Life on Earth swims at the bottom of an ocean of air. Visitors from elsewhere in the solar system would not find Earth's atmosphere inviting. Even Earth's earliest life forms would find Earth's current air mass toxic. Yet Earth's inhabitants thrive in this unique nitrogen-oxygen mixture that humans call air.

The existence of air on Earth, like the atmospheres of other planets, began before the planet even formed. Earth's current atmosphere developed through a sequence of events that started with the coalescing solar system.

Earth's first atmosphere, like the dust and rocks forming the early Earth, came together as the solar system formed. That first atmosphere was a thin layer of hydrogen and helium that blew away from the chaos of hot rocks that would eventually become the Earth. This temporary hydrogen and helium atmosphere came from the remnants of the gaseous ball that became the sun.

The hot mass of rock that became the Earth took a long time to cool. Volcanoes bubbled and released gases from the Earth's interior for millions of years. The dominant gases released consisted of carbon dioxide, water vapor, hydrogen sulfide and ammonia. Over time these gases accumulated to form the Earth's second atmosphere. After about 500 million years, the Earth cooled enough for water to begin to accumulate, further cooling the Earth and eventually forming the Earth's first ocean.

Earth's first recognizable fossils, microscopic bacteria, date back approximately 3.8 billion years. By 2.7 billion years ago, cyanobacteria populated the world's oceans. Cyanobacteria released oxygen into the atmosphere through the process of photosynthesis. As the oxygen in the atmosphere increased, the carbon dioxide decreased, consumed by the photosynthetic cyanobacteria.

At the same time, sunlight caused atmospheric ammonia to break into nitrogen and hydrogen. Most of the lighter-than-air hydrogen floated upward and eventually escaped into space. Nitrogen, however, gradually built up in the atmosphere.

About 2.4 billion years ago, the increasing nitrogen and oxygen in the atmosphere led to a shift from the early reducing atmosphere to the modern oxidizing atmosphere. The current atmosphere of 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.03 percent carbon dioxide and small quantities of other gases remains relatively stable due to photosynthesis of plants and bacteria balanced by animal respiration.

Most of Earth's weather and life occur in the troposphere, the atmospheric layer closest to the Earth's surface. At sea level, the force of air pressure equals 14.70 pounds per square inch (psi). This force comes from the mass of the entire column of air above each square inch of a surface. So where does air come from in a car? Since cars aren't airtight containers, the force of the air above and surrounding the car pushes air into the car.

But where does air come from in a plane? Airplanes are more airtight than cars, but not completely airtight. The force of the air above and surrounding the plane fills the plane with air. Unfortunately, modern airplanes cruise at or above 30,000 feet where the air is too thin for humans to breathe.

Increasing cabin air pressure to a survivable pressure requires redirecting some of the air from the plane's engines. Air compressed and heated by the engines moves through a series of coolers, fans and manifolds before being added to the air in the plane's cabin. Pressure sensors open and close an outflow valve to maintain a cabin air pressure between 5,000 and 8,000 feet above sea level.

Maintaining greater air pressure at higher elevations requires increasing the structural strength of the airplane's shell. The bigger the difference between the interior air pressure and the exterior air pressure, the stronger the outer shell required. While sea level pressure is possible, the pressure equivalent to 7,000 feet above sea level, about 11 psi, is often used in airplane cabins. This pressure is comfortable for most people while reducing the mass of the plane.

So where does air come from in boiling water? The answer, simply put, is dissolved air. The amount of air dissolved in water depends on temperature and pressure. As temperature increases, the amount of air that can be dissolved in water decreases. When water reaches boiling temperature, 212°F (100°C), the dissolved air comes out of solution. Since air is less dense than water, the bubbles of air rise to the surface.

Conversely, the amount of air that can be dissolved in water increases as pressure increases. The boiling point of water decreases with elevation because the air pressure decreases. Using a lid increases the pressure on the surface of the water, increasing the boiling temperature. The effect of lower pressure on boiling temperatures requires recipe adjustments when cooking at higher elevations.