Low-sun

These precautions being taken, there was no difficulty in recognizing, first, during frosty weather, when meteorologists know there is a minimum of moisture in the air, what should be the normal appearance of dry-gas lines or bands, for they are only then conspicuous, and are chiefly great B, the alpha band between C and D, and a remarkable band on the green side of the universally-known D-line of the regular solar spectrum, that band being remarkable not only for being situated as a dark shade in the otehrwise brightest part of the spectrum of daylight, but by being much more dependent than other dry bands on the lowness of altitude of the sun at the moment for its full and darkest development, and thence called in these inquiries the "low-sun band."

During the high-sun season the intertropical convergence zone with its thundershowery precipitation affects the areas much as it does in the Ar areas year-round; during the low-sun season the subtropical high pressure cells shift equatorward over the areas much as they occupy the subtropical desert areas on the poleward side of Aw areas year-round.

From November until April —the low-sun, or winter, months — the actual precipitation falls below this threshold, resulting in a deficit of moisture (the areas shown in white).