Wednesday, April 8, 2015

Weather System Mark 6 - Air Masses

Do not read this post without first reading its predecessor.

Step 2: Another Station

Let's leave Palmyra behind for the moment.  We will be coming back to it, but first we have across the desert about 2 degrees in longitude to another town called Rutbah in Iraq (station link).  We will need information from Rutbah in order to determine the conditions in Palmyra.  Rutbah is the nearest station to Palmyra's southeast.

Like Palmyra, we first work out Rutbah's Low and High, then calculate the Minimum, the Mean and the Maximum as we did before.  This is easy, giving us the following table:

As the reader can see, there is very little difference between Rutbah and Palmyra in February.  This is a good thing.  It helps establish a continuous baseline of temperature variance for a larger area than just one station.

Like Palmyra, we can also work out Rutbah's Drift and it's Mass.  These change every time we choose to calculate them, but we can pick some result that suits us.  Let's say that Rutbah's Drift is -2 (temperature 47F) and that means that Rutbah is also covered by a low air mass.

Hm.  That would seem to suggest that nothing is happening.  That is not, however, the case.

Rutbah has an elevation of 2,017 feet above sea level.  Palmyra is only 1,295 feet above sea level.  This means that when Rutbah's low air mass settles, it tends to follow the ground in the direction of Palmyra.  Now, this wouldn't be true if there were mountains between the two stations - but if there was a mountain range between the stations, it would have its own station dividing the two places.  As it happens, there is no substantial barrier between Palmyra and Rutbah, so yes!  We have a steady movement of air downwards from higher Rutbah into lower Palmyra.

This gives us a wind direction.  In this case, the movement of air is northwest.

We have enough information to calculate a windspeed as well, but I'm going to put that off for the time being - because we have to talk about air masses in this post.

Suppose that Rutbah had rolled a higher temperature, producing a high?  Suppose that Palmyra had a high and Rutbah had a low?  What then?

I calculate nine possible 'pairs.'  Lows and Highs are 'L' and 'H'; a mass that is neither is 'M' for mass or mean.  That gives us H-H, L-L, L-H, H-L, H-M, M-H, L-M, M-L and M-M.  We can presume, for shorthand purposes, that the station that is higher in elevation is always the first letter.

Thus, if Palmyra has a high air mass and Rutbah a low, we can call the pair L-H, or LH.  If Rutbah has a stable, unmoving air mass (M) and Palmyra a low, that would make the pair ML.  Follow?

Let's add some visuals.  The only pair type not included in the below is MM, since there is no movement of air between the two stations.

Each of the above pairs produces a specific meteorological result, all of which can be found in Wikipedia.  An updraft is a small-scale current of rising air, where one warm region slowly rises across the Earth's elevation.  Subsidence is the reverse.  We've already covered that.  With an inversion (those that are under pressure), the lifting effect of the high produces a wind shear over the cooler air below, creating thunderheads (this happens in the real world, though not always - but I already said to throw out the real world).  In a downburst, cold air drops through a warm air mass and creates micro- and macrobursts, which we can interpret here as varyingly intense windstorms.  Unstable and stable thermals are dependent on the adjacent air mass; when the still mass is lower in elevation from the thermal, the upper thermal cools and drops cold air into the unmoving, yet warmer mass, creating rain clouds without wind shear.  When the adjacent mass is higher in elevation, however, the rising thermals are stable, producing moderately cloudy skies and fine weather.  Downdrafts are milder forms of subsidence, often causing morning fog as it slips under stable air masses.  Cool subsiding air is subject to adiabatic warming that causes the evaporation of clouds, in this case happening when cooler air is warmed by an adjacent, high stable air mass.

All of that above is half truths and half lies - but hell, it works.  It makes a random system based on two separate calculations based one wholly local data.  What more could one ask?

We are not remotely done - but before I move onto Step 3, I'm going to wait for the morning.

1 comment:

Maliloki said...

Very cool stuff. I look forward to the rest. "Simple" but complete is hard to do, but the most interesting to me. Anybody with the willpower can make a table(s) that cover everything and take anything into account, but it takes real effort and elegance to make something cover a lot of ground with little information.

I don't know if that made any sense, or if your final result will be anything like what I'm imagining, but its late and my brain is only half working.