Vexillium's Atmosphere

Introduction to Vexillium's Atmosphere

The planet Vexillium sits in a vacuum with a thin atmosphere of its own slowly baking under the glare of Sol, Urun, the sun. As sunlight approaches the planet in parallel lines, the middle surfaces of the Vexillium sphere will receive far more intense sunlight than the poles.

Reduce, for a moment, the three dimensions of Vexillium to two, a circle. The same amount of sunlight hits the circle at Area A as hits Area B, yet Area A is smaller (or shorter on our two dimensional planet) than Area B.

Therefore, the light intensity at the equatorial regions is far higher than at any other latitude on the planet. This is most obvious at the poles at the equinoxes, when the planet is facing the sun "straight-on". The light at the poles is grey and very dull, whereas at the equator it is very intense and bright. And hot. For if the light intensity is higher, then the heat intensity is also higher, and the planet will inevitably be hotter at the equator than at the poles, which we know as fact.

We already know that hot air rises. Therefore, the heated air at the equator will rise. And because it is rising, there will be less air on the surface of the earth at the equator, and therefore, less air pressure (pressure just being a measure of the number of air molecules per volume). At the same time, cold air is more dense, and since it will be much colder at the poles, the air there will be a much higher pressure.

Simply put, high pressure air will attempt to reach low pressure regions, and a cycle of air flows develops on the planet.

More completely, since there will always be air on the surface of the equator being heated, there will be a steady supply of air rising. Therefore, the air that has already risen must move on, and it does: toward the poles. When it reaches the poles, the air is colder, and our heated air will sink to the ground. And because the air is sinking, it will create an area of high pressure over the poles. The air will then move along the surface, sucked into the low pressure zone around the equator.

Since the air at the surface of the planet is wetter than high, high, high in the upper reaches of the atmosphere, the warmer air dropping down on the poles will be dry, while the air coming into the low pressure equator will be moist, and thus humid. Now, where there is rising humid air, water in the air will condense as the air pressure decreases at higher altitudes. And condensed air is clouds. Conversely, descending dry air will create no clouds.

This is why the polar regions are cold, dry and cloudless, while the equator is hot, humid and frequently cloudy. It is also why high-pressure regions are associated with sunny weather, whereas low-pressure regions are cloudy and stormy.

Questions:

• Sorry, why do clouds only form over humid, low pressure places?

Spin that baby!

It is not so simple on Vexillium primarily because the planet is spinning. In reality, our neat single poles-to-equator air flow (at the surface) is interrupted by a reverse-direction flow in the middle.

The nett result is the same -- air will flow from the poles to the equator -- but now there will be an additional high-pressure region around 30º latitude and a low at 60º latitude.

Now, since we have already learned that a low-pressure region will be humid and a high-pressure will be drier, we can conclude that at 30º latitude it will be drier and sunnier, and that the 60º latitude will be more humid and thus more cloudy.

Since the 30º latitude is sunnier and drier, it is prone to deserfication, particularly in the centre of large bodies of land such as on Eras, where regional surface winds carrying ocean moisture cannot reach. This affects western Zartania and Whiland, Lysonia and neighbouring lands. Furthermore, any land down-wind of these 30º dry inland regions will experience hot dry winds, particularly in summer.

Planetary winds

We have already seen, there will be significant surface winds travelling from the poles and the 30º latitudes to the 60º latitudes, and from 30º to the equator. Very broadly speaking, these follow the pattern shown right.

The trade winds are winds from the north-east to the equator, and we used to great effect to facilitate trading in the futuronian ocean and elsewhere. North of the 30ºN latitude (and conversely in the south), there are the Westerlies, or roaring forties because of their proximity to 40º latitude. There is also the polar easterlies, north of 60ºN and south of 60ºS, that flow from the poles from the east. However, since these latitudes are heavily obstructed by sea ice, they have not been particularly utilised by sailors.

Of course, these winds would follow these patterns if there was no land. They are obstructed and redirected by land masses, by regions of higher and lower air pressure due to surface heating of the land (since land absorbs and dissipates heat far more easily than water).

Questions?

• Why do the trade winds move in an easterly direction?

ITCZ

You will note that the trade winds, those from the 30ºN and 30ºS regions flowing from the north- and south-east toward the equator, collide at the equator. The nett result was known to sailors as the doldrums, a hot, humid and windless region that could simply end a sailing ship's voyage.

It is known to climatologists as the Inter-Tropical Convergence Zone (ITCZ or "itch"), and is important to Vexillium because it moves, following the sun north in July, and south in January.

The ITCZ is the reason for the Eulos monsoons, for hurricanes, and a variety of other weather phenomenon.

What have we learned?

• That the equator receives more sunlight, is therefore warmer, has on-average lower air pressure, is more humid, cloudier and stormier.
• That the poles receive a lot less sunlight, and are thus cold, but, because they have higher air pressure, are sunnier, with dry, cold air.
• In addition, because the planet spins, there will be another low-pressure region at 60º latitude and a high pressure region at 30º latitude.
• The high pressure region at 30º latitude will be sunnier and the air will be drier, making the latitude prone to deserfication. This affects, most notably, the lands around north-eastern Eras, particularly Western Zartania and Whiland, inland Lysonia, and neighbouring lands.
• That the Trade winds flow from the north--east (in the northern hemisphere) toward the equator.
• That where the Trade winds meet at the equator, the hot, humid, cloudy region can be windless for days, and was known as the doldrums to sailors.
• That the Westerlies flow from the south-west (in the northern hemisphere) toward the 60º latitude.
• That there is Polar Easterlies winds.
• That the ITCZ is the hot, humid, cloudy region around the equator, also called the Doldrums, that follows the sun north and south, and causes the monsoons and hurricanes.