Weight of a Cloud

http://www.wsi.com/corporate/newsroo...oudWeight.html

Not as light and fluffy as they look.

Steve

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.

GC

Steve

My memory from my atmospheric physics days is that the water content of
a nice cumulonimbus is around 1 gram per cubic meter. Assume the
smallest cloud is a cubic kilometer and that gives you a billion grams,
or a million kilograms, or a thousand metric tons of water. That's
ignoring the weight of the air, too.

Mike


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.

GC

Steve

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.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Mike the Strike schrieb:

My memory from my atmospheric physics days is that the water content of
a nice cumulonimbus is around 1 gram per cubic meter. Assume the
smallest cloud is a cubic kilometer and that gives you a billion grams,
or a million kilograms, or a thousand metric tons of water. That's
ignoring the weight of the air, too.

Mike



More on the topic:

http://en.wikipedia.org/wiki/Water_vapor

The chart on saturation fraction suggests much mo around 2% (16
grams) at a possible cloudbase and 0.1% (0.8g) close to the top.

Even more amazing: the Energy it takes to lift that amount of water
from ground level to near tropopause height!!! It's in the region of
the annual output of a small nuclear powerplant!

Regards
Marcel

IIUC, Martin 's calculation is the mass of a thermal, and of the water
vapour in it.

Going back to the cloud (air saturated with water vapour and droplets),
to a first approximation, a small summer cu of say 1000 feet deep and
1,000 x 1,000 feet average Xsection would have a mass of around 36,000
tons at STP, I think.

One good for a gold height - say 10,000 feet tall and about that square
in cross section - would be 1,000 times more massive, say 36 million
tons.

The largest UK cu nim I have seen was about 7 miles high, 30 miles long
and say 1 mile deep. Call it equivalent to 5 miles high at STP (allowing
for rarified air at height) that's about 700 million tons mass, if all
air and water vapour. Dunno how to add an allowance for raindrops and
hailstones, but driving under it was like being in a waterfall.

Chris N.






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On 20 Aug 2006 21:09:37 -0700, "Mike the Strike"
wrote:

My memory from my atmospheric physics days is that the water content of
a nice cumulonimbus is around 1 gram per cubic meter. Assume the
smallest cloud is a cubic kilometer and that gives you a billion grams,
or a million kilograms, or a thousand metric tons of water. That's
ignoring the weight of the air, too.

Mike


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.

GC

Steve


luckily it only falls on our heads in very small pieces :-)

On 22 Aug 2006 01:30:19 GMT, Chris Nicholas
wrote:

IIUC, Martin 's calculation is the mass of a thermal, and of the water
vapour in it.

Going back to the cloud (air saturated with water vapour and droplets),
to a first approximation, a small summer cu of say 1000 feet deep and
1,000 x 1,000 feet average Xsection would have a mass of around 36,000
tons at STP, I think.

One good for a gold height - say 10,000 feet tall and about that square
in cross section - would be 1,000 times more massive, say 36 million
tons.

The largest UK cu nim I have seen was about 7 miles high, 30 miles long
and say 1 mile deep. Call it equivalent to 5 miles high at STP (allowing
for rarified air at height) that's about 700 million tons mass, if all
air and water vapour. Dunno how to add an allowance for raindrops and
hailstones, but driving under it was like being in a waterfall.

Chris N.

thank you chris.
I was about to post that the weight of a cloud was nonsense since it
floats around above us quite unaided.
weight is nonsense.
mass is more the issue.

Stealth Pilot