Driving, weather, and dew point. Strange occurrence.

I was driving to work this morning and experienced a strange phenomenon. While barreling down the toll road at ~74mph, the windows on my car fogged-up to 0-visibility levels. All the outside surfaces, including the mirrors, were covered in condensation within a matter of ~5 seconds. At first I thought it was just a waft of smoke from my kretek, but then it thickened and covered with surprising speed and determination.

I was able to hit the wipers and clear the forward view, but I couldn’t see anything else. Sure, I could roll-down the windows, but I couldn’t clear the rear window or side-views. This made lane-changing almost impossible to execute with any confidence. It began to dissipate after a minute or two, and all was gone within 5.

What just happened? I didn’t see any fog, so I assume I hit a patch of super-saturated air, though I’m interested in any meteorological specifics.

Current conditions and details.
Generally, the morning conditions were ~55F, low ceiling but no fog, one (~1.5 miles) patch of drizzle.
METARS for the area at the time (I was never more than 8-12 miles from the reporting station).
I was driving from about 13:45Z for 30 minutes (08:45 – 09:15 EST).
I had the heat selected (floor vents), temp just barely on the hot side of the mid point. A/C compressor on.

I doubt it was super-saturated. It appears you were driving through cool air and then hit a patch of warm air with sufficient humidity such that its dewpoint was below the temperature of your car. Warm, moist air against a cool surface creates condensation on that surface, just like breathing against a window on a cold winter day.

And indeed, your METARS data shows a very high dewpoint. You just need your car to be slightly cooler than that to make condensation happen.

I’ve had the same thing happen to my car, though not quite to the extreme that you describe.

Don’t know what the terrain is like around Raleigh-Durham, but if it’s at all hilly, then it wouldn’t surprise me to see substantial local variations as you move in/out of valleys.

On a related note, I find it annoying that the dumbing down of weather forecasts means that they now generally show humidity rather than dewpoint. Dewpoint is more informative in showing absolute water vapor content changing over time, and at what temperature interesting things happen.

Each measure has its purposes. RH is handy for knowing whether it’s going to feel muggy out, but dewpoint is perhaps more useful for technical endeavors in which actual condensation might be a concern.

Ok, very reasonable.

We have ~southern winds today bringing warm, humid air in here. And the commute from my wife’s house is very open, hilly, and rural. Additionally, much of my route is west and southwest of the airport (KRDU) and this happened at about 1400Z.

Interestingly, I’ve looked more closely at wx data for that period and noticed a change happening concurrently w/ my morning travels.
Metars from the next station southwest show significantly higher dew points and ~90 degree wind shift.
KTTA 291435Z AUTO 21008G14KT 10SM SCT022 BKN028 BKN037 22/19 A3001 RMK AO2 T02150192
KTTA 291415Z AUTO 20011G17KT 10SM BKN024 BKN031 BKN042 21/19 A3000 RMK AO2 T02140193
KTTA 291355Z AUTO 20004KT 10SM SCT020 BKN027 OVC046 19/19 A3000 RMK AO2 T01870185
KTTA 291335Z AUTO 00000KT 10SM SCT016 BKN022 OVC050 14/14 A2999 RMK AO2 T01400137

Also, this next question is not a dispute; merely curiosity. Why wouldn’t we have supersaturated air this close to the ground? Too warm? Too dirty?

Our local guy is quite dew-point focused. If he mentions RH, he’ll often say (paraphrasing), “Our temp is 67 with a dew point of 56, giving an RH of 69%” (or whatever it is).

I had this happen once. Windows suddenly fogged up, both inside and outside. Yes, I was in an area of rolling hills and I had just driven down into fog, but the fog itself didn’t look that thick.

It was really scary. I’m glad it never happened again.

Too dirty. Nucleation sites are always available for condensation to develop, so as soon as RH rises to a smidge above 100%, you get fog. Even if you filter the air to make it super-clean, apparently ion tracks from naturally occurring radiation (cosmic rays and other sources) can serves as nucleation sites. See the second page (labeled “page 412”) of this PDF, in the section titled “Supersaturation.” The pages are images, so I can’t easily copy/paste text here.

Strictly speaking, you can (and do) get atmospheric supersaturation, but only by the tiniest fraction of a percent; that’s what serves as the driving force to make condensation happen, and that’s when you get fog (because the air always has particles in it to serve as nucleation sites).

How long had you been driving?

With the A/C on and the heater control in the middle, the car is going to be blowing cooled air out of the vents until the car warms up sufficiently that the heater core counteracts it. If you’d only been driving a few minutes, it could be that the A/C cooled the inside of the car sufficiently that the windows were below the dewpoint, but then shortly afterwards the heater core was warm enough to heat them back up above it.

Very interesting thread. I didn’t know what a “Kretek” was, now I know all about them. I’d heard of clove cigarettes, even tried one or two in the 70s, but never kreteks. I even initially considered the possibility of it being a typo. I’ll have to stop by the tobacconist and get a pack, smoke one, then give away the rest (my usual pattern).
The weather stuff was interesting as well.

(this is why you shouldn’t post high, kids)

Hilarity, I’m glad the insides of my windows didn’t also fog. I was very relieved when a quick swipe of the wipers was all it took to clear the windshield.

GreasyJack, the heater core was toasty by then. I had been on the road (and at 70+ mph) at least 15-20 mins when this happened. The cabin was comfy and the windows clean and clear until… wham!

kayaker, It’s currently illegal to sell kretek/clove cigs in the US, so you may want to keep it on the DL with your tobacconist. It’s not illegal to posses them though. A friend from Indonesia brought me some during his visit last month; I’m not sure where to buy them.

Thanks for the link, Machine Elf. That stuff’s good to know and the paragraph you referenced is cool to wrap the mind around.

In that section they mention that you can get up to 1% supersaturation within thunderstorms. How is this possible when we have fog occurring at 0.01-0.03%? Is this related to pockets of higher water vapor pressure in the updrafts?

I think the increased degree of supersaturation is related to the ongoing updraft, which provides a sustained rate pressure/temperature reduction. Water vapor can only condense at a limited rate: once all the vapor near a nucleation site/particle has condensed onto it, more distant vapor must diffuse toward that droplet before it can condense. That diffusion process takes a finite amount of time. So the more rapidly you reduce pressure/temperature, the greater the degree of supersaturation you will be able to sustain. Shortly after you stop reducing pressure/temperature, the condensation process will catch up and achieve equilibrium, reducing the RH to 100%.

There’s also an issue related to the radius of curvature of the droplet: the larger the droplet, the slower the rate of condensation onto it. As a droplet gets larger, the condensation process slows down, meaning supersaturation conditions move toward equilibrium (100% RH) more and more slowly. So while a vision-obscuring fog may develop very rapidly, the removal of moisture from the air later on - when the droplets have grown much larger - can’t happen as quickly.

I disagree that RH is handy. To me, dewpoint is a solid indicator of mugginess. In summer, if the dewpoint is below 70, it will be comfortable at any temperature, even 100. But if the dewpoint is over 75, it will be uncomfortable at any temperature, even in the upper 70’s. If I need to go out in summer, I look only at the dewpoint, and ignore the temperature.

Huh. I’ve found them for sale online, and read about one brand (Djarum) that is labeled as a cigar to get around the ban.

I’ll be in the Caribbean next month. I usually buy some cigars to enjoy there, I’ll see what’s available.

And in fact, there are devices for detecting such particles based on this principle, called cloud chambers.