The Layers of Our Atmosphere - Part II
It is important however that you understand a few things about these layers. First of all, even though scientists have given special names to the boundary between each layer, this does not mean that there is an actual clearly visible border. If you were traveling up through the atmosphere in a rocket ship, you would not see any sudden changes as you went from one layer to the next. Instead, the change is gradual. The troposphere slowly changes mixing with the stratosphere. The higher you travel the less and less the air around you will resemble the air found in the troposphere and the more and more it will resemble the air found in the stratosphere.
Secondly, these layers of our atmosphere are not always at the same altitudes or heights all around the Earth, or throughout the year. The layers of our atmosphere expand with increased temperature, and contract with cooler temperature. Thus you can imagine that in the Winter the lower layer of the atmosphere contracts becoming more shallow, while in the Summer it expands, becoming deeper. When it is Winter in the Northern Hemisphere, such as in Europe, Asia, and North America, it is Summer the Southern Hemisphere, such as in Australia parts of Africa, and South America. Thus, while the troposphere is thin over the Northern Hemisphere, it is at the same time thicker over the Southern Hemisphere.
On average the troposphere extends from the surface of the Earth upwards to an altitude of 7 miles. Remember that the top of the troposphere is called the tropopause. The stratosphere then extends from the tropopause to an altitude of around 20 miles above sea level. The mesosphere extends from the stratopause to around 50 miles above sea level. The thermosphere begins at the top of the mesosphere, or at the mesopause. There is no definite top to the thermosphere. Instead it gradually mixes with the exosphere.