01 September 2009

A new unit of humidity?

Science Daily, the site that just recently gave us the news of the discovery of “Turkish Tablets” in a 2,700 year old “Turkish Temple” in Turkey, now seems to have invented a new unit of humidity. In a news item that came out today, they report that U.S. and Australian astronomers are thinking of building an observatory at a 4000-m high place called Ridge A on the mountains of Antarctica. Despite the extreme cold, the place appears to be quite suitable for a telescope, because there is no light pollution and the air is very still and dry. In fact, Science Daily reports that "the water content of the entire atmosphere there is sometimes less than the thickness of a human hair".

I am having difficulty understanding how one measures the humidity of air in terms of the "thickness of a human hair" or a linear dimension. I know that rainfall is normally measured using a linear dimension. So I suspect they mean that if all the water in the atmosphere above Ridge A (is that the "entire atmosphere"?) condensed and fell to the ground and stayed there, the depth of the water would be less than the thickness of a human hair. If that's indeed what they mean, that's a bizarre way of explaining it. Since most people are used to the expression of the atmospheric humidity as a percent of saturation, it would make more sense to simply express the air humidity at Ridge A the same way. 1%, or 0.1%, or whatever.

Revision added 2 September 2009: Here is the paper by Saunders et al. that was the basis for the Science Daily item. Science Daily's citation of the journal's title, "Publications of the Astronomical Society" is incomplete; the full title is Publications of the Astronomical Society of the Pacific.

If you take a look at the paper you will see that they are indeed talking about "precipitable water vapour". So my interpretation is correct. The annual mean PWW for Ridge A is given as 210 µm, that would indeed be negligible precipitation.

I don't quite see the conceptual difference, if there is any, between "precipitable water vapour" in the atmosphere (let's say, in millimeters) and absolute humidity, which is the total amount water present in a particular volume of water (grams/cubic meter).

And I still say that the Science Daily's "explanation" of the amount of atmospheric water above Ridge A is utterly confusing for the uninitiated.


Neil said...

But what does that work out to in London Bus Units?

Ralph hartley said...

"The water content of the entire atmosphere" is not the same as the humidity.

Astronomers really do measure the moisture content of the atmosphere in linear units (though not in hairs), because what they care about is the total amount of water above the telescope, not the humidity (and certainly not the relative humidity).

In visible light water vapor is not a problem; as long as clouds don't form, it's transparent. But to an infra-red telescope it's opaque; you can't look through too much, and less is always better.

A mountain top is better than a sea level location with the same relative humidity for two reasons. It is colder, so the same relative humidity represents less water, and it is above much of the atmosphere, so the total thickness of water between the ground and space is less (there is less air as well, which is also good).

Putting the telescope in space works, but has practical limitations.