Tuesday, April 11, 2017

Frost on your car but the temperature is above freezing. How can that be?

 I received several emails and a comment today from folks who were puzzled by frost on their cars when the air temperature was above freezing.  For example, a commenter (Grigsby) left this observation:


I also live in north Seattle and noticed frost on the roofs of nearby homes.  A plot of the minimum surface air temperatures around Seattle confirmed temperatures only dropped to 34-38F in north Seattle and the air temperature at my home never got below 35F

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.

The Explanation

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. 



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Masura said...

When is it going to stop being cloudy and rainy???

August Johnson said...

Radiative Cooling:


Radiative cooling is commonly experienced on cloudless nights, when heat is radiated into space from the surface of the Earth, or from the skin of a human observer. The effect is well-known among amateur astronomers, and can personally be felt on the skin of an observer on a cloudless night. To feel the effect, one compares the difference between looking straight up into a cloudless night sky for several seconds, to that of placing a sheet of paper between one's face and the sky. Since outer space radiates at about a temperature of 3 kelvins (-270 degrees Celsius or -450 degrees Fahrenheit), and the sheet of paper radiates at about 300 kelvins (room temperature), the sheet of paper radiates more heat to one's face than does the darkened cosmos. The effect is blunted by Earth's surrounding atmosphere, and particularly the water vapor it contains, so the apparent temperature of the sky is far warmer than outer space. Note that it is not correct to say that the sheet "blocks the cold" of the night sky; instead, the sheet is radiating heat to your face, just like a camp fire warms your face; the only difference is that a campfire is several hundred degrees warmer than a sheet of paper, just like a sheet of paper (at approximately air temperature) is warmer than the night sky.

The same radiative cooling mechanism can sometimes cause frost or black ice to form on surfaces exposed to the clear night sky, even when the ambient temperature does not fall below freezing.

Grigsby said...

Thanks for the thoughtful response. And I'm glad I'm not the only person who was curious. :-)

Brian said...

Makes sense Cliff, but the car in the photo had ice on the roof which was 5-6 feet off the ground, near the height of the thermometers that were reading in the mid-30's. So how did ice form on the roof?

Steve Mudge said...

Would specific heat be another avenue for frost to form at above freezing temps? Or is that really just a variation of the article's concept (ie-steel could be 31 degrees at air temp 34 derees but is that just because it re-radiates heat faster)?

RLL said...

Car was radiating into space, hence cooled below freezing point. Curiously first read about this years ago, you can make ice in some deserts in the summer when it cools by putting sheet pans of water on a roof.

Organic Farmer said...

Often notice frost on roof yet all stations reading above freezing??

IMHO.. The ground is now "warm" and prevents ground level frost. Frost is high up this time of year.

WIND 40mph gusts and a steady 30mph Right now. Way more rain,than predicted,.Plus this wind?!

JayW said...


The "frost on the car" effect is not just due to temperature inversions. The radiative cooling effect is strong enough to lower the temperature of surfaces exposed to the sky by several degrees below the ambient air temperature. So ice can form even if the temperature of the surrounding air is above freezing.

wff255 said...

I agree on the concept of radiative cooling. This explains why surfaces that face up-like the roof of a car freeze while the side windows may not, despite the side windows being closer to the ground. I see this often on clear, cool mornings.

Cliff said...

I think this is a little off.

Without the paper, the chllly night air feels cool to your skin. The paper simply traps some of your own body's heat from rising, and this is the the warmth you notice.

The temperature of the paper, or outer space, has nothing to do with it.

(BTW, my name is Cliff Alexander, not Cliff Mass. In the future I'll sign out as Cliff A.)

Cliff said...

The "temperature inversion" explanation may work if the low, cold layer of air contours objects on the ground. So maybe the air is warmer a few feet above the frosty roof or windshield.

This would be easy to test.

Another explanation is that sometime during the night the air dropped below freezing and ice formed. Maybe the warmer air of morning had simply not yet melted the ice.

Cliff A.

Ansel said...

I have heard that the effective "temperature" of the sky is less than minus 70 degrees C. in dry air. The sky emits only about as much IR radiation as a cake of dry ice! I have seen this myself with an IR thermometer.

So it is really true that if there is no wind, solid objects facing the night sky do indeed get cooler than the air by several degrees. This fact is not only the bane of drivers, but of gardeners as well. Tender seedlings can be frost-killed on 33 degree nights.

Doug said...

I often wonder about the frost patterns on cars in the morning. Some windows frosty, others not. I suppose it has to do with air currents.

donb said...

Another way to view this is differences in emissivity between the cooling air and the auto. Objects with lower emissivity lose heat at a lower rate than objects with higher emissivity, both being at the same initial temperature. If the auto has a higher emissivity, it will cool faster and reach a slightly lower temperature.
JayW above alludes to this effect.
Emissivity differences are what makes your low-e double pane windows work.