Panting heavily in the thin air of the Colorado Rockies, the jogger struggled up Cemetery Hill, the grueling climax of a five-mile race under a hot summer sun. Just when he began to think he wouldn't make it, a brass quintet, sitting in a grassy field at the summit, launched into a peppy marching tune. The music put a smile on the jogger's face and managed to pump enough spirit into his heart to propel him across the finish line.

How did a classical ensemble turn up in that mountain meadow? In Aspen, home of a renowned summer music festival, it's not at all unusual. Impromptu concerts in the out-of-doors are as much a part of the scene as the formal program inside the festival tent. The musicians had volunteered to toot when they knew the joggers needed it most.

Come summer and the American countryside blossoms into a vast open-air stage for the performing arts. From coast to coast, concert halls and theaters swing open their confining winter doors, and the musicians, the actors, the dancers and the audiences all troop outside for three months of culture under the sky. It's the season for festivals--opera, dance, Shakespeare, musical comedy, jazz, historical dramas. Celebrations of nature and the arts.

In amphitheaters, tents, bowls, sheds, parks (or whatever the stage may be called), communities large and small offer up a fascinating array of productions. Just to hear the names of a few chases away lingering cold-weather blues: The Symphony of the Hills Festival (Dodgeville, Wisc.), A Festival of Two Lakes (Ticonderoga, N.Y.), Symphony on the Prairie (Noblesville, Ind.); Festival of the Rockies (Estes Park, Colo.) and Music at the Vineyards (Saratoga, Calif.).

In plans for a summer trip, consider seeking out one or more of the many musical and dramatic presentations. As the sampling that follows suggests, the choice is wide:

For delightful outdoor ambiance, Music at the Vineyards ranks high.: On a dry, sunny weekend, climb through the Paul Masson vineyards to the top of the hill in the rolling Santa Cruz Mountains south of San Francisco. There, under the cooling shade of colorful umbrellas, listen to a wind ensemble from the Chicago Symphony play Mozart while your eyes take in the stunning view of the San Francisco Bay area. Ocean breezes riffle the performers' music, and the air is fragrant with ripening fruit. At intermission, tickle your nose with a sample of the winery's champagne.

Now entering its 26th year, the vineyard weekend festival this year includes 70 performances from mid-June to September, alternating between chamber music concerts; pop, folk and jazz entertainers (Rosemary Clooney, Joan Baez, Ray Charles); and Shakespeare ("As You Like It," "Love's Labor Lost"). There's seating for 1,000, but the programs on Friday evenings and Saturday and Sunday afternoons often sell out three months in advance.

Up north on the rugged coastline of Alaska, an American frontier that seems an unlikely spot for a classical music festival, the folks in the once-Soviet outpost of Sitka are presenting their 12th annual chamber music series June 3-24. Visiting artists, donating their time, will join violinist Paul Rosenthal (an Alaska transplant who originated the festival) in nine concerts in the 500-seat Centennial Building. Though this festival is indoors (it rains a lot), behind the stage are giant plate-glass windows with a view of Sitka's superbly scenic harbor. On opening night this season, listen to a Haydn string quartet and a Brahms piano quartet while the ships pass by on the horizon.

The story is that famed cellist Gregor Piatigorsky, a participant in 1974, was so impressed with caliber of the musicians (the festival has earned high praise from a major New York critic) that he refunded his $500 airfare and contributed an additional $500 to the festival budget. Since practically any performance series can be billed as a festival, the quality varies from amateur to promising student to the very best in the country_the latter to be found in programs at internationally famed festivals that include: the Berkshire Music Festival with the Boston Symphony at Tanglewood Shed in Lenox, Mass.; the Blossom Music Center series with the Cleveland Orchestra at Cuyahoga Falls, Ohio; the Hollywood Bowl Summer Festival with the Los Angeles Philharmonic

Travelers show up at Tanglewood, nestled between beach-fronted lakes and summer camps in the Berkshire Mountains, primarily for the Boston Symphony. For eight weeks it performs every Friday and Saturday evening and Sunday afternoon under Tanglewood Shed--sometimes drawing an audience of 30,000 over the course of the weekend. Rain or shine, listeners can recline on the lawn or sit under the shelter to appreciate some of the finest musical talent around. This summer's schedule includes performances by Leontyne Price, Alicia de Larrocha, Peter Serkin and Andre Previn.

Sharing the summer's music festival is Boston University's Tanglewood Institute, offering top-notch musical instruction to high school-aged musicians. And the Berkshire Music Center provides training for older students (over 18). On Aug. 25, the two student groups join the Boston Symphony to celebrate "Tanglewood on Parade," a gala evening performance. During the afternoon, there's ballooning in the mountains or almost continuous chamber music emanating from festival halls.

