There is a mountain on Venus higher than Mount Everest on a plateau at least as large as Tibet that lies north of two volcanic regions almost as big as the Hawaiian Islands.

This surprising discovery was made by a radar device aboard the Pioneer spacecraft in orbit around Venus and revealed yesterday at the annual meeting of the American Geophysical Union by Dr. Gordon Pettingill of the Massachusetts Institute of Technology, where the radar to penetrate the thick clouds over Venus was designed and built.

Pettingill said in an interview there are three mountain groups along the rim of the plateau - at the northern end, at the eastern edge and in the west. The highest mountain in in the east and rises to about 36,000 feet; the lowest are the western mountains, reaching 15,000 feet.

The plateau size was described as 2,000 miles east-west and 1,000 miles north-south, stopping at a point 60 degree north of the equator on Venus. Pettingill said the plateau is three miles higher than the terrain to the south.

"We feel pretty sure the plateau and the mountains are tectonic and not volcanic, meaning they were uplifted out of the planet like the Himalayas were on Earth," Pettingill said. "The mountain peaks do not seem different from the Himalayas, which maybe tells you that these heights are all the surface gravity of Earth and Venus can support. Anything higher and the crust just sags."

Still in the north but nearer the equator lie what appear to be two huge volcanic "islands," one 15,000 feet high and the other 18,000. The larger one is bigger than the island of Hawaii. Together, the two islands span 700 miles from north to south, making them only a little smaller than the Hawaiian island chain.

Pettingill said the "scientific suspicion" is that Venus is far more like Earth than people imagined, with continental plates moving and drifting and a hot core producing active volcanoes.

"We're not as sure of the volcanics as we are of the tectonics, the mountains," Pettingill said, "but the suspicion is that they are volcanic. It certainly makes Venus a lot more interesting than the smooth-surfaced planet people used to think it was."

Almost as surprising was the discovery by Pioneer that winds on Venus blow as hard as 400 miles an hour.

Dr. Charles C. Counselman of MIT said in an interview that winds of that speed were measured at an altitude of 34 miles, which is just about the middle of the 10-mile-thick sulfuric acid clouds that forever hide Venus from view. The winds Counselman measured at other altitudes were not as fierce as those in the middle of the clouds.

"The middle cloud layer is the densest part of the clouds, where most of the sunlight and most of the infrared radiation scattered back from the surface is being absorbed," Counselman said. "We're literally seeing the strongest winds where the strongest heating effects are being observed."

Counselman said the winds blow hard almost everywhere on Venus except near the surface, where Pioneer measured winds no faster than 10 miles an hour. The wind at the tops of the clouds blew 200 miles an hour and at the bottoms of the clouds about 100 miles an hour.

All the winds measured blew from east to west with only slight deviations in a north and south direction. The strength of the winds and their single direction takes the intense heat in the atmosphere of Venus so rapidly from its day to night side that there is almost no change in temperature on the dark side of the planet.

The experiment Counselman conducted to measure the winds was as ambitious as it was unprecedented. It involved months of computer calculations of the movements made by four Pioneer probes that descended into the atmosphere of Venus last December. The calculations were drawn from the way the probes were blown by the wind as they descended to the surface of Venus.

Counselman said science expects to reap rich rewards from the wind measurements, in large part because they can provide clues to the origins of Earth's climate.

"The atmosphere of Venus is much simpler than the Earth's because it doesn't have much water in it," Counselman said. "On Earth, the water in the atmosphere is always freezing, melting, vaporizing and condensing, which throws off all the measurements we try to make here on Earth. The situation on Venus approaches the pure and simple case that scientists love to deal with." CAPTION: Picture 1, 2, 3 and 4, Between clearly defined white polar caps on Venus, a relatively dark band of clouds in the equatorial region moves and changes under the fierce winds. NASA's Pioneer spacecraft sent back these pictures between Feb.2 and March 3. AP