The Hoh Rainforest, tucked into the western slopes of Washington’s Olympic Mountains, receives roughly 140 inches of rain in a typical year. That figure masks considerable swing: records from the watershed show annual totals ranging from about 92 inches to nearly 178 inches depending on the year.
What Those Numbers Actually Mean
A long-running precipitation record at the Hoh watershed found annual totals varying between 2,336 and 4,518 millimeters across the monitoring period, with the bulk of that rain arriving between October and May.1JAWRA Journal of the American Water Resources Association. TRENDS IN PRECIPITATION AND STREAM CHEMISTRY IN A PRISTINE OLD‐GROWTH FOREST WATERSHED, OLYMPIC NATIONAL PARK, WASHINGTON Converted to inches, that range spans roughly 92 to 178 inches, with about 140 inches as the long-term average. For perspective, the average across the contiguous United States sits around 30 inches a year. So even in a “dry” year, the Hoh gets more than three times the national average.
The range matters because visitors who show up in a low-rainfall year might see the forest looking surprisingly parched in late summer, while a high-rainfall year can turn every trail into a streambed by November. Year-to-year swings of 80 or more inches are not unusual, driven largely by how many Pacific storms track directly into the Olympic Peninsula during a given winter.
Why One Spot Gets So Much Rain
The Hoh Rainforest sits on the windward side of the Olympic Mountains, directly in the path of moisture-laden storms rolling off the Pacific Ocean. When that humid air hits the mountains, it is forced upward. As it rises and cools, the moisture condenses and falls as rain or snow. This process, called orographic precipitation, is the engine behind the Hoh’s extraordinary rainfall totals.
Research using high-resolution precipitation observations and mesoscale modeling has mapped how this works in detail across the Olympics. As stable airflow moves over the ridges of the western Olympics, regions of enhanced cloud-water condensation form above the windward slopes. Precipitation that is already falling from higher-altitude storm clouds collects additional moisture as it passes through these orographic clouds, growing heavier on its way down.2Quarterly Journal of the Royal Meteorological Society. The climatology of small‐scale orographic precipitation over the Olympic Mountains: Patterns and processes The result is that ridge crests in the western Olympics accumulate roughly 50 to 70 percent more precipitation than the valleys just a few miles away.3Quarterly Journal of the Royal Meteorological Society. The climatology of small‐scale orographic precipitation over the Olympic Mountains: Patterns and processes
This means that even within the Hoh watershed, the amount of rain varies significantly over short distances. The valley floor where you walk the Hall of Mosses trail is one of the wetter spots, but the ridges rising above it are wetter still. The peaks above the tree line receive much of their precipitation as snow, which feeds the glaciers that in turn feed the Hoh River throughout the summer.
The Rain Shadow on the Other Side
The flip side of the Hoh’s drenching is one of the most dramatic precipitation gradients in North America. Modern annual precipitation across the Olympic Mountains ranges from roughly 6,500 millimeters on the wettest western slopes down to about 500 millimeters in the driest areas of the northeastern rain shadow.4Earth Surface Dynamics. Spatially coherent variability in modern orographic precipitation produces asymmetric paleo-glacier extents in flowline models: Olympic Mountains, USA In plain terms, the western side can see over 250 inches of precipitation in a year, while the town of Sequim, barely 60 miles to the northeast, averages around 16 to 17 inches. Sequim often markets itself as a sunny retirement destination while the Hoh stays buried in cloud.
This gradient is not a gentle slope. Much of the drop-off happens in the span of 20 to 30 miles as you cross the mountain crest. By the time Pacific air has climbed the western slopes and dumped its moisture, there is very little left for the eastern valleys. The contrast shapes everything from vegetation to wildfire risk to local agriculture. The Hoh side grows moss-draped Sitka spruce; the dry side grows grass and madrone.
When the Rain Falls and When It Doesn’t
One of the more surprising things about the Hoh is that it has a genuine dry season. Most of the annual rainfall is packed into the cooler months, from October through May.1JAWRA Journal of the American Water Resources Association. TRENDS IN PRECIPITATION AND STREAM CHEMISTRY IN A PRISTINE OLD‐GROWTH FOREST WATERSHED, OLYMPIC NATIONAL PARK, WASHINGTON Summers are comparatively dry, and the rainforest goes through weeks with little or no significant rainfall. This is part of the broader Pacific Northwest pattern, where the jet stream and its associated storms shift northward in summer, leaving a high-pressure ridge parked off the coast.
