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Surrender Dorothy!
What's the largest billboard in the world? Answer: the sky. Skywriting is not used much today, but it was a very popular advertising medium for decades. The aerial technique was developed during World War I by John Savage, an English engineer and Royal Air Force pilot who used smoke from airplanes to send military signals over far distances. Skywriting was first used as an advertisement in the spring of 1922 when a pilot emblazoned "DAILY MAIL" over a horse racing track in Surrey, England (in October 1922, the same pilot wrote "HELLO USA" over New York City).
Skywriting is done at heights of between 9,000 and 20,000 feet, and is possible only in cloudless and moderately windy skies. A six- or seven-letter word can be over a mile high, two to seven miles long, and can be read over a 30 mile area. Most skywriting is done on weekends and over areas or events that draw large crowds—beaches, football stadiums, fairs, racetracks, etc. Under ideal conditions, a sky-written word can linger for 30 minutes or longer.
Want to find out more about skywriting? John Savage's informative patent can be viewed at the U.S. Patent and Trademark Office Web site (http://patft.uspto.gov/netahtml/search-bool.html). Since patents before 1975 are only searchable by patent number, you will have to enter 1489717 in the search term box and change the pull-down field to "patent number" (you will also have to change the "select years" pull-down menu to "1790 to present").
If you are really interested in skywriting, then you must download and watch the video "Sky Billboards" from the Internet Archive (http://www.archive.org/details/SkyBillb1935). "Sky Billboards" (runtime 7:32) was made in 1935 and does an excellent job of describing how skywriting works. A review at the Internet Archive site describes the film as having "exquisite aerial photography, fascinating information and breath-taking shots of sky writing in progress."
Sky-Writing.com (http://sky-writing.com/) answers basic questions about skywriting, including its history, the size and location of letters, how the smoke is made, etc. The site also answers the question: Was the skywriting in the movie "Wizard of Oz" real? The "Surrender Dorothy or die" scene—possibly the most recognized piece of skywriting ever done—was completely created by special effects that used a glass tank, miniature witch, water, oil, a hypodermic needle and a camera.
Skywriting's golden age was from the 1930s to the 1950s when Pepsi Cola and several other companies used skywriting as their primary way of advertising. Television and the advent of the highway billboard led to skywriting's decline.
Today, skywriting is a niche business, with only a handful of pilots possessing the skills necessary to do the job. But that does not mean that aerial advertising is dead. Airplanes towing banners, blimps with flashing lights and sky-typing aircraft that release computer-controlled puffs of smoke all fill the airways. This only goes to show that if you want to make an unusual and large impression, the sky truly is the limit.
—Don Hartman, University Libraries