Seattle’s reputation as one of the rainiest cities in the United States often suggests near-constant rainfall. However, the facts reveal a more nuanced story about the climate. Understanding the actual frequency of rain requires first establishing how meteorological science defines a “rainy day” to separate the city’s reputation from its measurable precipitation data.
How a “Rainy Day” is Measured
Meteorologists rely on a consistent, measurable standard to officially record a day of precipitation, rather than just a few drops or a brief sprinkle. The National Weather Service (NWS) defines a day with measurable precipitation as one where the accumulated liquid-equivalent moisture is \(0.01\) inches or greater within a 24-hour period. This minimum threshold is used uniformly across the United States for accurate climate records. This objective measurement excludes trace amounts of moisture, such as heavy fog or dew.
The Average Annual Frequency of Rain
Applying the \(0.01\)-inch standard, Seattle averages approximately 150 to 155 days each year with measurable precipitation. This frequency places Seattle among the US cities with the highest number of rainy days. The city’s location between the Olympic Mountains and the Cascade Range often leads to persistent, moisture-laden air masses. These masses produce light rain or drizzle over long periods, which is the primary source of Seattle’s reputation for persistent gloom.
Why Seattle’s Total Rainfall is Misleading
Despite the high number of rainy days, Seattle is not one of the wettest major US cities when considering total volume. The average annual precipitation is around 37 to 39 inches, which is only slightly more than the national average. This modest total is due to the characteristic nature of the rain, which is typically a light mist or drizzle rather than heavy downpours. Most precipitation events are prolonged but low-intensity.
Other major US cities experience fewer rainy days but accumulate a significantly greater volume of water. Cities like New York City and Atlanta, for example, each receive nearly 50 inches of total precipitation annually, recording measurable rain on only about 116 to 120 days. This difference highlights that Seattle’s high frequency of light rain is less about the sheer volume of water and more about the number of hours the sky remains overcast and damp. The low-intensity rain means Seattle rarely experiences the intense thunderstorms or heavy cloudbursts common in the Midwest or Southeast.
Seasonal Variation in Precipitation
The frequency of rainy days is not evenly distributed throughout the year, as the city’s climate follows a distinct seasonal pattern. The wet season spans roughly from October through March, accounting for the vast majority of the annual precipitation and rainy days. During this period, rain is often measured on more than half the days in a given month.
This concentration of moisture in the cooler months results in a notably dry summer season. The months of July and August consistently record the fewest rainy days and the lowest total rainfall, often leading to extended periods of sunny weather. This pattern is characteristic of a modified Mediterranean climate, featuring warm, dry summers following cool, wet winters.