IT'S SURE TO get cold any day now, and as temperatures drop, concern increases for the homeless in our midst. Spring, summer and fall we may be able to delude ourselves that all those people in the mid-city squares and parks are enjoying the fresh air; in winter, their need and desperation cannot be ignored. There have always been homeless in our communities -- the chronically unemployed, people with drug and alcohol problems, vagabonds -- but in recent years their numbers have been augmented by a new kind of displaced person: deinstitutionalized mental patients.

The American Psychiatric Association has just published a task force report on the subject and it should be read by all policy-makers seeking to understand and cope with this phenomenon. The doctors discuss the disappointments of the movement, begun 30 years ago, toward a system of community, rather than institutional care for the mentally ill. The development of psychoactive drugs, the well-intentioned efforts to make involuntary commitment more difficult and the belief that patients would receive better care in smaller settings close to home all encouraged planners to reduce institutional population. At the same time, increasing federal aid to disabled individuals and to local communities for mental health centers provided an incentive to try new forms of care. As a result, the number of mentally ill living in large state institutions has fallen from 559,000 to 132,000 in the past 30 years.

Deinstitutionalization would have worked for many patients if governments had provided sufficient funds for outpatient care. More than shelter is needed; supervision and treatment are essential, and the assumption that they would be available was part of the early planning. The APA task force report details the kinds of services and facilities that are needed and that must be increased and says that without these resources, the promise of community care is a hoax.

Finally, there is the fact that institutional care will remain the only alternative for many -- not only the dangerous but the severely disabled who are incapable of caring for themselves and coping with the stress of everyday life. The word "asylum," after all, means a place of sanctuary, safety and rest. Good state institutions provide these as well as food, shelter, medical care, respite for families, therapy and the social support of staff and companions. The psychiatrists remind us that this kind of care remains the best alternative for some of the homeless mentally ill on our streets.