It's been one of those rare summer Sundays in Washington, crystal clear and not too hot. The trees sway gently in the wind, making thatrich sound of shivering leaves.

As the sun sets in a fiery sky, tourists and natives gather to listen to free outdoor concerts - a ritual heresince 1838, when the Marine Band first performed on the Capitol steps. The music is varied: jazz, folk, rock, country and classical.

And theaudience is devoted.

Ken Steiner, a music programmer at WPFW radio, has been going to the jazz concerts at Fort Dupont for four years. The concert series opens Saturday with singer Stephanie Mills and the Laverne Reed Dancers. Others scheduled to appear this summer include Melba Moore, Lionel Hampton, Yusef Lateef and Gato Barbieri.

"It's great music in fresh air, under the stars, for free," says Steiner, adding that through the concerts, musicians who normally play at expensive Georgetown jazz clubs can reach the black community in Washington.

And the park has some distinct advantages, since the valley setting offers good acoustics and a temperature about 15 degrees cooler than the rest of the city. Best of all, it's not in the airplane flight pattern.

The roar of the jets is apparently no deterrent for those who crowd the Jefferson Memorial for the Army, Navy, Marine and Air Force band concerts. And people-watching has never been better. There are more joggers and bikers at this strategically located monument now that the gas squeeze is on, but the audience remains a true mix of old and young, male and female, domestic and imported. You can always spot the veteran concert-goers: They come prepared with blankets, cushions, folding chairs, sweaters, insect repellant and a couple of cans of their favorite brew.

As with all outdoor events, the concerts are canceled in case of rain. All are free unless otherwise noted.

BULL RUN REGIONAL PARK - Off I-66 Centreville exit. Free to residents of Arlington, Fairfax and Loudoun countries and of Alexandria, Fairfax and Falls Church. Non-residents $3 per vehicle. Call 278-8880. July 8, Chesapeake; July 15, Grass Reflection; July 22, Friendship Fire Co. & Chord Foundation; July 29, Santmyer Family.

CABIN JOHN REGIONAL PARK - OFF TUCKERMAN LANE, ROCKVILLE. CONCERTS BEGIN AT 7:30. JULY 10 ARMY BLUES; JULY 17, ROCKVILLE CONCERT BAND; JULY 24, COUNTRY CURRENT; JULY 31, AIRMEN OF NOTE; AUGUST 7, NAVY COMMODORES; AUGUST 14, ARMY BLUES BAND.

CAPITOL WEST TERRACE - Concerts begin at 8. Mondays, Navy Band and Sea Chantersf July 9, Country Current; July 16, Commodores.Tuesdays, Air Force Band; Wednesdays, Marine Band; Fridays, Army band.

CHERRY HILL PARK - 312 Park Avenue, Aflls Church. 241-5077. Concerts begin at 7. July 31, Army Chamber Singer; August 9, Falls Church Civic Band; August 31, Commodores.

COURTHOUSE SQUARE - Rockville, 424-8000, ext. 305. Concerts begin at 7:30. July 3, Hermian Hymm; July 10, Bill Holland and Rent's Due; July 12, Commodores; July 17, Chesapeake; July 19, Kinor Dance Company; July 24, Barbershop Quartet; July 26, Electric Brigade; July 31, Managerie; August 2, Dance Transfer; August 7, Celestones, First Missionary Baptist Church Gospel Chorus; August 9, Original Crabtown Stompers; August 14, Lovegrove Lane Jazz Quartet; August 16, Morning Train; August 21, Rockville Civic Ballet; August 28, Country Current; August 30, Airmen of Note.

FAIRFAX CITY HALL - West Lawn, 10455 Armstrong Street, Fairfax City. 385-7855.Concerts begin at 8. Programs will move to Fairfax High School if it rains. July 5, Bob Brown Marionettes; July 12, City Band; July 19, Band From Tin Pan Alley; July 26, City Band; August 2, City Band; August 9, David Vernon and Dixie Rebels; August 16, Denny and Lee; August 23, Fairfax Jubil-Aires.

FORT DUPONT - Minnesota Avenue and Randle Circle SE. Performances begin at 8:30. Call 426-7723. June 30 and July 1, Stephanie Mills and Laverne Reed Dancers; July 3, Marie Brooks Research Dance Theather; July 7 and 8, Freddie Hubbard with Grover, Margaret and Za Zu Zaz; July 14 and 15, Linda Hopkins and Dinizulu African Dancers and Drummers; July 21 and 22, Ramsey Lewis, Tim Eyerman and East Coast Offering; July 24, Melba Moore; July 28 and 29. Alvin Ailev Repetory Dance Ensemble and Benard Sweetney Quartet; August 4 and 5, Gato Barbieri, Terry Callier; August 11 and 12, Woody Shaw, Bryon Morris and Unity; August 18 and 19, Dexter Gordon and Irene Reed; August 23 Lionel Hampton Orchestra and Marice Lyles Jazz Quintet; august 25 and 26, Yusef Lateef and Wayne Davis.

