Weight of a Cloud

Graeme Cant wrote:
wrote:
http://www.wsi.com/corporate/newsroo...oudWeight.html

Not as light and fluffy as they look.

Interesting. When I started, my instructor told me the weight of upward
moving air in the core of a moderate thermal was about 80 tons. Haven't
ever done the figures to check the accuracy of that estimate.

I think its a lot more than that.

From first principles:

- dry air is roughly 20% oxygen and 80% nitrogen, giving a
"molecular weight for air" of 28.8.
- Avogadro's number of molecules weighs the same as the molecular weight
expressed in grams (28.8g) and occupies 22.4 liters at STP, so a liter
of air weighs 1.285 grams and a cubic meter weighs 1.285 kg
- Assuming a thermal is 3000 ft high and 330 ft in diameter (1000m high
and 100m in diameter), which is probably a bit smaller in diameter
than a UK summer thermal, we get a total volume for the thermal of
7.85 million cubic meters.
- this volume of dry air at STP weighs 10,000 metric tonnes
- correcting to 28 C and 1028 mb gives a weight of 9294 tonnes
for dry air.

Water vapor has a molecular weight of 18, so it weighs about half as
much as air. I'll leave the correction to air at 40% humidity as an
exercise for the reader but I'd be surprised if the corrected weight is
less than 9000 tonnes.

NOTE: for US pilots: a metric tonne is almost identical to an Imperial
ton, i.e. 2240 pounds.

Now you know why an airship can carry a decent sized load and why
thermals are unaffected by any number of gliders riding them.


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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