So what is going on? Can frost occur on an object whose temperature is above freezing? No way. The answer to this question can save your life by the way.
First a shocker. Official surface air temperatures are not measured at the surface. Rather, the standard is for thermometers to be placed 2-meters (roughly 6.5 feet) above the surface. Even most folks with home thermometers put the sensor a few feet above the ground.
Why is this important? Because on nights with low winds and relatively clear skies, the temperature at the surface and in the air a few feet above can be very different, with the ground much colder.
Winds last night were very weak, in fact, many locations reported calm conditions (calm indicated by circles on the weather map). Here is a plot of the temperatures and winds at 5 AM Tuesday. Nearly no winds. Low winds means there isn't much turbulence or mixing in the lower atmosphere.
The night was nearly clear, with only some thing cirrus, something shown by the infrared satellite image at 5 AM below.
With clear skies, the earth is able to effectively emit infrared radiation to space (see below). The atmosphere is not as good in emitting infrared radiation, so the earth cools down much faster. Clouds mess this up, since they emit infrared radiation down to the ground, greatly lessening the cooling.
Thus, on such clear nights we often develop surface-based inversions, with temperature increasing with height in the lower atmosphere. In fact, during calm, clear nights it is not unusual for the ground (or rooftops) to be 2-6 F cooler than the air at 6 ft. If winds are weak, the warmer air above is not mixed down to the surface.
Trust me, I have measured this myself with a high-accuracy thermometer. My neighbors were wondering what I was doing out there, but everyone knows that meteorologists are a bit eccentric.
So with 2-m temperatures in the mid-30s, it was not hard for the ground and other surfaces to get to freezing, allowing the formation of frost.
How can this factoid save your life? Easy. If local weather observations are in the mid-30s and it is relatively clear out, you better worry about ice on the roadway when you drive. Car thermometers are mounted several feet above the ground and thus can provide readings warmer than ground surface. So if your car thermometer reads 33-35F, you should slow down and be ready for ice, particularly in the early morning.
Keep in mind that the number one weather killer in Washington State is not floods, tornadoes or wind, but ice on the roadways. WSDOT has made huge strides in preventing roadway icing as has Seattle DOT (SDOT). But they can't by everywhere and caution is always wise.
How will Northwest Weather Change Under Global Warming? Help Us Determine the Local Impacts of Climate Change
Society needs to know the regional impacts of climate change and several of us at the UW are trying to provide this information with state-of-the-art high resolution climate modeling. With Federal funding unavailable, we are experimenting with a community funding to build this effort. If you want more information or are interested in helping, please go here. The full link is: https://uw.useed.net/projects/822/home All contributions to the UW are tax deductible.