Just as scientists (barring allegations of manipulated data and cover-ups) told us that they understand the atmosphere and man’s effects on it, along comes another surprise. The Thermosphere has shrunk so much and so quickly over the last few years that scientists cannot explain it.


The Earth’s atmosphere is divided into five layers whose heights are constantly changing:

  • The Troposphere comes first (average up to 11 miles above the surface of the Earth). This is the first layer above the Earth’s surface and contains half of the atmosphere. Our weather occurs in this layer.
  • The Stratosphere comes next (11 to 30 miles above the Earth). It is generally stable and this is the level in which jet aircraft generally fly. It also contains 90% of the Earth’s ozone (the ‘ozone layer’).
  • Then comes the Mesosphere (30 to 50 miles above the Earth). The Mesosphere starts above the maximum altitude for aircraft but below the minimum altitude for orbiting spacecraft. As information on it can only be gained by sounding rockets it is little understood.
  • The Thermosphere comes next and is the largest of layers. It can extend out to some 370 miles above the Earth’s surface.
  • Above this is the extremely low density Exosphere where atmosphere and space merge.

The Thermosphere is the height at which orbiting satellites such as the International Space Station operate. When this layer contracts the satellites and space debris fly through less dense atmosphere and experience less drag. They therefore stay in orbit longer. It can also throw SatNav positioning slightly.

The important part of the Thermosphere is that it blocks harmful ultra-violet rays. It also expands and contracts as the Sun’s solar activity increases and decreases. It also contracts when carbon dioxide levels rise and cool it due to increased radiation emissions.

But the contraction over the last couple of years has been the largest recorded for 43 years with density being 30% down. It coincides with the ‘solar minimum’ (2007-2009) when the Sun’s activity was extremely low. But the contraction is still greater than that expected even with the combination of low solar activity and the presence of carbon dioxide.

As ever, just when mankind thinks it knows the answers along come Mother Nature with another surprise!

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