AT THE SMITHSONIAN: Last year 22,401,341 visitors moved through various Smithsonian Institution museums; that represented a 2 percent increase over 1981. The busiest single month was August, with 3.2 million visitors. The slowest was January with 711,953 visitors. Nearly 40 percent of the visitors came during the summer months.

About 97 percent of those visitors went to the seven Smithsonian museums on the Mall; five museums are located elsewhere in the city. The National Air and Space Museum was the only museum open after 5:30 p.m.; between 5 and 9 p.m., the total attendance there was 854,920.

Attendance varies widely from museum to museum, season to season. Some typical days at six Smithsonian museums:

* American History: summer, 16,645; winter, 10,798.

* Natural History: summer, 19,012; winter, 12,270.

* Hirshhorn: summer, 3,842; winter, 2,664.

* National Portrait Gallery: summer, 1,017; winter, 1,009.

* African Art: summer, 246; winter, 240.

* Air and Space: summer, 42,649; winter, 18,693.

Yearly figures for all 12 museums:

* Air and Space: 8 million (up 28 percent over the past two years).

* Natural History: 5 million.

* American History: 4 1/2 milion.

* Hirshhorn, Arts and Industries Building and Smithsonian Institution Building (Castle): 1 million each.

* Freer Gallery of Art: 300,000.

* National Portrait Gallery/National Museum of American Art: 360,000.

* Renwick: 200,000.

* African Art: 90,000.

* Anacostia: 40,000.