
I've posted this graphic before only to be told that summer does not begin on May 6th but on June 21st. I've watched enough weather to know that the cycle of seasons differs quite a bit from place to place depending on a lot of things: mountains, vast plains, rain-forests, distances from the moderating influences of the ocean (or even large bays, lakes and rivers.)
Being a mere dilletante weather-watcher, obviously I'm compressing a lot of variables into a big generalization when I talk about the weather or climate. Also this is a pretty rambling and not particularly well organized look at the subject.
My theory about why summer begins on the West Coast on May 6th and not later as it does elsewhere is that the biggest influence on our coastal climate is the sun's "position in the sky" (that is obviously not a scientifically correct description). Of course the Pacific Ocean has an influence too but not as dramatic as the inland mountains and plains. The sea and the coast are so linked that there's not much difference between them but, as the weather moves eastward, it is altered drastically by the dry mountains and plains.
So, the sun is the real "engine" of our weather and here on the coast the sun has a "purer" effect than it has inland. By that I simply mean that it's "position in the sky" affects us more "purely" as it has not been altered by mountains and plains.
When I first posted this graphic last year at this time and was contradicted, I referred to Shakespeare's sonnet "Shall I compare thee to a summer's day?" He continues" "Thou art more lovely and more temperate; rough winds do shake the darling buds of May and summer's lease hath all too short a date."
May was summer to Shakespeare. His western England has a similar climate to the Northwest (as do Wales, western Ireland and Scotland.) They call it a "maritime climate" and they refer to inland Europe's weather as a "continental climate" which - except for the three coasts - is what the USA also has. (The west coast of Britain and Ireland are further north than here but they have a warm current coming up from the south that moderates their climate.)
The Mediterranean Sea has it's own influence on southern Europe which is said to have a "Mediterranean climate." Southern California has the same climate as southern Spain. Los Angeles is actually on the same latitude as Morocco. By the time you get to San Francisco the climate is like Portugal's and Oregon's climate is like that of Bordeaux - halfway between Mediterranean and maritime - which is why we produce such fine wines here.
So anyway, as I was saying: the sun has a more straightforward and "purer" effect on us at the coast since it is not yet effected by mountains and plains. It's "position in the sky" determines our seasons.
On December 21st the sun's effects are concentrated on the southern hemisphere and, here in the north, we only have a weak southern sun for about 8 hours a day. It's midwinter. Once the sun has "reached the furthest south" that it can go, it begins its "journey northward" and, by February 6th, it has once again begun to concentrate its energy on the northern hemisphere. Our days are 10 hours long. The sap starts rising in the deciduous plants. Tiny almost invisible buds appear. It's the beginning of spring.
By March 21st the sun's energy is equally divided between the two hemispheres and we have 12 hour days. By May 6th when the sun begins "to move even further north" we're having 14 hour days and every flower that can bloom is. It's the beginning of summer.
By June 21st the sun is as "far north as it can go" and it's permanent day at the North Pole. At our latitude, we have 16 hour days. It's midsummer and, from then on, it's down hill all the way. The days get shorter. By August 6th they've shrunk to 14 hours. The weather is balmy because the earth has been warmed for three months by now and everyone thinks "Ah, finally summer!" But really it is the beginning of fall and autumn is, as Keats said, a "close bosom friend of the maturing sun."
By September 21st the sun's energy is equally divided between the two hemispheres and we have 12 hour days. By November 6th the sun begins "to move even further south" and by now we're having only 10 hour days and the earth is starting to cool off. The sap begins to retreat in deciduous plants. It's the beginning of winter.
By December 21st the sun is as "far south" as it can go and it is midwinter - back to square one.
That's what I mean when I say that the sun's "position in the sky" is the biggest influence on our coastal weather. Between here and the East Coast there are all sorts of other influences. Mountains and plains have a big influence but probably the biggest difference between West Coast weather and the rest of the USA is because of the influence exerted by the Gulf of Mexico. The Great Lakes also alter the "pure" influence of the sun's "position in the sky." And the closer you get to the East Coast, the more influence the Atlantic has on the weather too.
But here on the West Coast our weather is "purely" a matter of the sun's "position in the sky" and our summer begins on May 6th as it did last Sunday when it was a balmy 70F and the mayflies, midges and mosqitoes suddenly swarmed.
By Monday it was 80F and the other major West Coast weather phenomenon began to kick in - wind. Yep, "rough winds do shake the darling buds of May." This wind happens whenever it's sunny here. As the air inland heats up and rises, cold air from the Pacific rushes in to fill the void. Without the wind, our temps would be over 90F most of summer. (The air from the Pacific is cold because of the Alaskan current which moves south along the coast and keeps the ocean below 50F all year round.)
But you really know that summer has arrived when the "fog" (or more correctly the "marine layer") is blown in from the ocean. (The "marine layer" is a fog-bank which hangs just off the coast over the cold Alaskan current all the time.) After two or three days of sunny, warm and windy days, the fog comes rolling in and keeps our summers cool and moist with daytime temps around 65F and night-time temps of 50F.
This is what the "marine layer" looks like from about 600 feet above sea-level at the top of the mountain on whose western slope I live.

The top of the "marine layer" hovers at about 300 feet above sea-level. My house is 170 feet above sea level and is completely enveloped by the fog. It's like living in a cloud-forest most of the summer.