Category Science

The exosphere is the uppermost region of Earth’s atmosphere as it gradually fades into the vacuum of space. Air in the exosphere is extremely thin – in many ways it is almost the same as the airless void of outer space.

The layer directly below the exosphere is the thermosphere; the boundary between the two is called the thermopause. The bottom of the exosphere is sometimes also referred to as the exobase. The altitude of the lower boundary of the exosphere varies. When the Sun is active around the peak of the sunspot cycle, X-rays and ultraviolet radiation from the Sun heat and “puff up” the thermosphere – raising the altitude of the thermopause to heights around 1,000 km (620 miles) above Earth’s surface. When the Sun is less active during the low point of the sunspot cycle, solar radiation is less intense and the thermopause recedes to within about 500 km (310 miles) of Earth’s surface.

Not all scientists agree that the exosphere is really a part of the atmosphere. Some scientists consider the thermosphere the uppermost part of Earth’s atmosphere, and think that the exosphere is really just part of space. However, other scientists do consider the exosphere part of our planet’s atmosphere.

Since the exosphere gradually fades into outer space, there is no clear upper boundary of this layer. One definition of the outermost limit of the exosphere places the uppermost edge of Earth’s atmosphere around 190,000 km (120,000 miles), about halfway to the Moon. At this distance, radiation pressure from sunlight exerts more force on hydrogen atoms than does the pull of Earth’s gravity. A faint glow of ultraviolet radiation scattered by hydrogen atoms in the uppermost atmosphere has been detected at heights of 100,000 km (62,000 miles) by satellites. This region of UV glow is called the geocorona.

Below the exosphere, molecules and atoms of atmospheric gases constantly collide with each other. However, air in the exosphere is so thin that such collisions are very rare. Gas atoms and molecules in the exosphere move along “ballistic trajectories”, reminiscent of the arcing flight of a thrown ball (or shot cannonball!) as it gradually curves back towards Earth under the pull of gravity. Most gas particles in the exosphere zoom along curved paths without ever hitting another atom or molecule, eventually arcing back down into the lower atmosphere due to the pull of gravity. However, some of the faster-moving particles don’t return to Earth – they fly off into space instead! A small portion of our atmosphere “leaks” away into space each year in this way.

Although the exosphere is technically part of Earth’s atmosphere, in many ways it is part of outer space. Many satellites, including the International Space Station (ISS), orbit within the exosphere or below. For example, the average altitude of the ISS is about 330 km (205 miles), placing it in the thermosphere below the exosphere! Although the atmosphere is very, very thin in the thermosphere and exosphere, there is still enough air to cause a slight amount of drag force on satellites that orbit within these layers. This drag force gradually slows the spacecraft in their orbits, so that they eventually would fall out of orbit and burn up as they re-entered the atmosphere unless something is done to boost them back upwards. The ISS loses about 2 km (1.2 miles) in altitude each month to such “orbital decay”, and must periodically be given an upward boost by rocket engines to keep it in orbit.

A pneumatic machine is one that is driven by compressed air. If no other forces are acting on them, the molecules in gases, such as air, spread out to fill the space that is available to them. If the space is sealed and then reduced, the air is compressed — the molecules are pushed closer together. This means that the pressure that the compressed air exerts on the inside of its container is greater than the atmospheric pressure pushing down on the outside of the container. A pneumatic drill, also known as an air-hammer or jack-hammer, uses compressed air to push its bit forcefully against the ground being broken up. The compressed air is supplied to the drill through a hose by a machine called a compressor.

A pneumatic system is a collection of interconnected components using compressed air to do work for automated equipment. Examples can be found in industrial manufacturing, a home garage or a dentist office. This work is produced in the form of linear or rotary motion. The compressed air or pressurized gas is usually filtered and dried to protect the cylinders, actuators, tools and bladders performing the work. Some applications require a lubrication device that adds an oil mist to the closed pressurized system.

Pneumatics is an application of fluid power—in this case the use of a gaseous media under pressure to generate, transmit and control power; typically using compressed gas such as air at a pressure of 60 to 120 pounds per square inch (PSI). Hydraulics is another form of fluid power, which uses a liquid media such as oil but at a much higher pressure with a typical range of 800 to 5000 PSI.

A big reason pneumatics are used is due to simplicity. With little experience, on-off control of machines and equipment can be designed and assembled quickly using pneumatic components such as valves and cylinders. With proper air preparation, pneumatics systems are also reliable, providing a long service life with little maintenance needed.

A parachute offers an enormous surface area on which air resistance can operate. The friction slows the descent of the parachutist, so that he or she can land safely. Without a parachute, a human body would accelerate towards the Earth, hitting it with fatal force.

Even before the advent of the airplane in the early 20th century, humankind had been striving to perfect the parachute. Indeed, rudimentary versions of these life-saving devices date back to at least the 15th century and Leonardo da Vinci. With applications ranging from recreational skydiving to military combat missions, parachutes today come in a variety of forms engineered for specific purposes and settings; accordingly, these work in related but distinct ways.

All parachutes are designed for one fundamental purpose: to slow the gravity-driven fall of an object — often a person, sometimes inanimate cargo — through the air. They do so by taking advantage of atmospheric drag, a physical quantity that to engineers is more often a nuisance than a boon. The greater the drag generated by a parachute, the more slowly a given object attached to that parachute will descend to Earth. In a vacuum a parachute would be worthless because it would have no air molecules to “pull” against.

