A little-known secret of many great-tasting wines made in Washington is literally the difference between night and day.

Called the diurnal shift, it marks the difference between the highest daytime temperature and the lowest temperature at night. This high-low variance can have a big impact on vineyard grapes during summer months.

Here in the Inland Northwest, the variance is fairly steep. In Yakima, the typical diurnal range is 35 degrees this time of year. In Walla Walla, a town recently described by a New York Times columnist as “Napa in blue jeans,” it’s 28. Conversely, the typical late summer range in Chicago is 17 degrees, and 20 in St. Louis.

How do temperature swings make wine taste so good? WSU viticulturist Michelle Moyer explains it this way: Our summer’s warm, sunny daytime hours boost sugar levels in the grapes, while the cool nighttime temperatures help them retain their acidity. The resulting sugar-to-acidity ratio ultimately gives rise to vibrant, fresh-tasting wines.

The main reason for the region’s dramatic temperature fluctuations is the dry air that makes our weather so comfortable. Moisture and humidity – like those levels seen in Chicago – produce smaller diurnal ranges because air with abundant water vapor is more resistant to temperature change than dry air. Also, in muggy areas like Chicago, clouds tend to block the sun during daytime and radiate heat back to the ground at night. This translates to cooler days and warmer nights, the opposite of what we experience here.

The next time you sip a delicious Washington-made chardonnay, merlot or syrah, you can thank your lucky stars for our cool summer nights that give way to warm and dry sunshine-filled days.

Nic Loyd is a meteorologist with Washington State University’s AgWeatherNet. Linda Weiford is a WSU news writer and weather geek. Contact: linda.weiford@wsu.edu or nicholas.loyd@wsu.edu.