Noah Clark, age 10, of Phoenix, Arizona, for his question:

What exactly is in the earth's atmosphere?

The enormous atmosphere is made of certain gases, just like the air we breathe in and out. These gases are mixed together in balanced amounts. The mixture is thickest near the ground and it gets thinner as we go higher. Hundreds of miles above our heads, the thin gases finally get lost in space. The basic mixture is more or less the same all through the atmosphere    though the layer near the ground also has a lot of smokey fumes, dust and a variety of chemical pollutants.

The most plentiful air ingredient is nitrogen and next is oxygen. Nitrogen makes up 78 per cent, or more than three fourths of the air, and oxygen makes up 21 per cent, which is almost one quarter. Together, these two gases make up 99 per cent of the atmosphere. This leaves only one part in 100 for all the other gases. Most of this one per cent is argon. The rest consists of small traces of neon and helium, xenon and kyypton, carbon dioxide and a few other gases. This balanced mixture is dry air, but in the real atmosphere it is always blended with large or small amounts of gaseous water vapor.