Research monitoring soil moisture in the Olympic Rainforest canopy and on the ground found sharp dry-downs during the summer dry season in both canopy soils (the mats of organic material perched on tree branches) and terrestrial soils below.5Botany. Patterns of moisture and temperature in canopy and terrestrial soils in a temperate rainforest, Washington Canopy soils stayed dry until the first autumn rainfall broke the drought, while the ground-level soils began rehydrating about a month earlier, possibly because tree roots were pulling moisture up from deeper layers and releasing it into the upper soil. This pattern means the lush epiphytes clinging to tree branches are actually the most drought-vulnerable part of the ecosystem.
For visitors, the practical takeaway is real: July and August are the driest months, and you might hike the Hoh for days without getting rained on. By mid-October, the spigot turns back on, and the forest returns to the dripping green it is famous for.
Atmospheric Rivers and the Big Floods
Not all of the Hoh’s rain comes in gentle drizzle. A significant portion of annual precipitation arrives via atmospheric rivers, narrow corridors of subtropical moisture that slam into the Pacific Northwest coast like fire hoses aimed at the mountains. These events can drop several inches of rain in 24 hours and are responsible for the most dramatic floods in the Hoh watershed.
A 4,000-year sediment record from the Olympics shows that the frequency and intensity of these flooding events has fluctuated over centuries, driven by shifts in ocean-atmosphere circulation patterns that steer atmospheric rivers toward or away from the peninsula. Researchers interpreted the sediment record as reflecting periods when atmospheric river landfalls were more or less common, with notable spikes in flood frequency around 2,350 to 2,450 years ago and again in the most recent century, from about 1910 to 2010.6The Holocene. A 4000-year record of hydrologic variability from the Olympic Mountains, Washington, USA The modern increase in flood layers may reflect natural climate variability, human-driven warming, or some combination of both.
These intense rain events shape the landscape in ways that gentle rain cannot. They trigger landslides, strip gravel from hillsides, and deliver enormous loads of sediment to the Hoh River, reworking the valley floor with each major flood.
What All That Rain Does to the River
The Hoh River is a braided, gravel-bedded system that responds almost in real time to the precipitation it receives. Its source is the Blue and Hoh Glaciers on Mount Olympus, and the river picks up water from dozens of tributaries as it flows west through the rainforest to the Pacific. Multi-decadal analysis of channel-planform changes in the Hoh and three other Olympic rivers found that the Hoh has been trending toward greater braiding over recent decades. Researchers attributed this to increased sediment supply caused by rapid glacial retreat: as the glaciers shrink, they expose fresh rock and sediment that washes downstream during high flows.7Earth Surface Processes and Landforms. Channel‐planform evolution in four rivers of Olympic National Park, Washington, USA: the roles of physical drivers and trophic cascades
The Hoh’s sediment-routing system is relatively short and lacks large reservoirs or floodplain areas that could buffer sediment before it reaches the lower river. This means climatic signals (more rain, more glacial melt, bigger floods) translate almost directly into changes you can see on the ground: the river widens, new gravel bars appear, and the main channel shifts. Homes and roads along the lower Hoh have been repeatedly threatened by lateral migration of the channel, a problem that will likely intensify as glaciers continue to recede and sediment supply stays high.
How the Forest Absorbs and Recycles Its Rain
The Hoh Rainforest is not just a passive recipient of rain. Its structure actively shapes how water moves through the ecosystem. The thick moss layer that blankets nearly every surface plays a major role. Mosses act as sponges: they absorb rainfall, slow its descent to the soil, and release it gradually. Research on moss-covered soils found that the water-retention properties of soil beneath moss cover were better than those of bare soil, with the moss layer’s own water-holding capacity influencing how much moisture reached the ground beneath it.8ResearchGate. Structural characteristics of the moss (bryophyte) layer and its underlying soil structure and water retention characteristics
In a place that gets 140 inches of rain a year, you might assume soil erosion would be a constant problem. But the combination of moss, thick duff layers of decomposing organic material, and a dense root network means the forest floor absorbs most of that water without losing significant topsoil. The soil chemistry reflects this efficiency: studies of precipitation as it filters through the canopy and into the soil of the Hoh found that the forest strongly retains nitrogen and phosphorus, with soil solution concentrations of these nutrients staying low even as rain washes through.9Soil Science Society of America Journal. Canopy and Soil Modification of Precipitation Chemistry in a Temperate Rain Forest The slightly acidic rain that falls on the canopy gets neutralized as it percolates through the soil, reaching a near-neutral pH by 40 centimeters depth.
