WELLINGTON, New Zealand (AP) _ The Antarctic's unpredictable weather forced the postponement today of a mission to pluck an American doctor with breast cancer symptoms from a South Pole research station.

Rising temperatures that had led to hopes Dr. Jerri Nielsen could be picked up today suddenly plummeted, forcing a New York Air National Guard crew to postpone their takeoff from McMurdo base on Antarctica's coast.

``This morning we assessed the weather and it looked favorable for a while,'' U.S. Air Force spokesman Capt. Victor Hines said from Christchurch, New Zealand.

But the temperature quickly plummeted to minus 69 degrees, ``and our forecasts said it was not going to get any warmer,'' he said.

At temperatures below minus 58, aircraft hydraulics _ crucial for operating undercarriages and flaps used in landing and taking off _ become sluggish, making flying risky.

Hines said the weather would be reassessed later today.

The postponement meant at least one extra day's wait at the bottom of the world for Nielsen, the only doctor at the U.S. National Science Foundation's Amundsen-Scott South Pole Station.

Nielsen, who discovered a lump on her breast, has reportedly been treating herself with chemotherapy since a daring airdrop of medical supplies in July.

When temperatures allow, a Hercules plane with skis for landing gear will set off on the 1,680-mile roundtrip to the pole in an operation that must be executed at high speed to prevent the freezing temperatures from crippling the plane.

The Hercules will land on a runway carved out of polar ice, pick up Nielsen, drop off a replacement doctor and fly out again _ all within 20 minutes and without turning off the plane's four propellers, officials said.

A second plane will remain at McMurdo as backup.

``We will make a final assessment tomorrow morning ... and hopefully we can go down and pick up Dr. Nielsen,'' Hines said. ``We want to go down and pick her up as soon as we can.''

After arriving at McMurdo, Nielsen is expected to transfer to another military aircraft for a flight to New Zealand, where she will catch a flight to the United States.