In Britain last year, Prime Minister David Cameron launched an initiative to measure the "national mood" along with other other vital economic statistics. France is looking into a similar statistic and while there is none in the U.S. so far, we think we have a good substitute: how many people are watching CNN? There's a familiar pattern to the CNN audience: in somber times ratings swell; when people are feeling happier, they tank. Call it the CNN Seriousness Index. 

CNN has long been a destination for breaking news, but so far it hasn't embraced the opinionated, personality-driven journalism of its rivals, the conservative-leaning Fox News and liberal-leaning MSNBC. What that means in practice, it seems, is that CNN's audience declines when people grow less interested in breaking news, as The New York Times points out today after looking at Wednesday night's ratings.

Here is how CNN stacked up with Fox News and MSNBC in primetime by week in March, using Nielsen data from TVNewser.

If you compare CNN's ratings to the top stories in Pew's News Coverage Index (which tracks "mainstream" print, online, radio, and TV outlets) and New Media Index (which tracks blogs and social media), an interesting pattern emerges: the cable news network appears to do well when "old" and "new" media are focused, laser-like, on the same breaking world events, and appears to struggle when the web is full of less substantial stories. Here's a recent history of our national obsessions:

WEEK OF FEBRUARY 28

CNN's Ratings: Low ratings, behind MSNBC

Mainstream Media: Media coverage is dominated by weighty topics like the Middle East uprisings and the U.S. economy (including the standoff between the governor and unions in Wisconsin), but Charlie Sheen's media blitz--a decidedly lighter topic--is the fourth most-covered topic.

New Media: Bloggers focus on the GOP budget and, to a lesser extent, on the last WWI veteran dying and Fox News suspending Newt Gingrich and Rick Santorum. Twitter users talk most about Facebook merging its "Like" and "Share" buttons while also discussing Apple products, the Oscars, and Libya. \

Mood: Feeling pretty good. 

WEEK OF MARCH 7

CNN's Ratings: High ratings, ahead of MSNBC

Mainstream Media: The media continues to focus on Middle East unrest and the economy until Japan's devastating earthquake and tsunami on March 11 eclipses everything else, especially on TV.

New Media: Twitter is consumed by major world events like the earthquake and the fighting in Libya (two thirds of Twitter news links on March 11 are about Japan's natural disaster), though users still manage to spend time talking about pop star Justin Bieber riding a skateboard in an airport terminal. Bloggers, curiously, spend the most time on the 2012 presidential campaign.

Mood: Sad and depressed

WEEK OF MARCH 14

CNN's Ratings: High ratings, ahead of MSNBC

Mainstream Media: The media focuses primarily on the destruction and mounting radiation fears in Japan until the U.S. announces that it will intervene militarily in Libya, at which point the media's attention whipsaws to the North African country.

New Media: It's all about the Japanese earthquake. For only the second time since Pew began measuring social media in 2009, the same story was the number one topic on blogs, Twitter, and YouTube.

Mood: Now sad and scared

WEEK OF MARCH 21

CNN's Ratings: Mid-level ratings, neck-in-neck with MSNBC

Mainstream Media: The coalition's military operation in Libya dominates the news, with coverage of the Japanese earthquake staying strong and other stories--like Elizabeth Taylor's death--getting significant play.

New Media: Bloggers focus primarily on Elizabeth Taylor's death, also devoting substantial time to Japan and Libya. No story dominates Twitter, but the microblogging site shifts back to its late February focus on technology. AT&T's acquisition of T-Mobile gets the most links. 

Mood: Feeling better!

WEEK OF MARCH 28

CNN's Ratings: Low ratings, behind MSNBC

Mainstream Media: Libya and Japan still drive the news cycle, but Pew notes signs that media attention to both stories is "beginning to plateau," with the U.S. economy and a potential government shutdown resurfacing as major topics.

New Media: The social media conversation grows more eclectic and veers further away from mainstream headlines. Bloggers discuss a Supreme Court decision on a former death row inmate and a BBC radio piece about closing public libraries. Twitter users meditate, as they tend to do, on technology and the web, chatting about Google's new +1 service and making April Fool's jokes.

Mood: Pretty okay