Stormy weather

The dogs don’t know why there’s no sun up in the sky. Seems like it keeps raining all of the time. Stormy weather.

The dogs agree: stormy weather is bad. Stormy weather, and especially thunder, necessitates seeking refuge, usually in a closet or at the feet of one of the humans.

Mama will keep us safe

This shot was from a few days ago when severe storms came through Georgia, causing at least one tornado. Fortunately the really bad weather missed us.

This shot was from Wednesday when another round of storms hit.

Under the table, between Mama and Daddy’s feet. Safest place around.

The storms on Wednesday were worse than those of a few days ago. We heard a report of another tornado south of us, which was where most of the really bad weather hit. The storms came through in the form of isolated thunderstorms rather than as a uniform wave of bad weather along a front. Here was one pretty severe storm that passed just north of us.

The red pushpin is our house. We got a light sprinkle from this storm, along with some lightning and thunder. The storm that passed south of us was also bad, but it gave us only distant thunder.

I have been working for a few days on channeling the runoff from the yard so that heavy rain won’t wash away what little topsoil remains. This is a partially-finished channel from one downspout of our newly-installed gutters.

It’s a little hard to tell, but there are ripples in the water. It was a fairly strong stream. I will eventually line both sides with stone from around the mountain and put commercial rocks in the actual stream bed. I will probably get some tennis-ball to softball-sized stone from a landscape company to make a bed further down the yard.

This shot doesn’t show the other canyons being dug in other places in the yard. They will get their own treatment soon.

I’m not sure how much rain we got, because our fancy tipping bucket rain gauge seems not to be tipping reliably. I guess I’ll just have to get one of the old-fashioned glass tube rain gauges.

As I write this there are still storm cells tracking towards us and the rest of Georgia, and Sam is under the table at my feet. I keep hearing thunder and Leah sees lightning. We may still get severe weather right here on top of the mountain.

The storm that passed north of us was producing hail around three-quarters of an inch in size. One of the Atlanta TV stations reported that the storm had a BTI of 2.5. Now you may be wondering, as I was, exactly what in the heck a BTI is. So I looked it up. It turns out that it is a commercially-produced index indicating likelihood of tornado production. It means Baron Tornado Index.

As it happens, I know where BTI came from. A Huntsville, Al, TV weatherman named Bob Baron decided to leave TV weather forecasting and form a company to sell weather visualization and analysis software, mainly to TV stations. I remember watching him when I lived in Huntsville. After searching for BTI, I went to the Baron company’s website. I was a little surprised to see how much the company has grown. I looked at their “leadership” listing, and was not surprised to find that everyone they list was basically in sales. They claim to have developed some pretty sophisticated software to forecast hurricane tracks, among other things. That kind of software development and meteorological capability requires a very good development team, but they were nowhere to be seen on the company website. That figures. The ones who do the work get no recognition. The ones who do the selling get all the glory. And, I suspect, most of the money.

They did mention their “chief scientist” in one news release. However, it would have been nice to see at least some mention of the rest of the staff that does all the work behind the glitzy products the company sells, rather than just the salesmen. Maybe they are mentioned somewhere on the company site. They are probably just hidden away so they won’t embarrass the salesmen.

Contrails and a sundog

Saturday morning just after 10 the eastern sky seemed to be full of contrails. There was also a faint parhelion, or sundog.

Shooting into the sun is not a great way to get much detail, but this worked fairly well. The sundog is a little to the left of the sun (about 22°, exactly where it should be). The small, bright blue dot further to the left and down a little is apparently an artifact of the lens of my iPhone.

I haven’t seen quite this many contrails at one time around here. We are close to some flight paths to the Atlanta airport, so airliners often fly overhead. Delta flies almost directly over our house on the Huntsville-Atlanta route; I once actually saw our old house from a Huntsville-Atlanta leg on my way out to California. Contrails usually dissipate before many more planes fly over, but not Saturday morning.

This sundog was a little higher in the sky than one usually sees. They are fairly common if you know when and where to look, usually seen in early morning or late afternoon, fairly close to the horizon. They are usually formed by plate-shaped ice crystals, which tend to orient themselves horizontally as they fall through the atmosphere. The sun’s rays have to pass through them edgewise to form the partial 22° halo, of which the sundog is a part. In order to see them at higher sun elevations, the crystals have to be column-shaped so that they have random orientations as they fall. If the ice crystals are randomly oriented at least some fraction will be at the right orientation to let sunlight pass through them so that a sundog will be visible. And so it was last Saturday.

I love looking out our windows.

Rough Ridge Fire

On Thursday morning we got up and, as usual, I looked out the window to see the sunrise. This is what I saw.

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I wondered why there was a cloud bank with such a clearly-defined edge when there was no passing front at the time. I learned later that it wasn’t clouds, it was smoke coming from the Rough Ridge fire burning in extreme northern Georgia near the Tennessee line. It has been burning since October 16. As of Thursday it had burned more than 10,300 acres. That’s about 16 square miles. The smoke has been blowing down towards the Atlanta area from the Rough Ridge fire and some others.

smokeplume

The smoke was thick enough that a Code Red air quality alert was issued for the Atlanta area. That means that the particulate matter was high enough to be unhealthy for all people.

The wind direction changed on Friday. This is what the view looked like Friday afternoon.

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Downtown Rome, which is usually visible, is almost completely obscured by smoke. There was a noticeable haze up on the mountain and a slight odor of smoke. I think the thickest part of the plume was east of us; it was not bad, but we were clearly within the plume.

