WeatherWhys: Did you know Boise is technically in a desert? Here’s how it affects weather
Take a drive south down Interstate 84 between Boise and Mountain Home and take a moment to glance out of the window at the side of the highway.
The lush green grass and trees around Boise will make way for a more arid landscape, with sparse vegetation and yellowing grass becoming prominent closer to Mountain Home.
If it weren’t for irrigation in and around the Treasure Valley, the scenery around Boise would look much the same as it does down I-84. Head even farther south to the Owyhee Mountains and trekkers will find themselves in the Owyhee Desert, which looks like something that belongs in Nevada as opposed to Idaho.
That’s because Boise is actually in a desert, or more accurately, a high desert. Idaho is on the upper edge of the Great Basin Desert, which is the largest desert in North America and spans much of Nevada and Utah.
While irrigation techniques can be used to alter the landscape, such as what’s seen around Boise, there’s no stopping the weather patterns that dictate the Treasure Valley’s climate.
Here’s exactly what it means to be in a high desert:
A wide range of temperatures
The Treasure Valley has a “typical high desert temperature regime,” according to the National Weather Service. That means the nights cool down much more quickly and the days heat up more quickly too than the average climate.
For example, the average high in Miami, Florida, in July and August is 87 and 88 degrees, respectively. For Boise, those high temperatures are 91 and 90 degrees, respectively. Meanwhile, the average low in Miami for those two months is 78 and 79 degrees but it drops as low as 60 and 59 degrees, respectively, in Boise.
Boise sees such fluctuations because of the low amount of moisture in the air, in part due to the mountain ranges to the west, as well as the high altitude of the city (2,730 feet above sea level).
“The higher the water content in the air, the humidity, the high temperature comes down and the low temperature comes up,” Idaho News 6 chief meteorologist Scott Dorval told the Idaho Statesman.
The lack of moisture in the Treasure Valley — Boise averages just 11.7 inches of precipitation annually — also leads to drier soil and in turn higher temperatures, Dorval said.
The wind and the rain
Around 75% of Boise’s annual precipitation comes during the winter and spring months, specifically between November and May.
That sounds like the perfect recipe for widespread snow, but Boise only averages about 20 inches of snow a year — about one inch of precipitation is equal to 10 inches of snow, Dorval said.
In comparison to the 200 to 250 inches of snow that Bogus Basin gets annually, Boise’s 20 inches seem like a measly amount. Being at a much lower altitude than Bogus Basin it makes sense for Boise to get less snowfall, but the Treasure Valley’s annual snowfall is further impacted by the warmer temperatures that persist year-round.
Boise is generally much warmer than other locations along the same latitude, according to the weather service.
The precipitation amount is also impacted by the rain shadow effect, which is when a patch of land has been forced to become a desert, such as the Treasure Valley, because of mountain ranges blocking rainy weather and forcing dry air into the area.
“The air will come over the Owyhee Mountains and sink into the valley and dries us out,” Dorval said. “So that’s kind of why we just don’t get much (precipitation).”
Which leads to the final point: How does wind affect Boise’s high desert climate status?
When a storm is coming in from the Pacific Ocean and into the Treasure Valley, wind is also blowing in from the east. As the wind is channeled through the mountains from the Idaho Falls region in east Idaho it will drop into the valley from about 4,000 feet to 2,730 feet above sea level.
When that happens, the inrushing wind increases the air pressure in the valley which in turn raises the temperature and clears out any potential storms that would enter the valley.
“What happens is that southeast wind that blows in ahead of a big storm dries the air out in the lower atmosphere,” Dorval said. “It can rain and snow in the mountains around us and very little, if anything, will happen in the valley.”
