The setting sun is electrifying the bud-swollen trees, the leaves might dare to peep out a week ahead of schedule, and spring officially is here as of 11:33 a.m. Sunday with the vernal equinox.

In Philly and elsewhere at our latitude, spring is a season for all seasons. Its idyllic image and its beauty belie its wonderfully complex and sometimes tempestuous personality. In all its temperamental glory it tracks uncannily with what we know as “spring fever.” It’s a real phenomenon, the experts tell us, and it seems that the atmosphere comes down with the most severe case of it.

To welcome the new season — with us for 92 days, 17 hours, and 55 minutes — we offer 10 reasons why spring is special around here. Even the rain has a bright side.

That was fast

If it seems as if winter went fast, it did. And not just this one. It’s the shortest season in our hemisphere; summer is almost five days longer. That’s astronomy, not climate change. In its annual 574-million-mile orbital journey, the Earth makes its closest approach to the sun in January, and accelerates slightly, speeding the trip to the spring equinox. To calibrate our calendars, 2.25 days get snipped off February. If you want a longer winter, try the Southern Hemisphere, but not many habitable places get lots of snow.

Brightening up

The sun is getting more powerful. That’s also astronomy, not climate change. At the equinox, the sun beams double the wattage over Philly as at the winter solstice, according to NASA. The biggest gains in the sun’s power occur in March, edging out runner-up February. That explains why late-season snow often doesn’t have a snowball’s chance.

Speeding up

The leaves might be opening five to seven days ahead of schedule, says Bill Cullina, director of the Morris Arboretum. He says his maples, cottonwoods, willows, and elms are already in flower. Locally, the growing season does appear well ahead of schedule, based on the Cornell University’s Climate Smart Farming calculator. Various studies have documented that the growing seasons are growing. That’s climate change, not astronomy. In Philadelphia, in the last 30 years on average the last official freezing temperature occurred on March 30; in the previous 30 years, it was April 8.

» READ MORE: Pa. releases climate action plan to lower greenhouse gas emissions

Spring fever

It hasn’t worked its way into the Physicians’ Desk Reference yet, but mental-health experts say that “spring fever” is a real phenomenon that can alternately energize us, make us lose interest, brighten our outlooks on life, and turn the lights out. This all has something to do with that increasing solar light, say psychiatrist Norman Rosethal, who first identified Seasonal Affective Disorder, and other experts. Says Rosenthal, “Spring fever aptly describes a feeling that seizes you with its urgency and impulses.”

Hay fever

It’s as much of a misnomer as “daylight saving” time, but hay fever also is real. Spring is the only season with two serious allergen periods. Tree pollen is already popping, and reached “very high” levels at points last week, said allergist and pollen counter Donald Dvorin, who practices in Mount Laurel, Burlington County. He says the tree pollen peak is probably two weeks away. In May, grass pollen will start taking flight and continue tormenting the allergic into June.

» READ MORE: A dramatic start for the tree-pollen season

It’s not all that clear

Rainy days aren’t going to win many popularity contests, but they are medicine for allergy sufferers since rain seriously dampens pollen flight. Fortunately for them, April and May have plenty of them. April ranks No. 1 for days with measurable rainfall in Philadelphia with 11.3, followed closely by May at 11.1. And for sunshine, May comes in dead last for numbers of clear days.

A season for all seasons

Perhaps it’s bitterness at being the shortest season, but winter is almost always reluctant to go away, which helps explain all the rain and storminess and why the spring atmosphere is ripe for terrorizing tornadoes. In Philly, spring has been the host of a 19.4-inch Easter blizzard and temperature ranges from 10 to 100 degrees.

» READ MORE: Here's why the weather is so topsy-turvy this time of year

It’s not all show

With the landscape exploding with blooms and blossoms, spring is also the prime time for those images that “flash upon the inward eye,” to quote Wordsworth. But don’t miss the subtle pleasures of admiring the buds that have filled the trees, especially when they catch fire at sunset, and those enchanting first delicate greens are soon to follow.

Calibrate your compass

For the next few days, the sun will be rising almost due east and setting almost due west, says Karen Masters, astronomy and physics professor at Haverford College, and that won’t happen again for six months. From here the sun’s path will gradually shift northward, rising well north of east by the summer solstice before heading back the other way.

About the moon

The season’s first full moon will rise April 16, a day before Easter, which always occurs on the first Sunday after the first full moon of spring.

It is known as the “Pink Moon,” rather prosaically named for the colorful flora common in mid-April, according to the Old Farmer’s Almanac. Native Americans had more descriptive names, among them, Breaking Ice Moon, Moon When the Streams Are Again Navigable, Budding Moon of Plants and Shrubs, and Moon of the Red Grass Appearing.

Whatever it is called, it doesn’t get much better than silvery moonlight on April blossoms. So feel free to turn off the TV and snap shut the laptop.