Does it really rain less at the airport?
October 27, 2022
If you compare recent monthly precipitation totals at the Haines Airport weather station with data from the townsite, you’ll see that it rains and snows significantly less at the airport, a couple miles away.
But does it?
As of Monday, it had rained 12.5 inches in town this month, but only 8.8 inches at the airport, according to the National Weather Service.
The common explanation for the discrepancy is that the airport is in a drier microclimate. In general, as you move up the valley from town, the less precipitation you’ll experience.
But local National Weather Service observer Jim Green is skeptical that the gap in recorded rainfall this month, and in many recent months, reflects a real discrepancy. More likely, he said, the precipitation gauge at the airport isn’t catching all the rain and snow that’s falling.
“There’s no physical explanation obvious to me for why (precipitation) would be less at the airport. In fact, if you used the topography and prevailing winds, it should be more there,” said Green, who has a degree in meteorology.
Green, who manually records weather observations at his house in town — listed on the National Weather Service’s website as “Haines #2” — measured a total of 11.2 inches last January. Meanwhile, the airport’s precipitation gauge — an automated measurement system located at 4 Mile Haines Highway — registered only 4.6 inches that month.
“The precipitation data from that station is highly suspect,” Green said. “It’s systematically too low from what it really should be.”
Green said he has done manual bucket tests to compare rainfall at the airport and at his house and has observed similar precipitation levels. He suspects an electrical box near the gauge might be blocking it from rain and especially snow, particularly on days when the wind blows from the west, or that the gauge records less precipitation because it’s windier at the airport than in town, and wind can affect catchment, especially during periods of light rain or snow. The recorded levels tend to be more similar when there’s heavier precipitation, Green said.
Some months, like this past September, the totals have been close — 10.7 inches at the airport, 10.2 in town. But other months the discrepancy sums to several inches. Since the weather station was installed at the airport, in 1988, the smallest discrepancy in monthly recordings at the two sites is August, when the airport gauge has reported 93% of the precipitation level in town, and the largest has been January — 58%.
There is also a weather station at the U.S.-Canada border — at 40 Mile Haines Highway — and Green observed that the January precipitation level at the airport drops off compared to the two other stations. Green said the airport has recorded, on average, more precipitation than the border in August, but less in January.
Kimberly Vaughan, National Weather Service Observing Program Leader in Juneau, said the service considers all concerns and analyzes data for irregularities, and electronic technicians check the Haines Airport weather station quarterly to ensure it’s working properly.
She had spoken with Green about his concerns in 2020 and with his help identified inaccurate readings at the time. Between January and July 2020 there was an issue with the heater on the airport rain gauge, which turns on in winter to melt snow into water that can be measured by the nearest hundredth of an inch. The heat inadvertently had been evaporating water, causing readings that were inaccurately low.
But “that's long been fixed,” Vaughan said. She said “wide discrepancies” in precipitation amounts can be expected at different observation sites in communities across Southeast due to natural variations from one microclimate to another. She pointed to National Weather Service data in different Juneau locations as an example. Mean monthly precipitation levels vary by a few inches between the forecast office in the Mendenhall Valley and the airport four miles away.
It’s possible the Haines Airport is merely in a drier microclimate than town. Haines Avalanche Center director and forecaster Erik Stevens said the Chilkat Valley’s “dry belt” could start at the airport and extend up to about 18 Mile. Downsloping — winds moving down mountainsides — can clear out clouds and have a drying effect, which could explain lower precipitation levels at the airport, based on where it sits in the valley, Stevens said.
Stevens observed the disparity tends to be largest during winter months. He said that supports his theory of downsloping, which is caused by strong southwest winds in fall and winter, and by “overrunning” of moist warm air over cooler air, a phenomena that could cause more precipitation in town during winter storms.
The avalanche center maintains a precipitation tracker using monitors at numerous locations in the borough, including atop Mt. Ripinsky and at Letnikof Cove, that show several different microclimates. Stevens said the microclimate argument aligns with his personal observations but that Green also might be right.
Green similarly said he understands Stevens’ perspective but that his own theory also explains why the gap in recorded precipitation levels is bigger in winter. Winds tend to be stronger, which could affect snow catchment, and they more often come from the west, when the electrical box threatens to obstruct the gauge.
“I think that it’s hard to definitively say one way or the other without some backup data. But my impression is that it’s not actually that much drier at the airport,” Green said. “I think the proximity of that (electrical) box is possibly the biggest factor that’s causing it to be as far off as it is.”
Vaughan said she would mention the concern to the technicians who check on the station.
With several days to go, this month has already been Haines’ rainiest October in more than two decades, according to National Weather Service data.
Three storms registered over three inches of rain in town, contributing to a monthly sum of 12.45 inches, as of Monday. In recent history, that’s third to 14.2 inches in October 2000 and over 16 inches in 2008, Green said, but numerous years last century had even higher October totals.
The weather station at 40 Mile Haines Highway, on the U.S.-Canada border, had recorded 15.92 inches of precipitation as of Monday, also more than any other October since 2000.
More rain and possibly snow are in the forecast for the remainder of the month.
The National Oceanic Atmospheric Association is predicting a third straight La Niña in coming months, a global weather pattern that could create a cooler and wetter winter in Haines.
*Correction: This article originally reported that the airport rain gauge is four miles from town. In fact, it is only about two miles from the station at Jim Green's house ("Haines #2").