why is it cold on the outside of an aeroplane and yet were nearer the sun?

When an aeroplane is in flight at high altitude, whilst it is
technically closer to the sun it is only closer by maybe 18000-20000
kilometres which when you consider that the sun is about 150 000 000
kilometres away, its not that much closer to make a difference.
(0.00012% closer).

The suns heat, passes through the air but hardly warms it as at higher
altitudes there are fewer molecules to warm. The heat is retained
better by solid and liquid objects such as the earth and the water.

As far as air is concerned, at lower altitudes the air is compressed
due to the weight of all the air above it and therefore the higher you
go the less the weight of the air above and therefore the air expands.

The temperature loss is an average of 6.4 degrees celsius for every
thousand feet. The decrease in temperature is known as adiabatic
cooling.

The reason for this loss of temperature is that;

1. In order for the air molecules to expand, they must do work on
their surroundings and because of conservation of energy, the
expansion of the molecules are at the cost of the heat.
2. The expanded molecules are less able to retain heat.

There is a good passage going into more detail on this page about
adiabatic cooling and even adiabatic heating.
http://en.wikipedia.org/wiki/Adiabatic_process

I hope this answers your question and trust that you will not hesitate
to ask for a clarification if any aspect of the answer needs further
detail.

Thanks

Johnny Phoenix

---

Clarification of Answer by johnny_phoenix-ga on 21 Aug 2006 03:23 PDT

Apologies, as pointed out by commenters, I can confirm that I should
have but 18km or 18000 metres rather than 18000km. I apologies for any
confusion and thank the commenters for highlighting this so that it
could be corrected.

Thanks. 

JP.

"... closer by maybe 18000-20000 kilometres"

Kilometres or metres?

Since 18,000 kilometres = 59,055,118 feet (Over 11,000 miles) and
commercial flights here in the USA fly mainly in Class A airspace
(between 18,000 feet and FL600) I'm guessing johnny_phoenix-ga meant
metres. Even at 18,000 metres, the planes would be at 60,000 feet
(FL600) and close to the limits of Class A airspace.

- artqs