While most roadways in contact with the earth have remained ice free, bridge icing has occurred all over the region. Yesterday, as I headed down to the Burke Gilman trail I was greeted by a very, very slippery bridge where another bicyclist had crashed the day before. No ice before or after the bridge.
Why are the bridges icing up when roadways and paths in contact with the earth are not? Because the earth is still relatively warm.
Here are the latest soil temperatures at 8 inches below the surface from the WSU AgWeatherNet website. The subsurface soil was in the upper 30s F over western Washington, near freezing in eastern Washington. 40s in the Willamette Valley and along the coast.
A few days ago the soil was even warmer after the extended warm period in mid-November. So roadways in contact with the soil are getting a lot of heat conduction from below. Bridges don't have that...the air underneath them acts to insulate them from the ground. On clear nights, the bridges radiate heat to space in the infrared and quickly cool without heat coming up from below.
I have done some forensic meteorology work and the number one type of lawsuits is associated with roadway icing, with bridges being the main locations of the accidents on cold, clear nights and during the subsequent morning hours. Many times the cars skid on the bridge and overturn or crash on the other side. Bridges te on curved roadways are particularly dangerous.
Perhaps the most tragic case involved some folks who crashed on the elevated 405 bridge cloverleaf near Woodinville during an icy night.. They got out of their damaged car and worried they would be crushed by other skidding vehicles. One individual jumped off the bridge, thinking it was just off the ground. He didn't survive the fall.
Tuesday night it was happening again. Here are the temperatures around Seattle, both over ground and on bridges, around 9:30 PM. The temps in rectangles are from road sensors, the others are air temperatures, generally about 6 ft above the surface. Look at the lower left corner...you see the two 35s? Those are on the West Seattle bridge, whose temperature is already a few degrees lower than surround air temperatures. There are other bridges with a similar situation and it will only get worse the rest of the night.
Let me show that. Here are the observations at 5:35 AM the next (Wednesday) morning. The West Seattle bridge has reached freezing, while the surrounding observation were 36. Look at the bridge temperature on the viaduct near the UW (the 30F in the NE quadrant for those not from Seattle)....substantially colder than nearby temps.
We are very fortunate that city of Seattle invested in roadway temperature sensors and the collection of all the weather data over the city. I suspect they will be checking and treating the bridges tonight to ensure safe travel tomorrow morning. WSDOT has also invested in roadway sensors and their personnel understand the threat of icy bridges, something that was not true 30 years ago.
How long will the cold temperatures last? Not long. We are near the end of the cold period and there may not be any freezing temperatures for many of you in western Washington for at least a week. But this is a Faustian bargain...we will give up the sun and get some rain in return.
REI asked me to do a few blogs for them. Here is my first on the upcoming winter snow forecast: