Does Seattle Have Humid Summers? Seasonal Breakdown of the City's Humidity

The Humidity Levels in Seattle Throughout the Year

Seattle is known for its mild temperatures and abundantly overcast skies. But when it comes to humidity, this Pacific Northwest city sees quite a bit of variation throughout the year. In the summer, Seattle transitions to warm and dry weather, with daytime highs in the 70s and low 80s Fahrenheit. However, the humidity tends to remain relatively low during these months. Once autumn rolls around, things start to change. The rainfall picks up dramatically from September through November, resulting in higher humidity levels. The humidity peaks during the winter rainy season before falling again in the spring.

So to answer the question “Is it humid in Seattle?”, the response depends on the time of year:

• Summer (June through August) – Not very humid. Average afternoon relative humidity around 60%.
• Fall (September through November) – Humidity increases. Average afternoon relative humidity 65-75%.
• Winter (December through February) – The most humid time of year. Average afternoon relative humidity 75-85%.
• Spring (March through May) – Humidity starts to decrease. Average afternoon relative humidity 65-70%.

The humidity in Seattle follows a predictable seasonal pattern of peaking during the winter rainy months and declining during the summer. While humidity levels can feel noticeable from fall through early spring, Seattle generally doesn’t experience the oppressive, muggy conditions that are common across the southern U.S. in summertime.

Weather Patterns That Influence Seattle's Humidity

Seattle's humidity is heavily influenced by its marine climate location in the Pacific Northwest. Proximity to the Pacific Ocean has a moderating effect on temperatures and also contributes moisture to the air mass. Here are some of the key weather factors that shape Seattle's distinctive seasonal humidity:

Winter Rainfall

During the winter months, Seattle experiences frequent rainstorms, with average precipitation of 5 to 6 inches each month. These rains saturate the air with moisture, resulting in high relative humidity levels. Overcast skies during the winter also prevent daytime heating and evaporation that would normally lower the relative humidity. The overall effect is high humidity levels persisting from late fall through early spring. Morning and late night humidity levels can approach 90-100% during rainy spells.

Marine Air

Seattle's adjacent location to Puget Sound and the Pacific Ocean has a major influence on humidity. When winds blow from the southwest, they transport moist marine air from over the ocean and sound into the Seattle area. This moist air mass increases humidity levels and also brings cloudy skies. The onshore flow is especially common during the transition seasons of spring and fall. Southerly winds can also transport moisture northwards from the Oregon coast into Seattle during fall and winter.

Dry Seasonal Winds

While marine winds enhance moisture and humidity during cooler months, Seattle experiences much drier winds patterns in the summer. As inland regions to the east heat up, areas of low pressure draw dry air from the Great Basin and Columbia Plateau towards the Pacific Northwest. These seasonal east and northeast winds, sometimes referred to as the "Eastside Eddy", bring hot and dry conditions that lower humidity in Seattle during the summer and early fall. Daytime relative humidity can fall below 40 percent at the height of summer.

The Rain Shadow Effect

The Cascade Mountains have a rain shadow effect that limits how much moisture from Pacific storms reach Seattle. As wet systems move inland from the Pacific and encounter the mountains, the first elevations like the Olympic Mountains and western slopes of the Cascades wring out much of the moisture. This leaves less precipitation to fall on Seattle and interior Puget Sound regions. Without as much rain re-moistening the air during summer, Seattle experiences more periods of lower humidity even for a coastal city.

Daily and Seasonal Humidity Cycles in Seattle

Over the course of a typical day or throughout the year, Seattle's humidity levels follow some consistent cycles. Here are the highs and lows:

Daily Cycles

• The highest humidity generally occurs during the overnight and early morning hours.
• After sunrise, the humidity starts to decrease through the late morning.
• Early to mid-afternoon is usually when the relative humidity reaches a daily minimum.
• The humidity begins rising again during the late afternoon and evening.

This cycle occurs as air temperatures warm during the day, allowing the air to hold more moisture. Then as temperatures cool at night, relative humidity increases. Cloud cover, wind shifts, and precipitation can alter this typical pattern.

Seasonal Cycles

• Summer has the lowest average humidity.
• Humidity increases during fall.
• Peak humidity occurs during the winter rainy months.
• A decline in humidity begins in spring.

This seasonal humidity cycle corresponds with rainfall patterns and the influence of marine air versus dry continental air masses through the year.

How Seattle's Humidity Compares to Other U.S. Cities

When looking nation-wide, Seattle experiences moderately high humidity levels during parts of the year compared to other major U.S. cities. Here is how it compares:

• The humidity is higher than desert and high elevation cities like Denver, Las Vegas, and Phoenix.
• Seattle sees more humidity overall than west coast cities like Los Angeles and San Diego.
• The summer humidity is lower than muggy southern cities including Houston, Memphis, Miami, and New Orleans.
• During winter, Seattle is more humid than many northern cities such as Chicago, Minneapolis, and New York.

So Seattle ends up with higher humidity than some peer Pacific coast and northern cities, especially during the wet season. Yet it lacks the oppressive humidity of southern U.S. summers. The city finds a middle ground with periods of noticeable humidity balanced by comfortable summers.

