This report describes the typical weather at the Seattle-Tacoma International Airport (Seattle, Washington, United States) weather station over the course of an average April. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.
Seattle, Washington has a mediterranean climate with dry warm summers and mild winters. The area within 25 mi of this station is covered by forests (61%), built-up areas (17%), oceans and seas (17%), and croplands (4%).
The month of April is characterized by rising daily high temperatures, with daily highs increasing from 54°F to 60°F over the course of the month, exceeding 70°F or dropping below 48°F only one day in ten.
Over the course of April, the length of the day is rapidly increasing. From the start to the end of the month, the length of the day increases by 1:35 hours, implying an average day-over-day increase of 3.2 minutes.
The shortest day of the month is April 1 with 12:53 hours of daylight; the longest day is April 30 with 14:28 hours of daylight.
The earliest sunrise is at 5:53am on April 30; the latest sunset is at 8:21pm on April 30; the latest sunrise is at 6:47am on April 1; the earliest sunset is at 7:39pm on April 1.
Daylight savings time is observed in 2012 but it neither starts nor ends in April, so the entire month is in daylight time.
The median cloud cover is 85% (mostly cloudy) and does not vary substantially over the course of the month.
On April 15, a representative day, the sky is clear or mostly clear 20% of the time, partly cloudy 23% of the time, and mostly cloudy or overcast 57% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around April 2, occurring in 61% of days. Precipitation is least likely around April 30, occurring in 55% of days.
Throughout April, the most common forms of precipitation are light rain and moderate rain.
Light rain is the most severe precipitation observed during 57% of those days with precipitation. It is most likely around April 3, when it is observed during 34% of all days.
Moderate rain is the most severe precipitation observed during 31% of those days with precipitation. It is most likely around April 4, when it is observed during 20% of all days.
The likelihood of snow falling is highest around April 1, occurring in 4% of days.
Either snow rarely accumulates at this location in April or snow depth measurements are unavailable or unreliable.
The relative humidity typically ranges from 49% (comfortable) to 91% (very humid) over the course of a typical April, rarely dropping below 32% (comfortable) and reaching as high as 100% (very humid).
The air is driest around April 30, at which time the relative humidity drops below 58% (mildly humid) three days out of four; it is most humid around April 1, rising above 87% (very humid) three days out of four.
Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.
Over the course of a typical April, the dew point typically varies from 34°F (dry) to 46°F (dry) and is rarely below 26°F (dry) or above 52°F (very comfortable).
Over the course of April typical wind speeds vary from 2 mph to 15 mph (light air to moderate breeze), rarely exceeding 21 mph (fresh breeze).
The highest average wind speed of 9 mph (gentle breeze) occurs around April 1, at which time the average daily maximum wind speed is 15 mph (moderate breeze).
The lowest average wind speed of 8 mph (gentle breeze) occurs around April 29, at which time the average daily maximum wind speed is 14 mph (moderate breeze).