AIR

The atmosphere surrounding Earth is full of air! The air in our atmosphere is made of molecules of different gases. The most common gases are nitrogen (78%) and oxygen (about 21%). There are other types of air molecules as well, but in very small quantities. Air is important for almost all life on Earth including plants and animals. Plants need gasses from air to do photosynthesis. Animals need to breath air to get the oxygen they need to survive.
As you move up in altitude through the atmosphere, the concentration of air molecules decreases. Some people call this "thin air". The air is thinner higher in the atmosphere because there is lower pressure the higher you go up.
There is a special layer of air molecules high in the stratosphere layer of Earth’s atmosphere, called the ozone layer. The composition of the atmosphere is different in the ozone layer. There are more ozone molecules than anywhere else. Ozone molecules help block some of the Sun’s strongest rays. Currently, scientists are monitoring this layer. It has recently become so thin at the South Pole where the molecules are being destroyed that we call it a “hole”.

AIR PRESSURE
Molecules in Earth’s atmosphere constantly bounce off each other and everything else around them. The force exerted by these air molecules is called air pressure. Molecules that are packed closely are at higher pressure than molecules that are more spread out. The molecules inside this balloon are at a higher pressure than the molecules outside the balloon.
What's a high pressure system?Sometimes, high in the sky, air slows its forward motion and piles up. This is called convergence. The piled-up air sinks. When the sinking air reaches the Earth’s surface, it spreads out. This is called divergence. In the Northern Hemisphere it spreads out with clockwise surface rotation and is called a high pressure system.
What's a low pressure system?High in the sky, air moves away from an area faster than air enters an area. When air spreads out (divergence) at high altitudes, air from below flows upward to fill the space, making a low pressure system. The system has surface convergence and anti-clockwise rotation in the Northern Hemisphere.

STABLE AND UNSTABLE
Weather is strongly affected by how stable or unstable the atmosphere is. Stable air means that the weather is likely to be calm. It may rain or snow slowly and steadily, it may be sunny, but the weather will not change quickly. Unstable air means that the weather might change quickly with very little warning. Unstable air leads to sudden thunderstorms.
What makes the atmosphere stable or unstable? Picture an invisible box of air called an air parcel. If we compare the temperature of this air parcel to the temperature of air surrounding it, we can tell if it is stable (likely to remain in place) or unstable (likely to move). Click below to see the animation of stable and unstable air parcels.

LAYERS
The atmosphere is a protective layer of gasses that surrounds the entire planet. It keeps temperatures within a relatively small range, blocks out harmful rays of sunlight, and provides plants and animals with air to respire. The atmosphere has five different layers that are determined by the changes in temperature that happen with increasing altitude.
TroposphereThe troposphere is where weather happens. The part of this layer that is closest to the Earth is warmer than the top of the layer. The air is heated by the surface of the Earth, which absorbs the Sun’s energy.
StratosphereThe stratosphere is where jet airplanes fly. Temperatures increase with altitude. The ozone layer within the stratosphere absorbs harmful ultraviolet rays of sunlight.
MesosphereBrrr! The coldest parts of our atmosphere are found in the mesosphere. It can reach–90°C up there!
ThermosphereThere are few air molecules is the thermosphere. The thermosphere can heat up to 1,500°C or higher when the Sun is active, making an aurora that lights up the night sky. Astronauts orbiting Earth in the space station or space shuttle spend their time in this layer.
ExosphereThe upper layer of our atmosphere, where atoms and molecules escape into space, is called the exosphere.