The Rough Ridge fire is burning in the Cohutta Wilderness in the Chattahoochee National Forest. It is thought to have started from a lightning strike. At this point it has not reached populated areas, which is good for residents, but its location in the wilderness area has caused problems for fire fighters. The terrain is rough and the area is isolated.

The unusually warm and dry weather and windy conditions have made fire danger particularly high. The relative humidity has been in the 20-percent range, which sounds to me more like what you might see in the western US.

This Web site has some photographs from the fire.

Atlanta’s air quality was upgraded on Friday to Code Orange, which is unhealthy for sensitive groups. It’s better, but still not good. As far as I can tell, our air quality index is Code Yellow, or moderate.

The weather forecast for Friday night was for more windy conditions. One of the firefighters said in an interview that the fire won’t be completely out until we get a long, soaking rain. There is no rain in the forecast for the next week, so we can expect the fire to continue to burn.

Distrail

We saw some nice clouds when we took the dogs out for an evening walk Wednesday.

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There were some waves and a couple of interesting features associated with the passage of high-altitude airliners. Just to the left of center there are two dark, linear features that are probably, but not certainly, contrails. If that’s what they are, they are probably old (where old means more than a few minutes). They look dark because we’re looking at the under, shaded side. They may not be contrails at all, just linear cloud features.

Just above them, however, is what appears to be a negative contrail, a linear feature of apparently clear air within a cloudy region of the sky. This is more likely to be an artifact of the passage of an airliner. A negative contrail is also called a distrail, or dissipation trail. Here’s a little closer look.

distrail

In searching around for an explanation for distrails, I found at least two. One is that the warm exhaust of a jet engine warms the air in the cloud, causing existing cloud droplets or ice crystals to evaporate. A second is that the small particulates in a jet exhaust become condensation nuclei which cause large enough droplets or ice crystals to form that they precipitate out of the cloud, leaving behind a clear region. The website with this particular explanation alternatively attributes the effect to the turbulence created by the aircraft’s passage causing greater condensation and precipitation. Other explanations don’t mention turbulence; I think it’s possible that under the right conditions, turbulence might cause a wake of sorts to appear in a cloud that looks like a negative contrail. I think that’s a different phenomenon from the particulate-condensation nuclei-precipitation explanation.

I think the second explanation (particulates-condensation nuclei-precipitation) is probably closer to right, and the warm air-evaporation explanation is wrong. My reasoning is that the same conditions that cause a condensation trail to form are being blamed for an evaporation trail. The warm (or hot, if you will) exhaust of a jet engine very quickly cools as the gases expand, which causes the water vapor it contains to condense, thus forming a contrail. I can’t think of a way that adding water vapor to air will cause evaporation.

The accuracy of the second explanation is bolstered by accounts of seeing virga or falling ice crystals from the region of the distrail.

third explanation includes some atmospheric optics along with the particulates-condensation nuclei-precipitation process that may or may not be necessary to the explanation of distrails. It relies on optical properties of ice versus water as well as on precipitation. If anyone is interested they can check out the site, which some readers are already familiar with.

On other days in other skies, I have seen what looks like a negative contrail, but these are usually a visual effect. What looks like a negative contrail in those cases is actually the shadow of a higher contrail falling onto a lower cloud deck. It’s sometimes hard to tell the difference because the lower cloud deck can hide the higher contrail, so all you see is a dark trail through the clouds that looks a lot like a view through the cloud to clearer sky.

In this case, I think what Leah and I saw was actually a distrail.

Sky with contrails

contrails

We have lots of commercial jets flying over us, most probably coming to or from Atlanta. Five of them left their contrails for us to see Sunday morning.

Contrails (condensation trails) are caused by water vapor condensing on tiny particulates in the exhaust of aircraft engines. Today we tend to think that means jet engines, but all engines produce at last some particulates. You may have seen photographs of World War II bombers flying at high altitude leaving contrails. Here’s an example taken by the Army Air Corps during WW II.

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The water vapor that causes contrails can come from the water vapor produced by the engine (burning petroleum fuel produces mainly carbon dioxide and water vapor) or water vapor in the surrounding air. The water vapor may not condense into visible droplets if there are no condensation nuclei. Particulates produced in the engine combustion provide the condensation nuclei that are needed. If the air at the altitude where the aircraft is flying is not very humid, a contrail can dissipate quickly. If it is humid, the contrail can stick around for a long time, or even continue to grow and form much larger clouds.

Our skies must have been fairly humid Sunday morning because these contrails remained visible for some time.

If contrails remain visible for long enough, you may be able to see how the wind at altitude can blow in different directions at different locations by the way the line of the contrail distorts. You could also estimate the wind velocity if you were so inclined.

Bob Roper, a professor at Georgia Tech who was on my reading committee, studied what were called meteor winds. He had a radar unit that looked up at very high altitudes, near the top of the atmosphere. Meteors that routinely crash into the upper atmosphere create ionization trails that reflect radar. By measuring the apparent motion of the ionization trails as seen by the radar, the wind velocity can be estimated. If you’re really interested in meteor wind measurements, you can get one of his articles here for $31.50. Or you could probably visit your local university library and ask for help finding atmospheric science journals. I imagine that neither course will be particularly attractive, and I’m afraid the article would be pretty dry anyway.

As one of the very few scientists doing meteor wind measurements in the West, Dr. Roper sometimes got to visit the USSR, where they also did such work. Back in those days, he would be debriefed by the US authorities when he returned. One time the Russians showed him a radar that could look over the horizon and detect, let’s just say for the sake of argument, ships in the Persian Gulf. He thought that was rather odd, since he was really interested only in radars that looked pretty much straight up. He figured that the Russians would expect him to be debriefed when he returned, and that the Russians probably wanted the US to know the USSR could watch US ships from afar.