An Ocean Of Air, by Gabrielle Walker

Our moon might be handy for a visit, but the brochure is unpromising. Intense cold alternates with baking heat; there is black sky, cosmic radiation - and no air. What a difference an atmosphere makes. Without its enveloping atmosphere, the Earth would be just as inhospitable. Old words may tell of thin air, or empty air, but in planet-shrouding quantity it is a rich, complex system, sustaining and protecting those who live in the bottom layer of gas.

Gabrielle Walker's engaging book, subtitled "A Natural History of the Atmosphere", is an antidote to taking our good fortune for granted. More history than natural history, it relates how air-breathing bipeds came to understand the importance of their invisible surround. Journeying beyond the atmosphere makes that obvious, and it is only a short trip. She opens with a gripping recreation of a 1960 skydive from a high-altitude balloon, a mere 32km up but already on the edge of space.

The rest of the book revisits the levels that the intrepid jumper, Joe Kittinger of the US Air Force, traversed as he fell to Earth. They come in reverse order, as scientific study started at the bottom, with Galileo and his successors realising the force of air pressure. Then the components of elemental air were teased out. Walker focuses on oxygen, which powers trillion-celled beasts like us, and carbon dioxide, the focus of our worries about global warming.

Other atmospheric constituents, such as nitrogen, water vapour and the trace "inert" gases, do not lend themselves so well to her interests, which are mainly the quirks of the investigators or threats to human life. So we learn about the atmosphere as temperature regulator and radiation shield, as well as a mostly benign bath of chemicals.

This is popular science, not a textbook, and it leads to a series of excellent stories, well told. Some are of slightly dubious relevance. The sinking of the Titanic is replayed, for example, because the ship was in radio contact as the disaster unfolded, and the atmosphere allowed Marconi to send signals round the curve of the Earth. But, on the whole, these vignettes illuminate as well as entertain.

Most of the science is straightforward, save for the way Earth's rotation powers trade winds and jet streams. Individual stories may be familiar, but they are well chosen to build Walker's overall theme of air awareness. You leave the book with a deeper appreciation of the stuff's life-giving qualities. Breathe deep, and enjoy.

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