Humidity refers to the amount of water vapor in the air, most commonly expressed as relative humidity. Relative humidity is a percentage indicating how saturated the air is with moisture compared to the maximum amount it can hold at a specific temperature. Since warm air holds more moisture than cold air, the same amount of water vapor results in a lower relative humidity percentage at a higher temperature. Oregon’s humidity experience is not uniform across the state, but is defined by distinct geographical regions.
Humidity West of the Cascade Mountains
The climate zone west of the Cascade Range, which includes the Coast and the Willamette Valley, is heavily influenced by the Pacific Ocean. This marine influence results in consistently high levels of relative humidity throughout most of the year. Along the immediate coast, relative humidity often reaches into the high 80s, especially during the wetter winter months.
Cities within the Willamette Valley, such as Portland and Salem, also experience high humidity, with morning readings frequently averaging between 84% and 87%. This high morning saturation is characteristic of an oceanic climate, often leading to fog and mist formation. The proximity to the ocean and the frequent weather systems moving inland keep moisture levels elevated during the fall, winter, and spring.
Summer brings a shift toward drier conditions, with a significant reduction in rainfall, though marine air still moderates the environment. During summer afternoons, relative humidity in the Willamette Valley typically drops to a range of 58% to 60%. The moderate temperatures in Western Oregon prevent the air from feeling heavy or oppressive, creating an environment that is damp and mild rather than muggy and hot.
Humidity East of the Cascade Mountains
A stark contrast in humidity exists east of the towering Cascade Mountains, which act as a formidable barrier to the Pacific moisture. This eastern area, encompassing the high desert and arid regions, is defined by a significantly drier climate due to the effect known as a rain shadow. As moist air is forced up and over the Cascades, it cools, condenses, and releases most of its precipitation on the western slopes, leaving the air dry by the time it descends eastward.
The extremely low relative humidity is a characteristic feature of this high-desert environment throughout the year. During the summer, afternoon relative humidity can plummet into the 25% to 30% range, with extreme conditions occasionally dropping as low as 10% to 20%. This low moisture content means that even when temperatures are high, the air feels much drier and less sticky than in the western part of the state.
This lack of atmospheric moisture contributes to a large diurnal temperature range, meaning a significant difference between daytime highs and nighttime lows. The dry air cannot trap heat effectively, allowing it to escape quickly after sunset, which causes temperatures to drop sharply. While morning humidity levels east of the Cascades can still reach 82% to 92%, similar to the west, the low daytime percentages make the overall climate experience substantially different.
Seasonal Shifts and Daily Variation
Humidity levels across Oregon are subject to pronounced seasonal and daily cycles. Winter is the most saturated period, particularly west of the Cascades, where the constant influx of Pacific weather systems leads to high relative humidity. In contrast, summer represents the driest time for the entire state, with the influence of the North Pacific High pressure system leading to clear skies and lower moisture content.
A distinct daily pattern in relative humidity is evident across all regions, driven by temperature fluctuations. Relative humidity consistently spikes overnight, with readings across virtually the entire state averaging between 82% and 92% in the early morning hours. This high nighttime saturation occurs because the air temperature is at its lowest, which reduces the air’s capacity to hold water vapor, causing the moisture present to nearly saturate the air.
As the sun rises and temperatures increase throughout the day, the air’s capacity to hold moisture expands, causing the relative humidity percentage to drop significantly. This drop is most noticeable during the afternoon, when temperatures peak and relative humidity reaches its lowest point. This daily fluctuation showcases the powerful temporal and geographic influences on Oregon’s moisture content.