High Humidity Cities

Here are some U.S. cities that experience higher humidity levels than Seattle nearly year-round:

• Houston - The combination of muggy Gulf of Mexico air, ample rainfall, and high summer temperatures make Houston one of America's most humid cities. Afternoon relative humidity averages between 60-80% year-round.
• New Orleans - New Orleans has high humidity nearly every day, with relative humidity levels frequently exceeding 75%. Like Houston, its Gulf Coast location provides moisture.
• Orlando - Frequent thunderstorms and moist tropical air flowing from the Atlantic Ocean and Gulf of Mexico make Orlando deeply humid. Humidity rarely falls below 60%.
• Miami - Miami experiences humid subtropical weather with high temperatures and abundant moisture off the Atlantic Ocean and Gulf of Mexico. Relative humidity averages in the 75% range.

Low Humidity Cities

These major U.S. cities have lower humidity levels than Seattle, especially in the summer:

• Las Vegas - A desert climate and abundant sunshine result in low humidity year-round. Typical summer relative humidity ranges from 20-40%.
• Phoenix - Phoenix only sees an average humidity around 30-50% through the year. The city's southwestern desert location stays hot and dry.
• Denver - Thin air, low precipitation, and sunshine add up to modest humidity in the Mile High City. Values usually top out near 50% in summer.
• Los Angeles - LA sees pleasantly dry weather, with summer humidity staying in the 50-60% range even on hot days.

Measuring Humidity in Seattle

Meteorologists track humidity levels in Seattle using automated sensors at locations like Seattle–Tacoma International Airport and weather stations around the region. These devices measure the air temperature and dew point temperature to calculate the relative humidity.

The dew point temperature indicates the atmospheric moisture content. It is the temperature to which air must be cooled for saturation and condensation to occur. The greater the difference between air temperature and dew point, the drier the air. Smaller differences correspond to higher relative humidity.

Computer models also estimate the humidity over Seattle and the Pacific Northwest using weather data, satellite observations, and our understanding of atmospheric physics. This information helps forecast the humidity in the coming days.

Apps and websites like Weather.com and Apple Weather provide current humidity readings and forecasts for neighborhoods across the Seattle metro area. Home weather stations can display indoor and outdoor humidity as well.

Typical Seattle Humidity Levels

To get a snapshot of Seattle's humidity, here are the average daily relative humidity values by month:

• January – 84%
• February – 82%
• March – 79%
• April – 75%
• May – 71%
• June – 63%
• July – 61%
• August – 63%
• September – 69%
• October – 76%
• November – 80%
• December – 83%

The cycle of lower summer humidity and higher readings during the wet season is evident. Within each month, the humidity also shifts higher at night and lower in the afternoon.

Tips for Dealing with Humidity in Seattle

While Seattle's seasonal humidity may not reach extreme levels, higher moisture levels can still be uncomfortable. Here are some tips for managing humidity:

Home Humidity Control

• Use air conditioning to lower indoor humidity on muggy days. Set the AC to around 70° F for a comfortable moisture level inside.
• Run a dehumidifier to remove excess moisture from the air.
• Seal air leaks and properly insulate walls to prevent moisture intrusion and mildew risks.
• Ventilate bathrooms, laundry, and cooking areas to let humid air escape.

Personal Comfort

• Dress in breathable natural fabrics like cotton and linen to stay cooler.
• Stay hydrated and avoid strenuous work on very humid days to prevent overheating.
• Take a cool shower to lower your body temperature and refresh.
• Spend a few hours in an air conditioned building to give yourself a break from the moisture.

Health Precautions

• Watch the weather forecast so you can plan for humid days. Visiting the doctor? Pick a lower humidity day.
• Comply with any medical device operating humidity ranges so moisture doesn't impact care.
• Have allergy medication on hand, as higher humidity can worsen symptoms.
• Stay up-to-date on immunizations, as humid conditions favor growth of some infectious diseases.

Paying attention to weather forecasts allows you to anticipate periods of high humidity in Seattle. Making smart preparations allows you to keep the moisture in check and stay comfortable indoors and out.

The Outlook for Seattle's Future Humidity

Looking ahead at long-term climate projections, Seattle may see some changes in humidity levels and patterns:

• Rising global temperatures are expected to increase evaporation and overall atmospheric moisture content.
• Seattle may experience higher humidity if the region becomes wetter under climate change.
• At the same time, the summer dry season could intensify and partially offset humidity increases.
• Heat waves fueled by climate change may cause spikes in perceived humidity through the warm season.

Research also suggests a warming planet could modestly alter the characteristics of Pacific Ocean weather patterns like El Nino and La Nina cycles. This would introduce some year-to-year variations in Seattle's humidity levels.

While the specifics remain uncertain, Seattle will likely continue to see its familiar seasonal humidity cycle but with some periods of enhanced moisture content. The near-term weather forecast will still be the best guide to prepare for humid or dry conditions.

The Bottom Line

Seattle experiences a distinct seasonal cycle of humidity owing to its mid-latitude marine climate. While not exceedingly muggy, humidity levels are noticeably higher from autumn through early spring compared to the summer dry season.

Within each day, nighttime and early morning hours tend to be the most humid. Later in the afternoon is when the relative humidity reaches a minimum. Tracking humidity forecasts allows you to plan outdoor activities during lower moisture periods. Or take measures at home to offset the effects of Seattle's familiar humid, misty weather when it moves in.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult with a healthcare professional before starting any new treatment regimen.