While Venus's weather at ground level is pretty boring - a steady 800 degrees Farenheit and heavy sulfuric acid clouds - it all gets a bit more interesting further up, say NASA scientists.
While Earth has seasons because its rotation axis is tilted by about 23 degrees, Venus has been tilted so much it's almost completely upside down. This gives it a net tilt of less than three degrees from the sun, so the seasonal effect is negligible.
"Also, its orbit is even more circular than Earth's, which prevents it from getting significantly hotter or cooler by moving closer to or further away from the sun. And while you might expect things to cool down at night - especially since Venus rotates so slowly that its night lasts almost two Earth months - the thick atmosphere and sulfuric acid clouds act like a blanket while winds move heat around, keeping temperatures pretty even," says Dr Tim Livengood, a researcher with the National Center for Earth and Space Science Education.
"Finally, almost all the planet's water has escaped to space, so you don't get any storms or precipitation like on Earth where water evaporates and condenses as clouds."
However, a new study of old data has revealed strange things going on at about 68 miles above the planet's surface, in the mesosphere and the thermosphere.
"Although the air over the polar regions in these upper atmospheric layers on Venus was colder than the air over the equator in most measurements, occasionally it appeared to be warmer," says Dr Theodor Kostiuk of NASA Goddard.
"In Earth's atmosphere, a circulation pattern called a 'Hadley cell' occurs when warm air rises over the equator and flows toward the poles, where it cools and sinks. Since the atmosphere is denser closer to the surface, the descending air gets compressed and warms the upper atmosphere over Earth's poles. We saw the opposite on Venus."
In addition, although the surface temperature is fairly even, the team observed changes of up to 54 degrees Fahrenheit within a few Earth days in the mesosphere. While the poles appeared more stable, there were still changes of up to 27 degrees Fahrenheit.
This variability could have many possible causes, according to the team. Turbulence from global air currents at different altitudes flowing at more than 200 miles per hour in opposite directions could exchange hot air from below with cold air from above to force changes in the upper atmosphere. The giant vortexes which swirl around each pole could also generate turbulence and change the pressure, causing the temperature to vary.
And because the atmospheric layersare above the cloud blanket, they may be affected by changes in sunlight intensity as day transitions to night, or as latitude increases toward the poles - or even be affected by solar activity.
Changes were seen over periods spanning days, weeks and a decade. Temperatures measured in 1990-91 are warmer than in 2009.
"In addition to all these changes, we saw warmer temperatures than those predicted for this altitude by the leading accepted model, the Venus International Reference Atmosphere model," said Kostiuk. "This tells us that we have lots of work to do updating our upper atmospheric circulation model for Venus."