Fallen trees add another dimension. In the Hoh, nurse logs (large trunks from fallen trees) serve as elevated platforms for new tree seedlings. A study in a northern temperate rainforest found that tree seedling density on nurse logs was about 4.6 times greater than on the forest floor.10PubMed Central. Plant–plant interactions change during succession on nurse logs in a northern temperate rainforest In a forest this wet, germinating on a raised log keeps seedlings above the saturated ground and gives them access to light gaps in the canopy. The decomposing wood holds moisture without becoming waterlogged the way the forest floor sometimes does. This is one of the reasons the Hoh’s most iconic feature, the colonnade of trees growing in a row on a vanished nurse log, exists at all: the rain itself creates conditions where growing on dead wood is better than growing on the ground.
Summer Drought and Climate Projections
The idea that a rainforest could be vulnerable to drought sounds paradoxical, but the Hoh’s summer dry season already stresses parts of the ecosystem, and climate models suggest the stress will get worse. Regional projections for the Pacific Northwest point toward wetter winters and drier summers, meaning total annual precipitation might hold steady or even increase while the seasonal gap between the wet and dry months grows wider.5Botany. Patterns of moisture and temperature in canopy and terrestrial soils in a temperate rainforest, Washington
For the Hoh, this pattern has specific consequences. The epiphyte communities that give the forest its character, the ferns, mosses, and lichens draped over every branch, depend on moisture year-round. Extended summer droughts could push canopy soils past the point where epiphytes can survive, gradually thinning the moss mats and changing the forest’s appearance. A Hoh Rainforest with 150 inches of rain a year but a four-month dry summer would look and function differently from the current version, which gets that rain distributed over eight to nine months.
Warmer temperatures also affect the snow-to-rain ratio at higher elevations. More winter precipitation falling as rain rather than snow means less snowpack to feed glaciers and sustain summer stream flows. The Hoh River, already showing signs of increased sediment from glacial retreat, could see lower summer flows and more extreme winter floods, a combination that would alter habitat for salmon and other species that depend on cold, clean, year-round water.
How the Hoh Compares to Other Rainforests
Temperate rainforests are rare globally, found in narrow strips of coastline where mountains intercept moist ocean air. The Hoh’s 140 inches of annual rainfall puts it among the wettest spots in the contiguous United States, but the definition of a temperate rainforest depends on more than just total rainfall. The key is a combination of heavy precipitation, mild year-round temperatures (freezing is rare at valley-floor elevation), and relatively low seasonality in temperature even if the rain itself is seasonal. The Hoh meets all three criteria.
Tropical rainforests typically receive their rain more evenly throughout the year, though many have wet and dry seasons as well. The Hoh’s sharp concentration of rainfall into the winter months, followed by a pronounced summer drought, makes it ecologically distinct. Trees in the Hoh grow slowly compared to tropical counterparts because winters are cool and dark, but individual trees live far longer, with Sitka spruce and western red cedar routinely reaching 500 years or more. The forest’s biomass per acre rivals that of tropical forests despite the slower growth rate, simply because the trees have centuries to accumulate wood.
The moss and epiphyte load in the Hoh is also exceptional by temperate-forest standards. In some areas, the weight of moss and ferns on a single tree exceeds what you would find on an equivalent tree in a tropical cloud forest. This is partly a function of the rain but also of the fog that rolls up the valley even during the dry season, providing supplemental moisture to canopy communities when rain is absent. Fog drip, where tiny water droplets collect on leaf and moss surfaces and drip to the ground, adds a small but ecologically meaningful amount of water to the forest’s total moisture budget that never shows up in rain gauge data.