If you're driving from Ashland in winter enjoying the sunshine, you've probably noticed a thick blanket of fog greeting you as you get closer to Medford.
Welcome to the Rogue Valley's inversion layer. Since the valley sits in a bowl surrounded by mountains, there sometimes is very little wind to mix up the air.
"We have particularly poor ventilation here because of the geography," said Daniel Hough of Jackson County Environmental Health.
High mountains and a relatively deep valley keep out winds, resulting in moist, cold air settling to the bottom. Gain a little altitude anywhere in the valley and the mists clear up. The temperature at higher altitudes can be warmer than at lower altitudes when this condition occurs.
"The temperature is switched," said Ryan Sandler, meteorologist for the National Weather Service. "It's switched from what it normally is."
When a high pressure system sits over the valley for a week or so, the colder air drains down into the valleys, particularly during the long winter nights.
During these episodes the low temperature in Medford can be 33, while the high is 39.
Often the temperature in Medford can be about 38, but at Sexton Summit north of Grants Pass the temperature can hit 55 at an elevation of 3,800 feet, said Sandler.
Grants Pass, because it has a long narrow valley, can have worse inversion problems than Medford. Sandler said the sun often breaks through in Medford while it remains foggy in Grants Pass.
During the summer, the inversion layers aren't as problematic. Sandler said there is usually an afternoon wind that clears out the valley.
However, particularly in the early summer and fall, temperatures in the valley can dip down to 45 to 50 degrees, but remain at 70 degrees on ridge tops, said Sandler. This can cause problems for firefighters.
Inversion layers also make it more difficult for meteorologists because computer systems don't do well at predicting temperatures at these times. Sandler said meterologists try to compensate for inversions when making temperature predictions.
The main problem with inversion layers both in the Rogue Valley and in, say, Los Angeles is that they also trap polluted air.
"Our inversion layers are a major cause of our pollution here," said Anna Kemmerer, environmental specialist with the Oregon Department of Environmental Quality.
In summer, the main pollutant in Jackson County is ozone. In winter, when woodstoves are in use, spewing out tiny particles that can cause health problemsthe air in Medford can be worse than any other time of the year.
Jackson County and most cities have passed ordinances requiring that old non-EPA certified woodstoves be replaced with new certified units at the time a home is sold.