Can you imagine waking up to below freezing temperatures of 14F, with ice and frozen ground, needing a thick coat and something to keep your head warm?
A few hours later, the temperature rises to 83F, and you’re dreaming of air conditioning, shorts, and ice cold drinks?
Daily temperature change is 69F!
You may think you are crazy or dreaming.
This happened in Seneca, Oregon yesterday. But why?
The station in question is the Seneca BLM Agrimet facility located at 4600 ft in an alpine basin in eastern Oregon (location marked with red marker on map).
Being in such a basin turned out to be very important.
To prove it, here are the minimum temperatures that morning (again, August 29th). The coldest temperatures were in the Seneca Basin, with an observed low of 14F in the lowest part of the basin. A few miles away in the surrounding hills, the temperature only dropped to the low 40s!
So what’s going on?
Our nights are getting longer, so there is more time for cooling at night. The sky was clear yesterday morning and afternoon, allowing for good solar heating during the day and good infrared cooling at night (see satellite imagery from Thursday morning). Clouds both reflect solar radiation (reducing the daily maximum) and act like blankets at night, reducing the ability of the surface to cool by emitting infrared radiation.
Dry air in eastern Oregon, relative humidity yesterday morning was below 100% (see map)
Dry air helps cool better at night and reduces the risk of warm fog.
So Seneca had nice cool temperatures at night and full sun during the day. Thursday’s cool temperatures were helped by a cold front that moved in the day before (see the temperature map, which was 5,000 feet above normal on Wednesday). The blue is cooler than normal.
Cold air is denser and heavier than warm air and tends to settle in basins and valleys—like the one around Seneca. There is definitely warmer air a few hundred feet above ground.
The cold air in the valley is not very deep and during the day the sun shines on the surface causing the cold air to mix, resulting in a sudden temperature rise of up to 80 degrees.
This is a view from the surface near the Seneca weather station. You can see the surrounding hills in the background.
The high mountain valleys of eastern Oregon are famous for their cool nights and wide temperature ranges during the summer months. Here is a map of typical daily (or daytime) temperature ranges across the US in July. Eastern Oregon is probably the “best” place in the country to freeze at night and fry during the day. Not sure if that’s a tourist draw.
The temperature change during the day in Seattle is only from 65 to 79F, only 14 F.
Shy.