The main part of the parachute is called a canopy, which balloons outward as its payload begins to fall. The canopy’s shape is the biggest determinant of a parachute’s behavior.

For many purposes, the original round or conical parachute has been supplanted by the ram-air, or parafoil, parachute. This type of chute has a self-inflating canopy; as a result, on deployment, it creates a much larger drag-force resistance than does a round model, and its terminal velocity is also slower. In addition, the slower descent gives the parachutist greater control over the direction of the fall.

For fliers in aircraft traveling at supersonic speeds, which could result in the aforementioned chutes breaking apart, ribbon or ring parachutes are the tool of choice. These have holes built into the canopy to lessen the pressure to which the material is subjected, but these holes are not so large that the chute itself is ineffective as a safety tool.

The earliest round parachutes were circular when flattened out, and this made them notably unstable in action because they resisted forming a dome shape; this led to a high number of fatal accidents. Later, military-built round parachutes worked far better because they were parabolic in shape. Some round parachutes are not steerable, so they travel in accordance with prevailing wind conditions. Steerable round parachutes, however, have holes cut in the edges of their canopies, so their passengers can exert a degree of landing control. Round parachutes are often used in medical missions and in the dropping of military cargo.

Although we cannot see the air, it is still made of atoms and molecules, just like everything else. When an object passes through the air, these molecules push against it, causing a force of friction called air resistance.

Imagine dropping an apple from the top of a skyscraper and watching it fall. Intuitively, you might guess that it will be falling at a faster speed right before hitting the ground than at the moment you released it. In other words, the apple sped up while falling. However, is there a way to quantitatively describe an object’s motion through air? If we were to drop the same apple from an airplane, would it ever reach a maximum downward velocity?

Air resistance is the force that air exerts on objects moving through it. Scientists often refer to this force as drag or drag force, a term we’ll use interchangeably throughout the lesson. Note that in many cases, ‘drag’ can refer to any type of fluid, not just air. However, for the purposes of our discussion, we’ll always refer to air as the fluid.

Typically, this force is directed opposite to the object’s motion, thereby slowing it down. If you’ve ever held your hand out of a speeding car’s window, you’ve noticed how the air pushes your hand in a direction opposite to that of the car’s movement. When a piece of paper is dropped to the ground, air resistance slows down its fall.

Although we’ll only be focusing on the force that air molecules exert on solid objects, it’s important to mention that this force may exist between different fluid layers in addition to existing between a solid object and a fluid.

As the sun’s light passes through the atmosphere, its rays are scattered by the tiny particles of pollen, soot and dust to be found there. As blue light is scattered most, the sky appears blue. At sunset and sunrise, sunlight has further to travel to reach us. Only red light can be seen because the blue light has been absorbed by the atmosphere.

To understand why the sky is blue, we need to consider the nature of sunlight and how it interacts with the gas molecules that make up our atmosphere. Sunlight, which appears white to the human eye, is a mixture of all the colors of the rainbow. For many purposes, sunlight can be thought of as an electromagnetic wave that causes the charged particles (electrons and protons) inside air molecules to oscillate up and down as the sunlight passes through the atmosphere. When this happens, the oscillating charges produce electromagnetic radiation at the same frequency as the incoming sunlight, but spread over all different directions. This redirecting of incoming sunlight by air molecules is called scattering.

The blue component of the spectrum of visible light has shorter wavelengths and higher frequencies than the red component. Thus, as sunlight of all colors passes through air, the blue part causes charged particles to oscillate faster than does the red part. The faster the oscillation, the more scattered light is produced, so blue is scattered more strongly than red. For particles such as air molecules that are much smaller than the wavelengths of visible light the difference is dramatic. The acceleration of the charged particles is proportional to the square of the frequency, and the intensity of scattered light is proportional to the square of this acceleration. Scattered light intensity is therefore proportional to the fourth power of frequency. The result is that blue light is scattered into other directions almost 10 times as efficiently as red light.

When we look at an arbitrary point in the sky, away from the sun, we see only the light that was redirected by the atmosphere into our line of sight. Because that occurs much more often for blue light than for red, the sky appears blue. Violet light is actually scattered even a bit more strongly than blue. More of the sunlight entering the atmosphere is blue than violet, however, and our eyes are somewhat more sensitive to blue light than to violet light, so the sky appears blue.

The air around the Earth is pulled towards it by the planet’s gravity. This causes it to press down on the Earth with a force known as atmospheric pressure. This is measured in units called millibars (mb). At sea level, the average atmospheric pressure is around 1000mb. It is changes in atmospheric pressure, caused by the air being heated by the Sun, that make the air flow from place to place, causing the winds and weather that we experience on Earth. Most weather events happen in the lowest layer of the Earth’s atmosphere — the troposphere.

The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.

Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.

Atmospheric pressure drops as altitude increases. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai’i. Honolulu is a city at sea level. Denali, also known as Mount McKinley, is the highest peak in North America. As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.

Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness, also called “the bends”, is also a problem for scuba divers who come to the surface too quickly.

Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying. Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.