The bizarre and catastrophic weather that caused severe flooding earlier this spring has transmuted and spread a severe drought that now threatens food supplies of tens of millions of people in Africa.

Leaders of six African nations have appealed to 160 relief organizations in the United States for an unprecedented level of emergency aid. Food production in Botswana, Lesotho and Swaziland has dropped to less then half the normal yield and 75 percent of those nations' livestock may be lost to drought and accompanying disease, Lesotho's ambassador to the United States, M'alineo Tau, said yesterday.

Eighteen African countries, from Ethiopia in the north to Zambia, Botswana, Swaziland, Malawi and Zimbabwe in the south, have been hit by drought. Some of the nations have curtailed development programs and are putting national efforts into feeding people and saving livestock.

The Overseas Relief Agency of the National Council of Churches last weekend urged churches to make the largest donation ever requested to help in the disasters around the world. The organization has requested U.S. member churches to donate $6.5 million in cash and goods, the largest appeal in the organization's 37-year history.

Paul McCleary, who heads relief effort and the special plea for money, said requests for help from around the world "are unprecedented and beyond our current resources."

The drought in Africa, considered to be the worst in this century, has forced some nations to begin importing massive quantities of food. In Botswana, for example, expected crop production for 1983 will be about 17,000 metric tons, compared with an average of 50,000 tons and a national consumption of 130,000 tons.

In Lesotho, about half of the population has been affected by the drought, with the country's crop yield a quarter of what it should be and some people eating seed they would otherwise use to plant next year's crop.

A spokesman for the Office of U.S. Foreign Disaster Assistance in the Agency for International Development said that although weather conditions vary widely in Africa and the world, overall the weather and crop disasters are more widespread and more locally severe than any seen for many years.

In addition to Africa, the worse droughts in 50 years or more have hit such widely scattered areas as Bolivia, Australia, the Philippines and parts of Southeast Asia.

In southern Africa, conditions have been made worse because some nations already have had two years of dry weather, and now fear that their land may become as desolate as the Sahel, the region south of the Sahara Desert that is characterized by chronic drought, the ambassador said.

Conditions in Africa and South America have prompted unusual action from U.N. Secretary-General Javier Perez de Cuellar, who will convene a meeting on Wednesday to help mobilize aid for the stricken countries in South America. Bolivia, Peru and Ecuador have been worst hit by floods in some areas and droughts in others. Hundreds have been killed, tens of thousands are homeless and damage is estimated to total about $1 billion.

The Agency for International Development has requested just under $100 million in disaster aid to help those South American nations.

The bizarre weather is believed to be caused by El Nino--winds and ocean currents that swept against the South American coast in unusual patterns this spring. El Nino, meaning "the child," was named by South American fishermen because it normally begins near Christmas.

The winds have dropped 300 percent more rainfall than normal in some parts of South America and left other parts of the world facing severe droughts. The same weather pattern causes both floods and droughts, in effect moving a large proportion of the world's moisture from drought-stricken areas and dropping it on flooded regions.