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Atmospheric Cells II
Cell Boundaries and Pressure
At the boundaries between cells bands of high or low pressure form. Low pressure bands form where air is rising; on Earth, or any comparable three-cell system, at the equator (the ITCZ Inter-Tropical Convergence Zone) and at the Polar Front (boundary between the polar and Ferrel cells). Storms and generally unpredictable weather are associated with these low-pressure bands. High pressure bands form at boundaries where air is falling (e.g. the Ferrel/Hadley cell border). These areas are associated with stable weather and very low precipitation - moisture in these areas tends to manifest as fog. More on this later - for now, we'll just work out where the damn things are.
Seasonal Migration of Cell Boundaries
When the circulation of air is driven by solar heating, the most important seasonal migration is that of the hot equatorial zone that forms the border between the two air circulation hemispheres - the thermal equator, which in turn forms the ICTZ low pressure belt. Over land (or high specific heat capacity areas) this band tends to follow the sun's zenith, or the latitude at which the sun may be directly overhead. This may sound like it's leading to some complicated trigonometry, but it isn't - broadly, the sun will be overhead at 0 degrees latitude during the equinoxes, at a latitude equal to the planet's axial inclination at the solstices, and move smoothly from one stage to the next (equinox [0 lat] -> summer solstice [max lat] -> equinox [0 lat] -> winter solstice [min lat], and so on). Over oceans (or low specific heat capacity areas) the movement will be much less pronounced, due to the less variable temperature.
On Blobworld, the ITCZ would lie about along the black line during north hemisphere summer (top) and winter (bottom):
In the northern hemisphere summer (top map) the ICTZ moves north - slightly over the oceans, more so over land, and at its greatest, approaching the boundary of the tropics, where there is the largest landmass nearby. In the northern hemisphere winter (bottom map) it moves south - the effect is less pronounced because the landmasses in the southern hemisphere are smaller and the ocean area is greater.
As long as the areas of least heating remain about at the poles, the boundaries of other cells are pushed up or pulled down proportionally in latitude. Proportionally is the key word here - regardless of anything else, a cell will occupy the same proportion of latitude (say, a third), between the ICTZ and the pole. The moving ICTZ causes the specific latitudes and absolute sizes of the cells to expand and contract, but not these proportions. The presence or absence of land also does not significantly affect these boundaries [confirmation needed]. On Blobworld:
In the northern hemisphere summer (top), the Ferrel/Hadley cell borders (orange), and thier associated wind and rain patterns, are pushed up in latitude, as are the Polar/Ferrel boundaries (purple). Broadly, in the northern hemisphere this means that only a small area is affected by polar winds and the warm/stable Ferrel/Hadley border weather pattern reaches higher latitudes, as does the rains of the ICTZ - all part of what forms "summer" conditions. In the southern hemisphere the situation is reversed - a larger area under the polar cell, the Ferrel/Hadley high pressure systems moving further towards the equator, and the stormy ICTZ moving away from outer tropical areas and taking the rains with it - all part of the "winter" pattern. In the second map, the basics are reversed, with a "summer" southern hemisphere and a "winter" northern one.
If you have a reasonably Earthlike world, especially if it has an Earthlike axial tilt, you probably have all the information you need to add these cell boundaries/pressure bands to your maps. If you have a more exotic axial tilt, you might want to stay tuned for the next section, in which I explore their effects.
GIMP Summary:
New layer - Cell boundaries
Paths tool
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"It is quite certain, in particular, that I have always been insane." ~ Aleister Crowley
"Save us all from arrogant men/And all the causes they're for/I won't be righteous again/I'm not that sure any more." ~ Shades of Grey, Billy Joel
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