FORT WARD PARK - 4301 West Braddock Road, Alexandria.750-6325. Concerts begin at 7:45. July 3, Country Current; July 5, Snakes Alive with Dennis and Carol Moore; July 10, George Mason University Summer Orchestra; July 12, Army Blues; July 18, Kingsmen; July 19, Jazz Masters; July 25, Falls Church Civic Band; July 26, Bobbe's School of Dance; August 2, Reunion; August 9 and 10, Friends and Company Variety Show; August 14, Air Force Mach One; August 16, Blue Ridge Travellers; August 23, Bavarian Austrian dance Company; August 30, Alexandria Harmonizers; September 6, Riverside Ramblers; September 13, All Scottish Night.

FOUNDRY MALL - C & O Canal at 30th Street NW. Sundays at 1:30. July 1, Celtic Thunder and Federal Jazz Commission; July 15, Aaron and Joel Marcus, None of the Above; July 29, Charlie Byrd Trio, M Street Brass Quintet; August 12, Richard Smallwood Singers and Tom Cunningham Big Band; August 26, Double Decker String Band and Rick Hencerson Quartet; September 9, Irish Tradition and Dottie Dodgion Quartet; September 23, Londonaires and The Ambassodors.

GLEN ECHO PARK - MacArthur Boulevard, Glen Echo. Call 492-6282. Continuous ethnic and folk music from 1 to 4 each Sunday. July 1 at 1:30, African Dancers; July 8 at 1:30, Rachel Pena Traditional Spanish Dance Company; July 15 at 3:30, Cuban Voices and Rhythms; July 22 at 3, Armen Boyajian and and Air Apparent; July 29 at 3, Hamid Hossain and Afro-American Jazz Group; August 5 at 3, Maria Rodriguez Latin American Jazz Band; August 12 at 3, Capitol Jazz Quintet; August 19 at 3, Sweet Honey in the Rock; August 26 at 3, Mantwilla Nyomo; September 2 at 1, Machu Pichu, Archie Stewart, Noble Jolley.

GREENBELT LAKE PARK - Kenilworth Avenue and Cresent Road, Greenbelt. 474-6878. Concerts begin at 7. July 4, Greenbelt Concert Band; July 8, Country Current; August 5, Greenbelt Concert Band; August 19, Airmen of Note.

HISTORY AND TECHNOLOGY MUSUEM - Mall entrance, 14th Street and Constitution Avenue NW. Wednesdays at 6, beginning July 11, Navy Band.

JEFFERSON MEMORIAL - Through August, Friday at 8, Air Force Band; Sundays at 8, Marine Band; Mondays at 8, July 9 and 30, August 20 National Concert Band; Tuesdays at 8, Army Band; Wednesdays at 8, Torchlight Tattoo; Thursdays at 8:30, Navy band.

LIGHTSHIP CHESAPEAKE - East Potomac Park. July 23 at 7, Country Current; July 30 at 7, Commodores.

LUBBER RUN AMPHITHEATER - North Columbus Street, Arlington. Call 558-2161. All performances at 8 July 1, Ambassadors Jazz Ensemble; July 19, Navy Commodores; July 27, Navy Country Current; July 28, Airmen of Note; July 29, Army Blues; August 4, Army Brass Quintet; August 14, Arlingtones; August 15, Sweet Adelines.

MEADOW POND BAND SHELL - Burke Center, Wilmington Drive , Burke, Virginia. 978-7940. Concerts start at 7:30. July 15, Jazz Homecoming; July 29, 4th Maryland Volunteers Regimental Band; August 12,Ambassadors; August 26, Sousa's Stars and Stripes.

MINIPARK - Maple Avenue and Center Street, Vienna. 938-800 ext. 275. July 20 and August 5 at 7, Kingsmen.

NETHERLANDS CARILLON - Across from the Marine Corps Memorial, Arlington. Carlillon concerts, Saturdays at 2, through September.

NEWTON WHITE MANSION - 3006 Enterprise Road, Mitchellville, Md. 277-2200, ext, 396. Concerts begin at 7:30. July 11, Bluegrass festival; July 25, program to be announced; August 8, Show music.

TAKOMA PARK - Library Parking Lot, 101 Philadelphia Avenue, Takoma Park, 270-4048. Concerts begin at 7:30. July 17, Country Current; July 31 Commodores; August 21, Army Blues.

WASHINGTON NAVY YARD - Wednesdays at 8:40, except July, Navy Baand.

WOLF TRAP - Vienna. 938-3810. July 4 at 2:30, Air Force Band, Singing Sergeants, Airmen of Note. CAPTION: Picture, THE MARINE BAND, HERE SHOWN WITH LEADER JOHN PHILIP SOUSA AT THE TURN OF THE CENTURY, CONTINUES ITS TRADITION OF FREE SUMMER CONCERTS.