(When the students and symphony members aren't practicing or performing, they crowd the nearby lakes, swimming, sailing and sunbathing.) Tanglewood and the other big festivals are (mostly) serious music, but there's a lot of fun (and a bit of hokum, too) in festivals of drama, often major presentations uniting professional performers in the leading roles and aspiring amateurs in the lesser parts. And usually there's no stinting on numbers: huge choruses and crowd scenes that are really crowded. Typically, they are original works, romantic histories with a touch of tragedy. The tragedies, it seems, sell better.

Out in the Texas Panhandle near Amarillo, deep inside a rugged canyon, a cast of 80 actors, dancers and singers nightly reenacts the struggles and joys of the early pioneers to the region in a lively musical drama called "Texas." With the 600-foot-high canyon walls as a backdrop to the 1,600-seat amphitheater, they offer up songs and dances, simulated brush fires and dust storms and a spectacular lightning display that cascades down the cliffside.

It's cowboys and Indians stuff mixed with Texas whoopie that has been drawing crowds for 17 years. There's a campground nearby if you want to spend the night, and a barbecue (what else?) dinner before every show.

The oldest of the outdoor dramas is "The Lost Colony," the tragic story of America's first English colony first presented on July 4, 1937 (in celebration of the 350th anniversary of the birth of Virginia Dare, the first child of English parentage born in America). Staged summer-long on North Carolina's Roanoke Island, it's now seen by about 80,000 visitors annually.

In Beckley, W. Va., the famous 19th-century duel of the "Hatfields and McCoys" is recreated, alternating with "Honey in the Rock," the tale of a mountain family caught in the anguish of the Civil War that created the state of West Virginia (picnic-style dinner before the show). Bardstown in Kentucky is the home of "The Stephen Foster Story," a big, colorful musical rioffering one night's entertainment (usually at a quite reasonable price) that is also part education, an introduction into the lore of the land more compelling than any guidebook. Other festivals hope the visitors will linger to take in several performances while they explore the community's other attractions.

Such is the case with the Oregon Shakespearean Festival in Ashland, where you could see six plays in three days if you really wanted to, but you would be better off spreading them out over a week or two as many people do. This is mountain country (the Siskyous). Crater Lake National Park and the Rogue River are nearby, so between productions there is plenty of opportunity for hiking, white-water raft trips and sightseeing.

This festival actually runs from February to October in two small indoor theaters, but it is transformed into a summer fest with the seasonal opening of the large Outdoor Elizabethan Stage in June. Rotating plays this summer are "Much Ado About Nothing," "Richard III" and "Cymbeline." "Hamlet" will be performed at an indoor stage along with several non-Shakespearean warhorses and one new play.

Audiences are invited to attend workshops on the plays; they can tour backstage and even dress up in some of the costumes; and frequently actors and other members of a company are available for informal conversation in the park that surrounds the theaters.

If the mountain setting of the Siskyous doesn't strike a proper chord, back East on the sea you can join the champagne crowd at Newport, R.I. A love for opulence, lavish parties and, perhaps, a trunk full of evening gowns and tuxedos is all it takes to be "in" at the Newport Music Festival.

In mid-July, the community buzzes with lovers of romantic chamber music. The festival also opens its door to obscure, seldom-heard works. On alternating nights, Newport's exquisite estates (the Breakers, Bellcourt Castle, Rose Cliff) play host to the best of Reineke, Ries and Reissiger. There's a cash bar at every performance and "apres concert" parties at other mansions or private beach clubs.

For outdoor splendor, though, it would be hard to beat the Aspen festival, and the music is pretty good, too. This restored silver mining community, nestled at almost 8,000 feet in the Rockies, is famous for its skiing. But once the mountains shed their snow, they are ideal for hiking, and there's rafting, horseback riding, ballooning, swimming, golf and other sports activities readily available.

Like a number of other musical celebrations, the Aspen Music Festival is part music school (120 faculty members, 800 students), with professional-caliber newcomers auditioning for a place on one of five of the festival's faculty and student orchestras. From June 24 to Aug. 21 this year, there's at least one concert a day, with the major performances scheduled for weekends, and lots of open rehearsals, master classes, recitals, seminars and panel discussions. For all but the big events, tickets are usually readily available in the 1,600-seat tent.

But the real charm of festival time is the spontaneous concerts erupting all over town when the students are out of class. Sit on a park bench in the mall, where small streams trickle down the street through well-tended flower gardens, or relax with a beer at an outside cafe. Sooner or later, a group of musicians will be serenading you.

In Aspen, Tanglewood and Texas (and points in every direction), the hills--to borrow from Broadway--are alive with the sound of music. CAPTION: Picture 1, The San Jose Symphony at Paul Masson's Summer Series in California. Picture 2, In Alaska, Sitka'sfestival features chamber music with a stunning in the composer's songs.

Why Bardstown, which bills itself as the bourbon capital of the world? It's the hilltop site of "My Old Kentucky Home" State Park, the brick mansion belonging to a relative of Foster's that inspired one of his most-popular songs. The outdoor stage is in a wooded area on the state park grounds. Like the "Texas" extravaganza, the historic dramas aim for the traveler passing through,