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Re: (meteorobs) streetlights in the cold....
On Tue, 4 Jan 2000, Robert Gardner wrote:
> Perhaps you can explain something to me. If there is no condensation in the air,
> does the air become less transparent? After all moist air is less dense than dry
> air, i.e. less mass per unit volume.
>
If given cold conditions which are sufficiently dry in that no
condensation occurs, I would think that the air becomes more transparent
than for warmer conditions.
Before I explain myself, I want to make a comment about your last
statement, Robert. As I am unaware of your (and others) background in
things concerning weather and the like, I hope I don't insult your
(or anyone elses) intelligence here with the following details.
It is true that moist air is less dense than dry air, and thus, should
rise above dry air. But strangely enough, when fog forms, it starts from
the ground up, sort of implying the air near the ground is moister, which
goes against what was stated above.
I'm going a bit off the original question here as I'm going to consider
condensation, but this may prove interesting.....perhaps???
I suppose a more useful description of the air is a quantity called the
relative humidity, as it determines how close the air is to becoming
saturated. It is a ratio of the mass of water vapour in the air compared
with the _maximum possible_ amount of water vapour that the air can hold
_at that temperature_. That is, when the relative humidity = 100%, the air
can hold no more water vapour at that temperature, and condensation will
occur. This maximum possible amount depends on temperature as mentioned in
an earlier email, ie the colder the air, the less water vapour can be held
before condensation takes place.
So, there may be a case where the temperature = 5 deg C (41 deg F) and
relative humidity = 100%. However, if this same parcel of air is heated to
a temperature of 10 deg C (50 deg F), the relative humidity becomes 50%.
[Note: These numbers have been made up. Do not take them as gospel.
However, they do show the behaviour that would occur.]
But what about the fog?
During the night, the ground re-rediates the heat it obtained during the
day, and cools much faster than the rest of the atmosphere. Through
conduction, the air immediately adjacent to the surface also cools, and by
dawn, the temperature in the lowest 200-500 metres [~600-1500ft]
(depending upon location), is found to increase linearly with height.
Now, although the air near the ground contains less water vapour,
the cooler temperatures bring the relative humidity closer to 100% than
for the air with a greater mass of water vapour, higher up where the
temperature is greater. Thus the condensation, eg fog, begins at
ground level and progressively increases in height.
So, in summation although the air near the ground contains less water
vapour, due to lower temperatures, it affects the transparency of the
atmosphere to a greater deal.
If that hasn't confused everyone, there's more to come *8^P
Now, back to the original question with regards to the transparency of
the atmosphere and _no_ condensation, or relative humidities ~ 40% (say).
In this circumstance, cooler nights would provide more transparent skies
than warmer nights with the same relative humidity.
The reason being that, as mentioned above, the warmer air can hold more
water vapour by mass. For example, consider a warm and a cool air mass
which have identical relative humidities = 40%, at sea level
(pressure=1000hPa)
Case 1 Case 2
air temperature: 15 10 (deg C)
amount of water vapour held: 0.12 0.09 grams/kilogram
maximum that can be held: 0.31 0.23 grams/kilogram
I should state that the units of water vapour are:
grams of water vapour held per kilogram of air
and these values are indeed fair dinkum!
So from the second row of the above table, at 10 deg C, the cool air mass
holds only 3/4 the total water vapour of the warm air mass even though
they have the same relative humidities. This means there is less water
vapour and thus scattering bodies in the cool air mass compared with the
warm air mass.
Whoa, that was long.
I hope this email cleared things up, literally!
Cheers
--
Robert A. Goler
E-mail robert@neumann.maths.monashdot edu.au
http://www.maths.monashdot edu.au/~robert/
Department of Mathematics and Statistics
Monash University
Clayton, Vic 3168
Australia
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