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20-04-2003, 01:56 PM

(John Brock) wrote in message ...
In article ,
Jaak Suurpere wrote:
(Nick Maclaren) wrote in message ...
In article ,
(Jaak Suurpere) writes:
| (pete) wrote in message ...

| The air density in such a basin, that near the equator, would make the
| conditions unthinkably intolerable. You would have rivers flowing
| down into the basin and evaporating, yielding a blistering hot
| desert which would be very high humidity were it not for its high
| temperature and pressure.

| Pressure would not do anything for humidity.
| If there is open water to evaporate, the air does get near saturated,
| even if the water is boiling hot. Which probably would not have been
| the case in Messinian times. It was still not that deep.

By which I assume that you mean that air flow through the basin
was significant? The "open water - saturation" effect is
conditional on the air movement not counteracting it.

Correct.
If you have archives extending to mid-December, I discussed it
before...
Basically, Mediterranean deep basin would be in a bad situation
regarding air movement.
It would be 3 or 4 kilometres above

Sorry - below, of course!

any surrounding area.
Any wind descending into basin would have to heat at the dry adiabatic
lapse rate. Which, IIRC, is about 10 degrees per kilometre. Over that
height, it would be 30 to 40 degrees.

In summer, it would mean heating from 25 degrees to around 60. If the
basin is only ventilated by winds of over 60 degrees, any cooling of
air inside the basin, whether through evaporation or radiation, would
tend to form cooler air mass, which would be stable and therefore
stagnant. And this would mean thatr it will reach saturation.

This is very interesting, and I want to make sure I understand it:

About 5 million years ago the Mediterranean not only went dry but
turned into a desert far hotter and more hostile to life

This I discussed before. I'll repeat it below for those who do not
have archives.

than anything on Earth today. The reason it was so hot was that as air
descended 3 or 4 kilometers into the basin (someone else said 3
miles -- which is right?)

Both are right, kind of.
The current deepest spots of Mediterranean are at a depth of around 5
kilometres - approximately 3 miles.

But this is on a small area. If there was any water remaining in the
Mediterranean, it could be expected to fill the deepest spots.

But it does seem likely that the remaining water levels could have
been around 3 or 4 kilometres below ocean.

it heated about 10 degrees per kilometer
due to increasing air pressure (is that what the "dry adiabatic
lapse rate" refers to?). In the summer the temperature would reach
60 degrees (Celsius I assume; = 140 Fahrenheit). The air didn't
circulate much, being at the bottom of a basin, and if there were
any significant standing bodies of water at the bottom (and I guess
there had to be, since a number of rivers empty into the Mediterranean)
they would cool the air just slightly, and this air would form a
hot, stagnant air mass with humidity at 100 percent that was
basically nailed to the bottom of the basin.

Is that a reasonable summary?

Summary of what was recently said; but there is more to this.

Synoptic-scale winds, the only thing that could have ventilated the
air in the bottom and kept it dry in the presence of standing water,
would have had to heat at the dry adiabatic lapse rate, 10 degrees per
kilometre, leading to summer temperatures in 60s of Celsius.

However, the air was humid. I have argued that if there was no
large-scale wind capable of blowing the steam away from basin bottom,
the vapour buildup would have favoured radiative heating of basin
bottom since steam is powerful greenhouse gas. Eventually, the vapour
would have caused wet convection - in other words, rainfall probably
in form of thunderstorms.
The wet adiabatic lapse rate is much lower than the dry adiabatic
lapse rate. I have not found out how much lower, especially in the
temperature range I am interested in. Does anyone in s.g.m know?

Thus, it seems likely that the temperatures may have been below 60-s
and accompanied by some rainfall.

Now regarding hostility to life:
Yes, a man whose body temperature is in low 40-s is in mortal peril. A
man whose body temperature is in upper 40-s is a corpse.

The absolute maxima of temperature on modern Earth have occurred in
Azizija - 58 Celsius - and Death Valley - 56,7 degrees.

Death Valley has July averages of 34 degrees, IIRC, and Azizija has
July averages of 28 degrees.

Now if you go to Death Valley or Azizija, in both places you will find
fairly diverse plant life. Part of the plants vegetate throughout
thesummer.

Plants cannot flee heat like animals. Nor can plants, especially
desert plants, cool themselves very much by evaporation like mammals
do, because they cannot afford the water.

It follows that many plants now growing in Azizija, or their
ancestors, have heated to 58 degrees and lived. Actually, sunlight
ground and plants on it could heat to higher temperatures than air in
shadow!

Now, both in Death Valley and Azizija, the extreme heat is usually
accompanied by low humidity. Mammals enjoy it, since it helps
evaporative cooling.

However, plants are not helped much by evaporation. On the contrary -
it tends to dry them up.

It would seem likely that plants which can endure 58 degrees and low
humidity could also endure 58 degrees and high humidity.

Now, the area with currently the highest dewpoint temperatures -
southern Red Sea region - does not really have that great absolute
temperature maxima. What makes the heat troublesome for men - not
intolerable, since they do live there, and long have - is the fact
that there are consistent high temperatures with high humidity.

I don't remember the exact data. But I imagine that in summer of Red
Sea region, you could have night temperatures of upper 20-s and day
temperatures of upper 30-s, with high humidity - whereas in Death
Valley or so, you'd encounter night temperatures of lower 20-s and day
temperatures in lower 40-s, with low humidity, and with occasional
temperatures in 50-s.

Now try to imagine a region where night temperatures are in lower 40-s
and day temperatures in upper 40-s, with high humidity. Occasional
maxima might be somewhere in 50-s, with high humidity...

Men would not endure. On the other hand, many plants might.

Not all the plants, though. I wonder which plants would choose to grow
in such conditions?

One more thing: these conditions might be unfavourable to many
animals, including herbivores.

This is really quite bizarre -- like
something on another planet. If this situation still existed the
worst parts would be as hostile as the South Pole. Worse even,
since extreme cold is easier to deal with than extreme heat. It's
interesting to think about what the history of exploration into
this region might have looked like had it existed until today!

We could expect some seasonality, since the Mediterranean basin is far
from equator. The heat would be less in winter. How much less is very
unclear to me!