Venus's atmosphere is the heaviest of those of all the planets. It is made up of carbon dioxide, with small quantities of nitrogen and water vapor, and traces of argon, carbon monoxide, neon, and sulfur dioxide. After the sun's heat reaches the planet's dense atmosphere, the carbon dioxide and sulfuric acid in its thick clouds trap it and prevent it from escaping into space. As this phenomenon is very similar to the operation of a greenhouse, it is known as the greenhouse effect.

The planet's atmospheric pressure (the force per unit area exerted against a surface by the weight of the air above that surface) is calculated to be 1,323 pounds per square inch (9,122 kilopascals)—about 90 times that on Earth. The surface of the planet is completely and permanently shrouded by thick clouds made up of droplets of sulfuric acid. The atmospheric pressure is 90 times that at sea level on Earth. In the upper atmosphere, winds of up to 225 miles per hour (362 kmh) blow continuously from east to west in a broad band above the equator.