Freezing Drizzle Causes I-90 Accidents and Closure

27 Jan 2009

On the afternoon of Jan 27th, 2009, a mult-vehicle accident closed the eastbound lanes of Interstate 90 between the Highway 41 and Northwest Blvd exits.

The images below are courtesy of KHQ-TV.

The image below was provided courtesy of KREM-TV.

In all of these images, it's apparent that the amount of snow was not a major factor in the road conditions, as there was just a dusting on the roads. The cause of the accidents was icy roads, which was the result of a freezing drizzle event. Freezing drizzle is caused by a somewhat unusual set of circumstances in the atmosphere.

The 3 images below were provided courtesy of KXLY. In these images, note the accumulation of small water droplets on the windshield and side windows. These are drizzle drops, which are smaller than rain drops.

Most clouds are composed of tiny liquid droplets. These droplets are so small that they can exist as liquid even in temperatures well below freezing. The image below shows a typical cloud which is not precipitating (i.e. raining or snowing). The upper portion of the cloud which is colder than -20C is generally made up entirely of ice crystals. But in the lower, warmer part of the cloud, there is a mix of ice crystals and water droplets. For the portion of the cloud warmer than about -10C, the cloud is made up entirely of water droplets.

When the cloud begins to precipitate, the snow which is produced in the upper part of the cloud, falls through the lower, warmer part of the cloud. But since this part of the cloud is still below freezing, the drops adhere and freeze to the snowflakes. If the temperature remains below freezing all the way to the ground, the result is snow.

But in the case of a shallow cloud that is warmer than -10C, the cloud is made up entirely of water droplets. If the cloud begins to precipitate, it does not form snow. Rather, the tiny droplets fall in the form of liquid drizzle. If the temperatures at the ground are below freezing, the result is freezing drizzle.

During the morning hours (4 am PST), the atmosphere over Post Falls was generally colder than -10C. Below is a graph of the vertical distribution of temperature (right green line) and moisture (left green line). Where the two green lines are close together, the atmosphere is very moist indicating the presence of a cloud. The upper portion of the cloud was around -25C, which was primarily made up of ice crystals.

By the afternoon (4 pm), the lower levels of the atmosphere had warmed up, as seen in the graph below. The clouds were now mainly warmer than -10C.

Below is a set of radar images for various times during the day. In these 4-panel images, the 4 panels show the radar reflectivity at 4 different altitudes. The table below shows the altitude of the 4 panels, along with the approximate temperature at these levels. Note that in the afternoon, the 2500' and 6000' panels are generally too warm to have ice crystals, while the 2 higher elevations (9,000' and 13,000' AGL) are cold enough have have ice.

The first panel is taken at 11:48 am. The high reflectivity in the upper-left, upper-right, and lower-right images is indicative of snow.

By 1:35pm, an area of reflectivity was concentrated between Post Falls and Coeur d'Alene. Notice how this reflectivity is mainly apparent in the top two panels (2500' and 6000'). But the Post Falls area doesn't have any reflectivity in the bottom two panels (9000' and 13,000'). Thus, this cloud was largely warmer than -10C and was composed of liquid droplets. This indicates freezing drizzle.

You can access the entire radar loop by clicking here. But this is a rather large file (3.1 Mb).

There is another interesting indication of the freezing drizzle event. The web camera at Mt Spokane actually shows three freezing drizzle events. Since this camera is located around 5000' elevation, and I-90 is around 2300' elevation, this shows that the clouds even at 5000' were too warm to have snow crystals in them. The loop below is provided courtesy of Mt. Spokane Ski Resort.

The first freezing drizzle at Mt Spokane started at around 10:15 am. The camera lens melted the ice by 12:45pm, but more freezing drizzle occurred at 1:15pm. The lens again melted the ice by 3:00pm, but a third episode of freezing drizzle started at 4:15pm and continued until sunset.

The freezing drizzle event the caused icy roads on Interstate 90 near Post Falls was a very localized event. While freezing drizzle was observed in other locations, the intensity of the drizzle was not enough to cause driving problems.