‘i at Mānoa in Honolulu. Luo and her colleagues have now used precipitation data from 1990–2019 to define climate divisions for the 50th state.
Hawaii is the final state to receive official climate divisions, said Thomas Giambelluca, a climate scientist at the University of Hawai‘i at Mānoa and member of the research team. “We’re the last.” The lack of climate divisions in Hawaii means that the state has been omitted from analyses such as the U.S. Gridded Standardized Precipitation Index and the National Temperature Index.
“We are excluded from a huge percentage of the so-called national products related to weather and climate,” Giambelluca said.
From Stations to a Grid
To define climate divisions for Hawaii, Luo and her colleagues mined monthly precipitation data from more than 600 rain gauges spread across seven of Hawaii’s eight major islands: the Big Island, Kauai, Oahu, Maui, Molokai, Lanai, and Kahoolawe. (Niihau, Hawaii’s westernmost island, was not represented because of a lack of data.) The researchers then interpolated between those measurements to define a gridded data set of precipitation measurements with a resolution of 250 meters.
That was a critical step, said Chris Daly, a geospatial climatologist at Oregon State University in Corvallis not involved in the research. “You need spatially complete information, not just station data.” And it’s a challenging undertaking, said Daly, who helped to define the climate divisions for Alaska. “It takes a lot of data and a lot of expertise to be able to create these data sets.”
Looking for Patterns
Luo and her colleagues next used an algorithm to group the gridded data into clusters. The goal was to group together regions that exhibit similar precipitation patterns, said Abby Frazier, a climatologist at Clark University in Worcester, Mass., and a member of the research team. “We tried to figure out which regions in Hawaii have similar climates.”
The team reran the analyses allowing for anywhere from 8 to 16 clusters before settling on a dozen clusters. “The 12 divisions capture the entire state’s variability in rainfall,” Luo said. As a sanity check on their results, the researchers verified that the 12 regions also reflected differences in surface temperature.
“We have these incredibly wet places, but we also have these extremely dry desert locations.”
Twelve climate divisions might, at first glance, seem excessive for such a small state. After all, Idaho, Michigan, Mississippi, New York, Ohio, Pennsylvania, Texas, Washington State, and Wyoming each have only 10 climate divisions, and every one of those states is substantially larger than Hawaii. The only state with more climate divisions is Alaska, but it’s also more than 60 times larger than Hawaii.
However, it’s important to remember that Hawaii is extremely diverse, climatically speaking, said Frazier, who completed her graduate studies at the University of Hawai‘i at Mānoa. “We have these incredibly wet places, but we also have these extremely dry desert locations.”
This investigation is long overdue, Daly said, and the findings make sense. “The people working on it know a lot about the climate of Hawaii.” These results were published in the Bulletin of the American Meteorological Society.
There’s still more to do, Frazier pointed out. Climate division data sets that are billed as national aren’t quite there yet, she said. “There’s still nothing for Puerto Rico, Guam, and American Samoa.”
—Katherine Kornei (@KatherineKornei), Science Writer