Why are there so many contrails today?

Driving in to work this morning, I noticed there were an awful lot of contrails in the sky. Why, I wondered, were there so many today?

[Holy contrails, Batman!]

Is it because it's a cold day today? Perhaps because we just had a front come through? To find out, I gave my old meteorology professor from Embry-Riddle Aeronautical University a call.

My guess is that you have a lot of moisture in the air there right now, Dr. Randell Barry said. Really? That's it? It's all because of water vapor? I'll explain it to you how he explained it to me. From the beginning...

Contrails (or "Cirrus aviaticus" if you're feeling snooty) are the long, skinny clouds that form behind airplanes. As with any other cloud, they need condensation and a little dust to form. Luckily for them, passing airplanes provide both. All the air has to do is be full of moisture.

Air can only hold so much water vapor - water in its gaseous form - before the water condenses, turning into a liquid. As air cools, the amount of water vapor it can hold decreases. Once air cools to a certain point - called the dew point - the air is saturated: It is full of water. That's when condensation happens. Clouds form in saturated air when water vapor attaches to fine particles like dust, salt or ice (formally known as "condensation nuclei") and condenses.

As long as the air temperature is close to the dew point, we know the air is full of water vapor and close to being saturated. At airplane cruising altitude, this air is ripe for contrail formation. The plane does the rest of the work.

Airplane exhaust contains both water vapor and some fine particles, Dr. Barry explained. When added to the surrounding moist air, the water vapor is enough to saturate the air, allowing the water vapor to condense on the small particles. And voila: Just like that, a cloud is born!

To prove his point, we pulled up an image of the United States taken by a weather satellite. On a water vapor image, bright white areas indicate moist air while dark gray areas indicate dry air. Sure enough, sitting over the eastern half of the U.S. was a huge area of moist air.

[This water vapor image, taken by a GOES weather satellite, shows an area of moist air covering the eastern half of the United States.]

To get 3D perspective, we then consulted a sounding - a chart that shows the data taken by a weather balloon. In the sounding below taken from the Dulles Airport area in Virginia, the temperature is represented by a black line while the dew point is shown as an orange line.

Right around 26,000 feet, the two lines approach one another, telling us that the air temperature and the dew point temperature there were close. The air at that height was full of water vapor. The two lines stay near one another all the way to the top of the sounding, so we can expect that airplanes flying at just about any cruising altitude in the Dulles area would find perfect conditions for contrail-making.

[This sounding taken from the University of Wyoming website shows the temperatures and dew points recorded by a weather balloon as it rose above Dulles Airport.]

That's all there is to it, then. As long as the air and dew point temperatures are close in the region where airplanes fly, contrails are likely. If the air is moist enough, the contrails may linger for a while. Otherwise, they may vanish almost as soon as they came. This morning, though, they lingered.