Relating to a sphere; shaped like a sphere or one of its segments.

Something spherical is like a *sphere* in being round, or more or less round, in three dimensions. Apples and oranges are both spherical, for example, even though they're never perfectly round. A *spheroid* has a roughly spherical shape; so an asteroid, for instance, is often *spheroidal*—fairly round, but lumpy.

(1) The part of the earth's atmosphere that extends from about seven to about 30 miles above the surface. (2) A very high or the highest region.

The stratosphere (*strato-* simply means “layer” or “level”) lies above the earth's weather and mostly changes very little. It contains the ozone layer, which shields us from the sun's ultraviolet radiation except where it's been harmed by manmade chemicals. The levels of the *atmosphere* are marked particularly by their temperatures; *stratospheric* temperatures rise only to around 32°—very moderate considering that temperatures in the *troposphere* below may descend to about -70° and those in the *ionosphere* above may rise to 1000° .

(1) The part of the world in which life can exist. (2) Living things and their environment.

The *lithosphere* is the solid surface of the earth (*lith-* meaning “rock”); the *hydrosphere* is the earth's water (*hydro-* means “water”), including the clouds and water vapor in the air; and the *atmosphere* is the earth's air (*atmos-* meaning “vapor”). The term *biosphere* can include all of these, along with the 10 million species of living things they contain. The biosphere recycles its air, water, organisms, and minerals constantly to maintain an amazingly balanced state; human beings should probably do their best to imitate it. Though the word has a new sound to it, it was first used over a hundred years ago.

Half a sphere, especially half the global sphere as divided by the equator or a meridian.

*Hemisphere* includes the prefix *hemi-,* meaning “half.” The northern and southern hemispheres are divided by the equator, the circle halfway between Earth's two poles. The eastern and western hemispheres aren't divided so exactly, since there are no poles in the Earth's east-west dimension. Often the dividing line is said to be the “prime meridian”—the imaginary north-south line that runs through Greenwich, England, from which all longitude is calculated (itself being the 0° meridian). But for simplicity's sake, the eastern hemisphere is often said to include all of Europe, Africa, Australia, and Asia, while the western hemisphere contains North and South America and a great deal of ocean.

(1) To turn from one purpose or course to another. (2) To give pleasure to by distracting from burdens or distress.

The Roman circus was used to provide *diversion* for its citizens—and sometimes to divert their attention from the government's failings as well. The diversion was often in the form of a fight—men pitted against lions, bears, or each other—and the audience was sure to see blood and death. A *diverting* evening these days might instead include watching the same kind of mayhem on a movie screen.

A device that changes something (such as radio signals, radio frequencies, or data) from one form to another.

Converters come in many forms. Travelers to foreign countries who bring along their electric razors or hair dryers always pack a small electric converter, which can change direct current to alternating current or vice versa. In 2009 millions of Americans bought digital-analog converters, small box-shaped devices that change the new broadcast digital signal to the analog signal that older TV sets were made to receive. A *catalytic converter* is the pollution-control device attached to your car's exhaust system that converts pollutants such as carbon monoxide into harmless form.

(1) To turn (your eyes or gaze) away or aside. (2) To avoid or prevent.

Sensitive people avert their eyes from gory accidents and scenes of disaster. But the accident or disaster might itself have been averted if someone had been alert enough. Negotiators may avert a strike by all-night talks. In the Cuban missile crisis of 1962, it seemed that nuclear catastrophe was barely averted. *Aversion* means “dislike or disgust”—that is, your feeling about something you can't stand to look at.

(1) To go back or return (to an earlier state, condition, situation, etc.). (2) To be given back to (a former owner).

Since the prefix *re-* often means “back” (see RE-), the basic meaning of *revert* is “turn back.” *Revert* and *reversion* often show up in legal documents, since property is often given to another person on the condition that it will revert to the original owner at some future date or when something happens (usually the death of the second person). In nonlegal uses, the word tends to show up in negative contexts. Many reformed drinkers, for example, eventually revert to their old ways, and most people revert to smoking at least once or twice before succeeding in quitting for good.