Two belts of calm, light, and variable winds in the global pressure system: the doldrums near the equator and the horse latitudes near the subtropical highs at about 30° north and south.
- The doldrums lie along the equatorial low-pressure belt (the equatorial trough), where the trade winds of both hemispheres meet at the Inter-Tropical Convergence Zone (ITCZ).
- In the doldrums air rises (convection) rather than blowing horizontally, giving calm conditions, frequent thunderstorms, and heavy convectional rainfall; sailing ships could be becalmed here.
- The horse latitudes are the subtropical high-pressure belts near 30° north and south, where descending air gives calm, dry, settled weather and the world's major hot deserts.
- The name "horse latitudes" comes from the tradition that becalmed sailing ships threw horses overboard to save water; the doldrums are also called the "belt of calm."
- From the subtropical highs, trade winds blow toward the equatorial low (the doldrums) and westerlies blow toward the subpolar low, so both belts are sources or sinks of the planetary wind system.
The pairing of doldrums (equatorial low, rising air, rain) with horse latitudes (subtropical high, sinking air, deserts), and the link of the doldrums to the ITCZ, are recurring climatology facts.
Doldrums (equator, low pressure, rising air, wet) versus horse latitudes (about 30°, high pressure, sinking air, dry); both are calm-wind belts, but for opposite pressure reasons; the doldrums coincide with the ITCZ.
Calm belts: doldrums at the equatorial low (rising, rainy) and horse latitudes at the subtropical highs (sinking, dry, deserts).