IN HOT, humid August, fall foliage fanciers dream of a cold snap and shortened days that will speed a chemical reaction and cause leaves to change from quiet chlorophyll green to vibrant red, yellow and orange. It's quite a process--but autumn buffs don't need a lecture about carotene, xanthrophyll, anthocyanin and falling leaves.

What many of them want to know is how to get up-to-the-minute reports on where and when to see nature's annual deciduous color show, which will begin soon.

Vermont officials point out that "No one can accurately predict when the changing leaves will be 'best.' Colors will vary in different areas of the state, from mountains to valleys and from day to day. Usually a hot, dry summer will hasten the season, while more rain delays the onset of color. Early days of cold autumn weather can also affect the timing." That's worth keeping in mind, whatever area you decide to visit.

Listed below are the phone numbers for current leafy information from nine states. Some are special lines providing only foliage conditions, others are regular tourism information extensions. A few have recorded messages, and some are toll-free (800) numbers. In most cases, reports on conditions are not expected to begin until mid-September:

Virginia: 703-999-2266.

West Virginia: 800-624-9110.

Delaware: 800-441-8846 (out of state); 302-282-8667 (in state).

Pennsylvania: 800-VISIT PA.

New York: 518-474-4116 (out of state); 800-342-3810 (in state).

Connecticut: 203-566-2496.

Massachusetts: 617-727-3201.

New Hampshire: 603-224-2525 or 2526.

Vermont: 802-828-3239

WILLIAMSBURG FAIR: Combining a military encampment and an 18th-century fair, Colonial Williamsburg has scheduled its first "Publick Times and Fair Days" for the weekend of Oct. 22-23. In the past, the encampment was held over the Labor Day holiday and the fair ran in November. The enlarged show will extend over the entire Market Square and Courthouse Green areas.

Originally Publick Times embraced a period in the fall when Virginia's legislature and General Court held sessions in Williamsburg, then capital of the colony. The sessions attracted merchants, planters, socialites and vendors, making it an ideal time for a market or fair.

In a reenactment of an 18th-century encampment of Continental soldiers of the Revolutionary War, costumed military units--about 300 men and an equal number of camp followers--will erect tents on Friday evening, Oct. 21. For two days they will perform camp duties while 18th-century characters and 20th-century visitors mix. There will be craft demonstrations and live entertainment of the period, booths will feature 18th-century-style products, peddlers will hawk other items to the crowd, and an auctioneer will periodically sell off merchandise.

Food and drink will be sold at several locations on or near the Square and Green, ranging from light refreshments, baked goods, fruits and other items from booths and carts, to more solid fare at Chowning's Tavern.

The two-day program will open at 9:30 each morning and continue until after dark. With bonfires and cressets lighting the night on Saturday, visitors will be invited to join in learning 18th-century dance steps. Among Sunday's events will be fiddling contest, and fire drills using Colonial Williamsburg's new replica of an 18th-century fire engine.

More information: Phone toll free (from Virginia) 800-582-8976; (from other areas) 800-446-8956. Or write George Wright, Colonial Williamsburg Foundation, Post Office Drawer C, Williamsburg, Va. 23187.

SAN FRANCISCO BUSES: Repairs on San Francisco's famed cable car system won't be completed until next summer, but tourists can still get to Fisherman's Wharf, Nob Hill and Ghirardelli Square from Union Square.

Visitors can take the Muni Railway's No. 62 shuttle bus through Chinatown, down Columbus Avenue in Neapolitan North Beach to the Wharf and Ghirardelli. Gray Line runs London double-decker buses over a longer scenic route to the Embarcadero (waterfront), the Wharf and Square. The Muni (Municipal Railway) also serves the Wharf with several regular bus routes.

In addition, the Chamber of Commerce has been sponsoring a Historic Trolley Festival this summer using antique streetcars on Market Street. The festival ends Sept. 26.

For more information on the city's transportation and attractions, visit the Information Center operated by the Redwood Empire Association at the foot of Market Street in the Spear Street Tower of One Market Plaza.

JUNIOR LEAGUE TOUR: "The culture of a people is expressed through its cuisine," notes the introduction to a brochure announcing an up-scale food and wine tour of Italy and France scheduled next month by the Junior League of Washington, D.C.

Designed by International Cuisines of Birmingham, Ala., each 16-day trip--one departing Sept. 16 and the other on Sept. 30--is limited to 25 people and includes air fare, overnight accommodations in all cities, continental breakfasts, 14 lunches and ll dinners, all transfers, service charges and tips. Cost from Washington, based on double occupancy, is $2,995.

Travelers "will explore the homes of Brunello, Chianti, Barolo and Barbaresco in Italy and Beaujolais, Burgundy, Chablis and Champagne in France," notes the tour operator. They will also savor food created in the two and three-star restaurants of Troisgros in Roanne, Boyer in Reims, Lameloise in Chagny, L'Esperance in Vezelay and Enoteca Pinchiorri in Florence.

The League is a non-profit women's volunteer organization providing service to the Washington metropolitan area in the fields of health and education. A portion of the tour price is paid by International Cuisines to the League for use in its local projects.

Information or reservations: Write the Junior League of Washington, 3039 M St. NW, Washington, D.C. 20007, or phone International Cuisines toll free at 800-633-4734.